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U.S. NAVY TOWING MANUAL

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					SL740-AA-MAN-010
0910-LP-101-2029                                                                     REVISION 3




         U.S. NAVY
      TOWING MANUAL
                                              IT   ED STATES NAV
                                           UN                   Y
                                     NA




                                                                  D




                                       AL
                                                                AN
                                       V




                                            SEA                 MM
                                                   SYSTEMS CO




      THIS DOCUMENT SUPERSEDES NAVSEA SL 740-AA-MAN-010 DATED 1 SEPTEMBER 1988




                   THIS DOCUMENT HAS BEEN APPROVED FOR PUBLIC RELEASE AND SALE;
                                   ITS DISTRIBUTION IS UNLIMITED




                    Published by direction of Commander, Naval Sea Systems Command




                                                                        1 JULY 2002
SL740-AA-MAN-010
                                                  U.S. Navy Towing Manual

Date of Issue for original page is 1 July 2002

                                        LIST OF EFFECTIVE PAGES

Page No.                                    * Change No.           Page No.                                    * Change No.

Title and A . . . . . . . . . . . . . . . . . . . . . . . . 0      E-1 through E-6 . . . . . . . . . . . . . . . . . . .           0
Certification Sheet . . . . . . . . . . . . . . . . . . 0          F-1 through F-5 . . . . . . . . . . . . . . . . . . .           0
blank . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0    F-6 blank . . . . . . . . . . . . . . . . . . . . . . . .       0
Flyleaf-1 (Flyleaf-2 blank) . . . . . . . . . . . . 0              G-1 through G-19 . . . . . . . . . . . . . . . . . .            0
i Foreword . . . . . . . . . . . . . . . . . . . . . . . 0         G-20 blank . . . . . . . . . . . . . . . . . . . . . . .        0
ii blank . . . . . . . . . . . . . . . . . . . . . . . . . . . 0   H-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   0
iii through xix . . . . . . . . . . . . . . . . . . . . . . 0      H-2 blank . . . . . . . . . . . . . . . . . . . . . . . .       0
xx blank . . . . . . . . . . . . . . . . . . . . . . . . . . 0     H-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   0
xxi through xxxvi . . . . . . . . . . . . . . . . . . . 0          H-4 blank . . . . . . . . . . . . . . . . . . . . . . . .       0
1-1 through 1-5 . . . . . . . . . . . . . . . . . . . . 0          H-5 through H-19 . . . . . . . . . . . . . . . . . .            0
1-6 blank . . . . . . . . . . . . . . . . . . . . . . . . . 0      H-20 blank . . . . . . . . . . . . . . . . . . . . . . .        0
2-1 through 2-10 . . . . . . . . . . . . . . . . . . . 0           I-1 through I-22. . . . . . . . . . . . . . . . . . . .         0
3-1 through 3-27 . . . . . . . . . . . . . . . . . . . 0           J-1 through J-9 . . . . . . . . . . . . . . . . . . . .         0
3-28 blank . . . . . . . . . . . . . . . . . . . . . . . . 0       J-10 blank. . . . . . . . . . . . . . . . . . . . . . . .       0
4-1 through 4-46 . . . . . . . . . . . . . . . . . . . 0           K-1 through K-9 . . . . . . . . . . . . . . . . . . .           0
5-1 through 5-29 . . . . . . . . . . . . . . . . . . . 0           K-10 blank . . . . . . . . . . . . . . . . . . . . . . .        0
5-30 blank . . . . . . . . . . . . . . . . . . . . . . . . 0       L-1 through L-11 . . . . . . . . . . . . . . . . . . .          0
6-1 through 6-37 . . . . . . . . . . . . . . . . . . . 0           L-12 blank. . . . . . . . . . . . . . . . . . . . . . . .       0
6-38 blank . . . . . . . . . . . . . . . . . . . . . . . . 0       M-1 through M-23. . . . . . . . . . . . . . . . . .             0
7-1 through 7-15 . . . . . . . . . . . . . . . . . . . 0           M-24 blank . . . . . . . . . . . . . . . . . . . . . . .        0
7-16 blank . . . . . . . . . . . . . . . . . . . . . . . . 0       N-1 through N-2 . . . . . . . . . . . . . . . . . . .           0
8-1 through 8-62 . . . . . . . . . . . . . . . . . . . 0           O-1 through O-13 . . . . . . . . . . . . . . . . . .            0
A-1 through A-3 . . . . . . . . . . . . . . . . . . . . 0          O-14 blank . . . . . . . . . . . . . . . . . . . . . . .        0
A-4 blank . . . . . . . . . . . . . . . . . . . . . . . . . 0      P-1 through P-12 . . . . . . . . . . . . . . . . . .            0
B-1 through B-15 . . . . . . . . . . . . . . . . . . . 0           Q-1 through Q-27 . . . . . . . . . . . . . . . . . .            0
B-16 blank . . . . . . . . . . . . . . . . . . . . . . . . 0       Q-28 blank . . . . . . . . . . . . . . . . . . . . . . .        0
C-1 through C-6. . . . . . . . . . . . . . . . . . . . 0           R-1 through R-12 . . . . . . . . . . . . . . . . . .            0
D-1 through D-18. . . . . . . . . . . . . . . . . . . 0            S-1 through S-13 . . . . . . . . . . . . . . . . . .            0
* 0 in this column indicates an original page.




A
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                                          RECORD OF CHANGES
                                             ACN/FORMAL

                   DATE
 *CHANGE                                                                             ENTERED
                    OF                 TITLE AND/OR BRIEF DESCRIPTION**
  ACN NO.                                                                               BY
                  CHANGE




*When a formal change supersedes an ACN, draw a line through the ACN number
**Only message or letter reference need be cited for ACNs

                                                                              Flyleaf-1/(Flyleaf-2 blank)
                                                     U.S. Navy Towing Manual


                                                     Table of Contents

Paragraph                                                                                                                             Page
Chapter 1 - OPERATIONS OVERVIEW
1-1 Introduction to Navy Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1
1-2 Harbor Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
1-3 Point-to-Point Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
      1-3.1 Inland Towing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
      1-3.2 Ocean Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
      1-3.3 Defueled Nuclear Powered Ships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
1-4 Rescue and Emergency Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
      1-4.1 Naval Task Force Standby Duty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
1-5 Salvage Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
      1-5.1 Combat Salvage and Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
1-6 Special Ocean Engineering Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
1-7 Tow-and-Be-Towed By Naval Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
Chapter 2 - OVERVIEW OF TOWING SHIPS
2-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2-2 Requirements Placed on Towing Ships. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2-3 Design Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
      2-3.1 Stern Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
      2-3.2 Tug Powering and Bollard Pull. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
      2-3.3 Fenders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
               2-3.3.1 Features and Characteristics of Fenders . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
               2-3.3.2 Operating Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
2-4 Yard or Harbor Tugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6
      2-4.1 YTL Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6
      2-4.2 YTB Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6
2-5 Ocean Tugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6
      2-5.1 ARS 50 Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7
      2-5.2 T-ATF 166 Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7
Chapter 3 - TOWING SYSTEM DESIGN
3-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3-2 Designing a Towing System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
      3-2.1 Tug and Tow Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3-3 System Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1



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                                                      U.S. Navy Towing Manual


3-4 System Design Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
      3-4.1 Calculating Total Towline Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
               3-4.1.1 Calculating Steady Resistance of the Towed Vessel . . . . . . . . . . . . . . . . . . 3-2
               3-4.1.2 Calculating Steady Towline Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4
               3-4.1.3 Calculating Steady Towline Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4
               3-4.1.4 Dynamic Loads on the Towline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5
               3-4.1.5 Factors of Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
               3-4.1.6 Predicting Dynamic Tensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
      3-4.2 Calculating Towline Catenary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9
      3-4.3 Reducing Anticipated Towline Peak Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10
               3-4.3.1 Using an Automatic Towing Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23
               3-4.3.2 Using Synthetic Towlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23
      3-4.4 Tug and Equipment Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24
               3-4.4.1 Tug Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24
               3-4.4.2 Towing Gear Selection Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26
Chapter 4 - TOWLINE SYSTEM COMPONENTS
4-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
4-2 Towline System Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
4-3 Main Towing Hawser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
      4-3.1 Wire Rope Hawser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
      4-3.2 Synthetic Hawser. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
4-4 Secondary Towline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3
4-5 Attachment Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5
      4-5.1 Winches and Towing Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
      4-5.2 Bitts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
      4-5.3 Padeyes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8
      4-5.4 Padeye Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-13
      4-5.5 Deck Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15
      4-5.6 Smit Towing Bracket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15
      4-5.7 Towing Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17
      4-5.8 Chocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17
      4-5.9 Fairleads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17
4-6 Connecting Hardware (Jewelry) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17
      4-6.1 Shackles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18
      4-6.2 Other Connecting Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21
      4-6.3 Wire Rope Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21
      4-6.4 Synthetic Line Terminations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23
      4-6.5 Synthetic Spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-25


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                                                     U.S. Navy Towing Manual


      4-6.6 Bridles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-25
      4-6.7 Pendants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29
      4-6.8 Retrieval Pendant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-30
      4-6.9 Chain Stoppers, Carpenter Stoppers, and Pelican Hooks. . . . . . . . . . . . . . . . . . . . 4-30
      4-6.10 Chafing Gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-33
4-7 Fuse or Safety Link Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-33
4-8 Line Handling Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-36
      4-8.1 Caprails . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-36
      4-8.2 Towing Bows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-36
      4-8.3 Horizontal Stern Rollers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-36
      4-8.4 Capstans and Gypsy Heads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-36
4-9 Sweep Limiting Devices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-39
      4-9.1 Vertical Stern Rollers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-39
      4-9.2 Norman Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-42
      4-9.3 Hogging Strap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44
      4-9.4 Lateral Control Wire. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44
4-10 Cutting Gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44
Chapter 5 - TOW PLANNING AND PREPARATION
5-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
5-2 Lessons Learned . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
5-3 Staff Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
      5-3.1 Towing Ship Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
      5-3.2 Operational Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
               5-3.2.1 Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
               5-3.2.2 Manned Tows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
               5-3.2.3 Tug Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2
               5-3.2.4 Unsuitable Tows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
      5-3.3 Selecting the Navigation Track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
5-4 Towing Responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3
      5-4.1 Sponsoring Command Responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
      5-4.2 Towing Command Responsibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
      5-4.3 Assisting Command Responsibilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
5-5 Review Instructions and Operational Orders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
5-6 Riding Crew Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5
5-7 Preparing the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6
      5-7.1 Installing Flooding Alarms, Draft Indicators, and Other Alarms . . . . . . . . . . . . . . . . . 5-6
               5-7.1.1 Flooding Alarm Sensor Mounting Requirements . . . . . . . . . . . . . . . . . . . . . 5-8


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               5-7.1.2 Wiring and Power Supply Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8
               5-7.1.3 Alarm Lighting Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9
               5-7.1.4 Audible Alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9
               5-7.1.5 Requirements for Other Alarms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9
               5-7.1.6 Draft Indicator Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9
               5-7.1.7 Towed Vessel Propeller Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9
               5-7.1.8 Removing Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9
               5-7.1.9 Locking Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
               5-7.1.10 Allowing Propellers to Free-Wheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
               5-7.1.11 Stern Tube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
      5-7.2 Ballasting or Loading for Proper Trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
      5-7.3 Ballasting for Proper Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10
      5-7.4 Two Valve Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13
      5-7.5 Inspecting the Tow for Structural Damage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14
               5-7.5.1 Barge Hull Thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14
      5-7.6 Locking the Rudder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14
      5-7.7 Installing Navigational Lights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18
      5-7.8 Selecting the Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18
      5-7.9 Preparing Tank Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21
5-8 Emergency Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21
      5-8.1 Electrical Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-22
      5-8.2 Fire-fighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-22
      5-8.3 Dewatering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-23
               5-8.3.1 Deciding to Use Dewatering Equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . 5-23
               5-8.3.2 Choosing Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-23
               5-8.3.3 Pre-staging Hoses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24
      5-8.4 Marking Access Areas on Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24
      5-8.5 Preparing for Emergency Anchoring of the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-24
5-9 Completing the Checklist for Ocean Tows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26
      5-9.1 Determining Seaworthiness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26
      5-9.2 Towing Machine/ Towing Winch Certification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26
      5-9.3 Tow Hawser Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26
      5-9.4 Commercial Vessels (U.S. Coast Guard Inspected) Master’s Towing Certificate . . 5-26
      5-9.5 Preparing for a Riding Crew . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-26
5-10 Accepting the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-28
      5-10.1 Inspecting the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-28
      5-10.2 Unconditionally Accepting the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29
      5-10.3 Accepting the Tow as a Calculated Risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29
      5-10.4 Rejecting the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29



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      5-10.5 Preparing for Departure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29
5-11 Completing the Delivery Letter or Message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-29
Chapter 6 - TOWING PROCEDURES
6-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6-2 Initiating the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
      6-2.1 Accelerating with a Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
      6-2.2 Getting Underway from a Pier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
      6-2.3 Getting Underway in the Stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
      6-2.4 Getting Underway while at Anchor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
      6-2.5 Recovering a Lost Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4
      6-2.6 Emergency Connection to a Disabled Vessel or Derelict . . . . . . . . . . . . . . . . . . . . . 6-6
               6-2.6.1 Using the Anchor Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
               6-2.6.2 Using Installed Bitts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8
               6-2.6.3 Using a Gun Mount or Foundation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9
               6-2.6.4 Placing a Crew on Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9
      6-2.7 Approaching a Drifting Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
               6-2.7.1 Establishing the Relative Drift. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
               6-2.7.2 Similar Drift Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
               6-2.7.3 Dissimilar Drift Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
      6-2.8 Passing the Towline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10
      6-2.9 Communications between Ships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12
6-3 Ship Handling and Maneuvering with a Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12
      6-3.1 Tug Steering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12
      6-3.2 Keeping a Tug and Tow in Step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14
      6-3.3 Controlling the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14
               6-3.3.1 Active Control of the Tow's Rudder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14
               6-3.3.2 Yawing and Sheering of the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
               6-3.3.3 Trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
               6-3.3.4 Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
               6-3.3.5 Use of Rudder or Skegs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
               6-3.3.6 Location of the Attachment Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
               6-3.3.7 Propellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16
               6-3.3.8 Steering Tug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17
               6-3.3.9 Sea Anchor or Drogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17
               6-3.3.10 Bridle vs. Single Lead Pendant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17
      6-3.4 Backing Down with a Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17
6-4 Routine Procedures While at Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17
      6-4.1 Setting Course . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17
      6-4.2 Towing Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17
      6-4.3 Towline Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-18


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       6-4.4 Towing Watch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-18
       6-4.5 Periodic Inspection of Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-18
                6-4.5.1 Boarding the Tow for Inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-19
                6-4.5.2 Inspection Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-19
       6-4.6 Towing in Heavy Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-19
       6-4.7 Replenishment at Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21
                6-4.7.1 Transferring Personnel and Freight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21
                6-4.7.2 Emergency Replenishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21
                6-4.7.3 Rigging and Use of Fueling Rigs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21
                6-4.7.4 Astern Refueling from Another Tug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21
                6-4.7.5 Replenishment Near a Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21
6-5 Terminating the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-22
       6-5.1 Requesting Assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-22
       6-5.2 Shortening the Towline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-22
       6-5.3 Disconnecting the Tow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-22
       6-5.4 Towing Delivery Receipt and Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-23
       6-5.5 Transferring the Tow at Sea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-23
6-6 Tow and Be Towed by Naval Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-23
       6-6.1 Towing Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-24
       6-6.2 Towing Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-24
                6-6.2.1 Procedure for the Towing Ship. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-24
                6-6.2.2 Procedure for the Towed Ship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27
                6-6.2.3 Quick Release of Towed Ship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27
       6-6.3 Getting Underway with Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27
6-7 Emergency Towing Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-28
       6-7.1 Fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-28
       6-7.2 Tug and Tow Collision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-28
       6-7.3 Sinking Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-29
                6-7.3.1 Beaching a Sinking Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-29
                6-7.3.2 Slipping the Tow Hawser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-30
       6-7.4 Disabled Towing Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-31
                6-7.4.1 Disconnecting the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-31
                6-7.4.2 Recovering the Towline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32
       6-7.5 Anchoring with a Tow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32
       6-7.6 Quick Disconnect System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-33
       6-7.7 Man Overboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-33
       6-7.8 Using an Orville Hook to Recover a Lost Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34
                6-7.8.1 Origin of the Orville Hook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34
                6-7.8.2 Use of an Orville Hook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34


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Chapter 7 - SPECIAL TOWS
7-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1
7-2 Target Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1
      7-2.1 Williams Target Sled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1
      7-2.2 Towing Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2
      7-2.3 Routine Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2
               7-2.3.1 Transporting the Target to the Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2
               7-2.3.2 Streaming the Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2
               7-2.3.3 Making Turns with the Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2
               7-2.3.4 Recovering the Target. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3
      7-2.4 Special Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3
               7-2.4.1 Passing the Target to a Combatant Ship . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3
               7-2.4.2 Recovering a Capsized Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3
      7-2.5 Target Towing Precautions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4
      7-2.6 Other Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4
7-3 Towing Through the Panama Canal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4
7-4 Towing in Ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5
      7-4.1 Short-Scope Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5
      7-4.2 Saddle Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6
      7-4.3 Rigging for Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6
7-5 Submarine Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6
      7-5.1 Towing Arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7
               7-5.1.1 Retractable Deck Fittings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7
               7-5.1.2 Tow Attachment Points. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7
      7-5.2 Personnel Safety Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8
               7-5.2.1 Protection for Work on the Deck. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8
               7-5.2.2 Boarding the Submarine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8
               7-5.2.3 Personnel Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8
               7-5.2.4 Submarine Atmosphere Problems Resulting from Fire . . . . . . . . . . . . . . . . 7-8
      7-5.3 Tendency to Yaw and Sheer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8
      7-5.4 Rigging for the Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8
               7-5.4.1 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9
               7-5.4.2 Underwater Projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9
               7-5.4.3 Towing by the Stern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9
               7-5.4.4 Use of the Sail as a Tow Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10
               7-5.4.5 Welding to the Hull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10
               7-5.4.6 Passing a Messenger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10
      7-5.5 Towing Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10
               7-5.5.1 Towing on the Automatic Towing Machine . . . . . . . . . . . . . . . . . . . . . . . . . 7-10
               7-5.5.2 Towline Tension and Towing Speeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11


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               7-5.5.3 Drogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11
7-6 Towing Distressed Merchant Ships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11
      7-6.1 Information Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-12
      7-6.2 Attachment Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-12
7-7 Ships with Bow Ramp/Door . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-12
7-8 Towing Distressed NATO Ships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13
      7-8.1 Standardized Procedures (ATP-43). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13
      7-8.2 Making the Tow Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13
      7-8.3 NATO Standard Towing Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-14
7-9 Unusual Tows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-14
      7-9.1 Dry Dock (Careened). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-15
      7-9.2 Damaged Ship (Stern First). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-15
      7-9.3 Inland Barge Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-15
      7-9.4 Other Tows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-15
      7-9.5 Towing on the Hip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-15
Chapter 8 - HEAVY LIFT TRANSPORT
8-1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1
      8-1.1 Repair Work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1
8-2 Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1
      8-2.1 Dry Docking Comparison. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1
      8-2.2 Commercial Fleet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2
      8-2.3 Choosing a Vessel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2
8-3 Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2
      8-3.1 Designating the Lift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2
      8-3.2 Request for Proposal (RFP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9
      8-3.3 Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9
               8-3.3.1 Choosing a Heavy Lift Team . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9
               8-3.3.2 Contractor Preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11
               8-3.3.3 Transport (Load) Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11
               8-3.3.4 Choosing A Load Site. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13
               8-3.3.5 Preparing The Deck . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13
      8-3.4 Pre-Load Conference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13
      8-3.5 Load Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-14
               8-3.5.1 Visual Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-14
               8-3.5.2 Support Tugs/Divers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-14
      8-3.6 Preparing the Asset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-15
               8-3.6.1 Arrival Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-15
               8-3.6.2 Transport of Damaged Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16


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8-4 Loading Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16
      8-4.1 Positioning of the Asset(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16
      8-4.2 Fendering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18
      8-4.3 Riding Crew Accommodations During Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18
      8-4.4 Deballasting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18
      8-4.5 Connection of Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18
      8-4.6 Blocking and Seafastening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-19
8-5 Seakeeping and Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-19
      8-5.1 Ship Motions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-21
                8-5.1.1 Wind Heel Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-21
      8-5.2 Stability of the Asset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-21
                8-5.2.1 Stability Afloat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-22
                8-5.2.2 Stability During Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-22
                8-5.2.3 Draft-at-Instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-24
                8-5.2.4 Draft-at-Landing Fore and Aft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-27
      8-5.3 Stability of the Heavy Lift Ship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28
                8-5.3.1 Intact Stability Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28
                8-5.3.2 Stability During Ballasting/Deballasting . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28
      8-5.4 Stability of the Heavy Lift Ship with the Asset Secured Aboard during Transit . . . . 8-29
                8-5.4.1 Damage Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-30
8-6 Blocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-30
      8-6.1 Preparing the Docking Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32
      8-6.2 Docking Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32
                8-6.2.1 Dynamic Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-33
                8-6.2.2 Loading of Keel Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34
                8-6.2.3 Keel Block Loading Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34
                8-6.2.4 Distribution of Asset’s Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-35
                8-6.2.5 Calculation of the Asset’s Loading on the Docking Blocks by the
                        Trapezoidal Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-36
                8-6.2.6 Knuckle Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-40
                8-6.2.7 Safe Allowable Compressive Stress of Blocking . . . . . . . . . . . . . . . . . . . . 8-42
8-7 Seafastening Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-43
      8-7.1 Side Blocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-43
                8-7.1.1 Stability of High Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-45
      8-7.2 Loading on Side Blocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-45
                8-7.2.1 Assessing the Loading on Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-46
                8-7.2.2 Loading on Side Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-46
                8-7.2.3 Dynamic Loads During Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-47
                8-7.2.4 Dynamic Loads from Ship Motions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-47
                8-7.2.5 Dynamic Loads from Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-49



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               8-7.2.6 Determining the total amount of side blocking required . . . . . . . . . . . . . . . 8-49
      8-7.3 Additional Side Block Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-49
      8-7.4 Spur Shores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-50
               8-7.4.1 Loading on Spur Shores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-51
               8-7.4.2 Determining the Number of Spur Shores . . . . . . . . . . . . . . . . . . . . . . . . . . 8-52
               8-7.4.3 Distribution of Spur Shores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-53
      8-7.5 Seafasteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-54
               8-7.5.1 Dynamic Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-54
               8-7.5.2 Assumed Friction Factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-55
               8-7.5.3 Sea Fasteners Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-55
8-8 Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-57
      8-8.1 Hydrographic Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-57
      8-8.2 Acceptance Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-57
      8-8.3 Structural Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-57
      8-8.4 Indicators and Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-57
      8-8.5 Pre-loading Block Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-58
               8-8.5.1 Wooden Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-59
               8-8.5.2 Block Securing Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-59
      8-8.6 Additional Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-59
      8-8.7 Asset Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-60
      8-8.8 Post Float-On Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-60
      8-8.9 Examination of the Seafastening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-60
               8-8.9.1 Prior to Transit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-60
               8-8.9.2 During Transit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-61
               8-8.9.3 Upon Arrival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-61
8-9 Offloading Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-61
      8-9.1 Prior to Arrival at Destination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-61
      8-9.2 Arrival Activities at Off Loading Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-61
      8-9.3 Off Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-61
Appendix A - SAFETY CONSIDERATIONS IN TOWING
A-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-1
A-2 Scope and Applicability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1
A-3 Basic Safety Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-1
A-4 Specific Safety Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-2
      A-4.1 Specific Approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2
               A-4.1.1 General Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-2
               A-4.1.2 Non-Emergency Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-2
               A-4.1.3 Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-2
               A-4.1.4 Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3


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      A-4.2 Contingency Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-3
Appendix B - WIRE ROPE TOWLINES
B-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-1
B-2 Traceability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-1
B-3 Strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-1
      B-3.1 Elongation (Stretch). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-1
B-4 Maintenance, Cleaning, and Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-3
B-5 New Hawsers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-3
      B-5.1 Unreeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-3
      B-5.2 Reeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-3
      B-5.3 Installing New Wire Rope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-5
B-6 Stowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-7
B-7 Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-7
      B-7.1 General Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-7
      B-7.2 Specific Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-7
B-8 Special Precautions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-10
B-9 Wire Rope Hawsers for Navy Tow Ships. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-10
B-10 Wire Rope Terminations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-10
B-11 Wire Rope Procurement Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-12
B-12 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-12
      B-12.1 Wire Rope Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-12
      B-12.2 Wire Towing Hawsers for T-ATF 166 Class Ships . . . . . . . . . . . . . . . . . . . . . . . . .B-12
      B-12.3 2-1/4-Inch Towing Hawsers for ARS-50 Class Ships . . . . . . . . . . . . . . . . . . . . . .B-13
B-13 Sockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-13
B-14 Lubrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-15
Appendix C - SYNTHETIC FIBER LINE TOWLINE
C-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
C-2 Traceability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
C-3 Strength and Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
      C-3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
      C-3.2 Specific . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
C-4 Elongation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-2
C-5 Maintenance and Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-2
C-6 Stowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3
C-7 Uncoiling or Unreeling New Hawsers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3
C-8 Breaking in New Hawsers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4



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C-9 Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .C-4
C-10 Types of Wear or Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .C-4
C-11 Special Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .C-6
C-12 Fiber Rope Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .C-6
Appendix D - CHAINS AND SAFETY SHACKLES
D-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-1
D-2 Traceability and Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-1
      D-2.1 Traceability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-1
      D-2.2 Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-1
D-3 Strength and Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-2
D-4 Elongation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-2
D-5 Maintenance and Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-2
D-6 New Chain and Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-3
D-7 Stowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-3
D-8 Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-3
      D-8.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-3
      D-8.2 Specific . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-3
D-9 Types of Wear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-4
D-10 Special Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-4
D-11 Chain Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-5
D-12 Connecting Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-5
D-13 Safety Shackles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-5
D-14 Proof Load, Safe Working Load, and Safety Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-6
D-15 Plate Shackles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-6
Appendix E - STOPPERS
E-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-1
E-2 Types of Stoppers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-1
E-3 Prevention of Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-1
E-4 Stopping Off a Wire Towing Hawser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E-1
E-5 Synthetic Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-2
E-6 Stopper Breaking Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-2
E-7 Fiber Stoppers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E-5
E-8 Stopper Hitches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-5
E-9 Securing the Passed Stopper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-6
E-10 Setting the Stopper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-6
E-11 Releasing the Stopper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E-6


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Appendix F - TOWING HAWSER LOG
F-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .F-1
F-2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .F-1
F-3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .F-1
F-4 Log. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .F-1
F-5 Failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .F-1
Appendix G - CALCULATING STEADY STATE TOWLINE TENSION
G-1 Self-Propelled Surface Ships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-1
      G-1.1 Hull Resistance Curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-3
      G-1.2 Additional Resistance Due to Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-3
      G-1.3 Wind Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-16
      G-1.4 Propeller Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-16
G-2 Floating Dry Docks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-16
      G-2.1 Frictional Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-16
      G-2.2 Wave-Forming Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-16
      G-2.3 Wind Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-17
      G-2.4 Total Tow Resistance for Dry Docks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
      G-2.5 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
G-3 Barges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
      G-3.1 Frictional Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
      G-3.2 Wave-Forming Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
      G-3.3 Wind Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
      G-3.4 Total Barge Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
      G-3.5 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18
                G-3.5.1 Frictional Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            G-19
                G-3.5.2 Wave Forming Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   G-19
                G-3.5.3 Wind Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            G-19
                G-3.5.4 Total Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           G-19
Appendix H - CHECKLIST FOR PREPARING AND RIGGING A TOW
Appendix I - TOWING RIGS
I-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1
I-2 Single Tug, Single Tow Configurations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1
      I-2.1 Pendant or Single Leg Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1
      I-2.2 Bridle Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-1
      I-2.3 Towing Alongside . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-5
      I-2.4 Liverpool Bridle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-7
I-3 Single Tug, Multiple Unit Tow Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-7


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      I-3.1 Christmas Tree Rig. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-7
      I-3.2 Honolulu Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-12
      I-3.3 Tandem Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-12
      I-3.4 Nested Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-12
I-4 Multiple Tug, Single Tow Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-12
      I-4.1 Side-By-Side Towing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-13
      I-4.2 Steering Assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-13
Appendix J - EMERGENCY TOWING OF SUBMARINES
J-1 Submarines Prior to the 688 and 726 Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-1
      J-1.1 Tow Attachment Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-1
      J-1.2 The Towing Rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-1
      J-1.3 Chafing Pendant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-4
J-2 SSN 688 Class Submarines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-4
J-3 SSBN 726 Class Submarines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-4
J-4 SSN 21 Class Submarines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-7
Appendix K - COMMERCIAL TUG CAPABILITIES
K-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-1
K-2 Tug Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-1
      K-2.1 Salvage Tug Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-1
               K-2.1.1 Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-1
               K-2.1.2 Draft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.1.3 Freeboard/Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.1.4 Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.1.5 Crew. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.1.6 Towing Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.1.7 Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.1.8 Bollard and Towline Pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
      K-2.2 Power, Bollard Pull, and Towline Pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.2.1 Power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-2
               K-2.2.2 Bollard Pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-3
               K-2.2.3 Towline Pull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-3
               K-2.2.4 Maneuverability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-6
K-3 Ocean-Going Tugs for Hire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-6
      K-3.1 Ocean-Going Tug Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-6
      K-3.2 Decline in Salvage Tug Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-8
      K-3.3 Availability of Ocean-Going Tugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-8
K-4 Obtaining Tug Assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-8
      K-4.1 Emergency Tug Assistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-8
      K-4.2 Restrictions in Contracting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-9


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      K-4.3 Contracting for Emergency Commercial Towing Assistance . . . . . . . . . . . . . . . . . . .K-9
      K-4.4 Arranging for Routine (Non-Emergency) Tows by Commercial Tugs . . . . . . . . . . . .K-9
Appendix L - TOWING MACHINERY
L-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1
L-2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1
      L-2.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1
      L-2.2 Functions of Towing Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1
               L-2.2.1 Attachment of the Hawser to the Tug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1
               L-2.2.2 Hawser Scope Adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1
               L-2.2.3 Storage of Unused Hawser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-1
               L-2.2.4 Quick Release of the Hawser under Tension . . . . . . . . . . . . . . . . . . . . . . . . L-2
               L-2.2.5 Protection of the Hawser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-2
L-3 Automatic Tension Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-3
      L-3.1 Tow Hawsers Surge Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-3
               L-3.1.1 Early Automatic Towing Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-3
               L-3.1.2 Electric Towing Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-3
               L-3.1.3 Wire Surge Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-3
      L-3.2 Automatic Features on Towing Machines - General . . . . . . . . . . . . . . . . . . . . . . . . . L-4
      L-3.3 Limitations in Quantitative Understanding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-4
L-4 Types of Towing Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-4
      L-4.1 Conventional Towing Winches and Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-4
               L-4.1.1 Drum Arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-4
               L-4.1.2 Drum Securing Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
               L-4.1.3 Drum Prime Movers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
               L-4.1.4 Automatic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
               L-4.1.5 Instrumentation and Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
      L-4.2 Traction Winches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
               L-4.2.1 General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
               L-4.2.2 Hawser Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
               L-4.2.3 Traction Winch Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-5
L-5 U.S. Navy Towing Machinery Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-7
      L-5.1 ARS 50 Class Towing Machinery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-7
      L-5.2 T-ATF 166 Class Towing Machinery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-8
Appendix M - ESTIMATING DYNAMIC TOWLINE TENSIONS
M-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-1
      M-1.1 Ship Motion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-1
      M-1.2 Wire Towline Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-1
      M-1.3 Synthetic Towline Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-1
M-2 Design of the Extreme Tension Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-2



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        M-2.1 Understanding Wire Towline Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-3
        M-2.2 Motions of the Tug and Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-3
        M-2.3 Description of Physical Variables and Influences on Extreme Tension . . . . . . . . . M-3
                M-2.3.1 Size of Tug and Tow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           M-3
                M-2.3.2 Wave Size, Angle, and Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                     M-4
                M-2.3.3 Average Towline Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               M-4
                M-2.3.4 Weight and Scope of Towline Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                      M-5
                M-2.3.5 Tow Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       M-5
                M-2.3.6 Yawing and Sheering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             M-5
        M-2.4 Display of the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-5
M-3 Use of the Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-6
        M-3.1 Allowable Extreme Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-6
        M-3.2 Interpolation Within the Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-6
                M-3.2.1 Ship Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   M-6
                M-3.2.2 Tow Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       M-6
                M-3.2.3 Towing Hawser Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               M-7
                M-3.2.4 Wave Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      M-7
                M-3.2.5 Wind Strength and Wave Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   M-7
                M-3.2.6 Adjust Calculations for Sheering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                M-7
M-4 Response to Worsening Sea Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-7
        M-4.1 Reduce Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-7
        M-4.2 Increase Towline Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-7
        M-4.3 Change Course . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-8
M-5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-8
        M-5.1 ATS 1 Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-8
        M-5.2 ARS 50 Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-9
        M-5.3 T-ATF 166 Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-9
Appendix N - REFERENCES
Appendix O - GLOSSARY
Appendix P - USEFUL INFORMATION
P-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P-1
P-2 Weights and Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P-1
Appendix Q - HEAVY LIFT SAMPLE CALCULATIONS
Q-1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-1
Q-2 Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-1
Q-3 Draft-at-Instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-7
Q-4 Draft-at-Landing Fore and Aft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-10
Q-5 Keel Block Build and Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-13
Q-6 Side Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-18


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Q-7 Positioning of Spur Shores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-26
Appendix R - CHECKLIST FOR PREPARING AN ASSET FOR HEAVY LIFT
Appendix S - INDEX




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                                                    List of Illustrations

Figure                                                                                                                                  Page
2-1. ARS 50 Bulwark Forward Limits. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
2-2. Typical Rubber Fenders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4
2-3. Pneumatic and Foam Fenders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5
2-4. ARS 50 Class Salvage Ship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8
2-5. T-ATF 166 Class Fleet Tug. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9
3-1. Towline Forces at Stern of Tow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5
3-2. Towline Tension vs. Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6
3-3. Available Tension vs. Ship’s Speed for U.S. Navy Towing Ships. . . . . . . . . . . . . . . . . . . 3-11
3-4. Catenary vs. Tension; 1 5/8-Inch Wire, No Chain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12
3-5. Catenary vs. Tension; 1 5/8-Inch Wire, 90 Feet of 2 1/4-Inch Chain.. . . . . . . . . . . . . . . . 3-13
3-6. Catenary vs. Tension; 1 5/8-Inch Wire, 270 Feet of 2 1/4-Inch Chain. . . . . . . . . . . . . . . . 3-14
3-7. Catenary vs. Tension; 2-Inch Wire, No Chain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15
3-8. Catenary vs. Tension; 2-Inch Wire, 90 Feet of 2 1/4-Inch Chain. . . . . . . . . . . . . . . . . . . . 3-16
3-9. Catenary vs. Tension; 2-Inch Wire, 270 Feet of 2 1/4-Inch Chain. . . . . . . . . . . . . . . . . . . 3-17
3-10. Catenary vs. Tension; 2 1/4-Inch Wire, No Chain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18
3-11. Catenary vs. Tension; 2 1/4-Inch Wire, 90 Feet of 2 1/4-Inch Chain. . . . . . . . . . . . . . . . 3-19
3-12. Catenary vs. Tension; 2 1/4-Inch Wire, 270 Feet of 2 1/4-Inch Chain. . . . . . . . . . . . . . . 3-20
3-13. Distance Between Vessels vs. Hawser Tension for 1,000 and
      1,800 Feet of 6x37 FC Wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-25
4-1. Typical Towline Connection Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
4-2. Secondary Towline System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4
4-3. Secondary Towline System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6
4-4. Aft End of ARS 50 Towing Machinery Room and Typical Towing Fairleads /Bitts. . . . . . . 4-9
4-5. Horizontal Padeyes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10
4-6. Vertical Free-Standing Padeyes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-12
4-7. Minimum Padeye Design Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14
4-8. Smit Towing Bracket. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16
4-9. Types of Chocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18
4-10. Shackles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-20
4-11. Types of Wire Rope Terminations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-22
4-12. Towline Termination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23
4-13. Pear-Shaped Detachable Links. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24
4-14. Synthetic Line End Fittings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-26
4-15. Synthetic Line Grommet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27
4-16. Towing Rigs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27
4-17. Chain Bridles Using Plate Shackles and Safety Shackles.. . . . . . . . . . . . . . . . . . . . . . . 4-28
4-18. Pelican Hook and Chain Stopper.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-34
4-19. Carpenter Stopper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-35
4-20. Chafing Gear and Stern Rollers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-37
4-21. Caprails. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-38
4-22. Towing Bows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-38
4-23. Capstans and Gypsy Head. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-40
4-24. Stern of T-ATF 166 Class. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-41
4-25. Stern Rollers (ARS 50 Class). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-42
4-26. Norman Pins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-43
4-27. Norman Pin Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45



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4-28. Rigging of Hogging Strap on Ships without Horizontal Stern Rollers. . . . . . . . . . . . . . . 4-46
5-1. Example of a Flooding Alarm Schematic.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7
5-2. Special Draft Markings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11
5-3. Securing the Propeller Shaft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
5-4. Reinforcing Bottom Plating in Barges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-17
5-5. Securing the Rudder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-19
5-6. Securing the Rudder. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-20
5-7. Sample Provisions for Emergency Boarding of Tow at Sea. . . . . . . . . . . . . . . . . . . . . . . 5-25
5-8. Billboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-27
6-1. Methods for Securing Messenger to Towline.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
6-2. Accepting a Tow in the Stream. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5
6-3. Sharing Towing Load Between Bitts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7
6-4. Across Sea/Wind Approach - Similar Drift Rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11
6-5. Downwind Approach Crossing the “T” to Ship Lying Broadside to Wind/Sea. . . . . . . . . . 6-13
6-6. Effect of Towpoint on Steering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15
6-7. Passing a Tow at Sea. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-25
6-8. Tow-and-Be-Towed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26
6-9. Orville Hook Retrieval Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-35
6-10. Orville Hook Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-36
6-11. Deployment of the Orville Hook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-37
7-1. Williams Target Sled Rigged for Tow with Righting Line Streamed. . . . . . . . . . . . . . . . . . 7-1
7-2. NATO Standard Towing Link.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-14
8-1. Heavy Lift Vessel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4
8-2. Heavy Lift Vessel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5
8-3. Heavy Lift Vessel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6
8-4. Plan of Action and Milestones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8
8-5. Assets Being Loaded.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-17
8-6. Phases of Stability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-20
8-7. Draft at Instability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-23
8-8. Limit of Stablility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-25
8-9. Draft at Landing Fore and Aft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28
8-10. GZ Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31
8-11. Load Distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-39
8-12. Keel Blocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-41
8-13. Heavy Lift Blocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-42
8-14. Cribbed Blocks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-44
8-15. Spur Shores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-45
8-16. Overturning Moment Due to Wind Forces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-48
8-17. Sea Fasteners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-56
B-1. Bird Caging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-2
B-2. Popped Core. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-2
B-3. Kinks and Hockles.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-4
B-4. Re-reeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-4
B-5. Wallis Brake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-6
B-6. Nomenclature of Wire Rope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-8
B-7. Measuring Wire Rope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-9
B-8. Poured Sockets FED Spec. RR-S-550D Amendment 1. . . . . . . . . . . . . . . . . . . . . . . . . .B-14
D-1. Types of Chains and Connecting Links. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-16
D-2. Detachable Link with Identifying Marks for Assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . .D-17



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D-3. Typical Method for Modifying Detachable Chain Connecting
      Links for Hairpin Installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-18
E-1. Crisscross Chain Stopper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-2
E-2. Typical Stopper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-3
E-3. Half Hitches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-4
E-4. Crisscross Fiber Stopper.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-4
E-5. Double Half-Hitch Stopper.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .E-5
G-1. Example 1 - Scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-5
G-2. Example 1 - Worksheet.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-6
G-3. Example 2 - Scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-11
G-4. Example 2 - Worksheet.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-12
G-5. Available Tension vs. Ship’s Speed for U.S. Navy Towing and Resistance Curve. . . . G-13
G-6. Hull Resistance Curve, RH/∆ vs. Tow Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-14
G-7. Wave Resistance Curve RS vs. Wave Height and Wind Force.. . . . . . . . . . . . . . . . . . . G-15
I-1. Towing Rigs (Plan View).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-2
I-2. Example of Chain Bridle with Chain Pendant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-3
I-3. Example of Wire Bridle with Wire Pendant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-4
I-4. Towing Alongside. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-5
I-5. Liverpool Bridle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-6
I-6. Use of Liverpool Bridle on Stranding Salvage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-8
I-7. Christmas Tree Rig.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-9
I-8. Example of a Christmas Tree Rig Configuration with Chain Bridle. . . . . . . . . . . . . . . . . . I-10
I-9. Example of a Christmas Tree Rig Configuration with a Wire Bridle. . . . . . . . . . . . . . . . . . I-11
I-10. Three-Barge Tow in Christmas Tree Rig Ready for Streaming. . . . . . . . . . . . . . . . . . . . I-14
I-11. Honolulu Rig. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-15
I-12. Tandem Rig. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-16
I-13. Two-Tug Tows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-17
I-14. Example of Chain Bridle and Pendant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-18
I-15. Flounder Plate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-19
I-16. Plate Shackle and Pin for 2-Inch Closed Socket. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-20
I-17. Plate Shackle and Pin.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-21
I-18. Plate Shackle and Pin.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-22
J-1. SSN 637 Class Towing Gear (Harbor Towing Only). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-3
J-2. Hinged Cleat for SSN 688 Class. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-5
J-3. Towing Gear Arrangement for SSN 688 Class (Harbor Towing Only).. . . . . . . . . . . . . . . . J-6
J-4. Towing Arrangement for SSBN 726 Class.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J-8
J-5. Emergency Towing Pendant Assembly for SSN 21 Class.. . . . . . . . . . . . . . . . . . . . . . . . . J-9
K-1. Towline Pull vs. Towing Speed for Tugs with
      Controllable-Pitch Propellers and Nozzles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-4
K-2. Anchor-Handling/Supply Tug. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-5
K-3. Point-to-Point Towing Tug. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-5
K-4. Salvage Tug. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-6
L-1. Multi-Sheave Traction Winch.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-6
L-2. ARS 50 Towing Machinery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-7
L-3. T-ATF Towing Machinery (SMATCO Winch).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L-9
L-4. T-ATF Towing Machinery (Series 332 Automatic Towing Machine). . . . . . . . . . . . . . . . . L-11
M-1. Types of Motion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-2
M-2. Extreme Tensions (Curves 0 to 24). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-10
M-3. Extreme Tensions (Curves 25 to 49). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-11
M-4. Extreme Tensions (Curves 50 to 74). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-12


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M-5. Extreme Tensions (Curves 75 to 99). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-13
Q-1. DDG 51 Draft Diagram and Functions of Form. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-5
Q-2. Draft at Instability Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-9
Q-3. Draft-at-Landing Fore and Aft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-12
Q-4. Load Distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-16
Q-5. Overturning Moment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-27




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                                                        List of Tables

Table                                                                                                                                  Page
3-1. Hydrodynamic Resistance of the Towline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3
3-2. Safety Factors for Good Towing Practice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8
3-3. Section Modulus for Wire Rope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22
3-4. Elongation of 1,500 Feet of 6x37, 2 ¼-Inch IWRC EIPS Wire Rope. . . . . . . . . . . . . . . . . 3-22
3-5. Operating Range for Automatic Towing Machines of Various Types of Ships. . . . . . . . . 3-26
4-1. U-Bolt Clips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-31
4-2. Applying U-Bolt Clips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-32
5-1. U.S. Navy Craft Not Recommended for Open-Ocean Tows. . . . . . . . . . . . . . . . . . . . . . . . 5-4
5-2. Minimum Plate Thickness for Forward One-Fifth of Barge Bottom. . . . . . . . . . . . . . . . . . 5-15
5-3. Minimum Plate Thickness for Mid-Section.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16
5-4. Battery Capacity Requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21
6-1. Information on U.S. Navy Bitts.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8
8-1. Commercial Submersible and Semi-Submersible Vessels. . . . . . . . . . . . . . . . . . . . . . . . . 8-3
8-2. Heavy Lift Ship vs. Submersible Barge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7
8-3. Sample Cc Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-27
8-4. Heave and Surge Motion Parameters for Calculation of Loading
     Factors for Conventional Surface Ships. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-35
8-5. Pitch Motion Parameters for Calculation of Loading Factors for
     Conventional Surface Ships.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-36
8-6. Roll Motion Parameters for Calculation of Loading Factors for Conventional
     Surface Ships.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-38
8-7. Allowable Block Stress (Assuming Douglas Fir). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-41
B-1. Wire Hawsers Carried by U.S. Navy Towing Ships.. . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-10
B-2. Nominal Breaking Strength of Wire Rope 6x37 Class, Hot-Dipped Galvanized. . . . . . . .B-11
B-3. Efficiency of Wire Rope Terminations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-13
C-1. Fiber Comparisons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-2
C-2. Synthetic and Natural Line Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6
D-1. Die Lock Chain Characteristics (MIL-C-19944).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-7
D-2. Navy Stud Link Chain Characteristics (MIL-C-24633). . . . . . . . . . . . . . . . . . . . . . . . . . . D-8
D-3. Commercial Stud Link Anchor Chain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-9
D-4. Commercial Detachable Chain Connecting Link.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-10
D-5. Commercial Detachable Anchor Connecting Link.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-11
D-6. Commercial End Link. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-12
D-7. Type I, Class 3 Safety Anchor Shackle (MIL-S-24214A (SHIPS)). . . . . . . . . . . . . . . . . D-13
D-8. Type II, Class 3 Safety Chain Shackle (MIL-S-24214A(SHIPS)). . . . . . . . . . . . . . . . . . D-14
D-9. Mechanical Properties of Shackles (FED SPEC RR-C-271D). . . . . . . . . . . . . . . . . . . . D-15
G-1. Calculation of Steady State Towing Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-4
G-2. Characteristics of Naval Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-7
G-3. Beaufort Scale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-10
G-4. Drydock Towing Coefficients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-17
J-1. Towing Arrangement for Higher-Population Submarine Classes. . . . . . . . . . . . . . . . . . . . J-2
K-1. Typical Commercial Salvage/Towing Vessels for Hire
      Compared with US Navy Salvage Ship. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .K-7
M-1. T-ATF Towing YRBM Barge Displacing 650 Tons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-14
M-2. T-ATF Towing FFG 1 Frigate Displacing 3,200 Tons. . . . . . . . . . . . . . . . . . . . . . . . . . . M-15
M-3. T-ATF Towing DD 963 Destroyer Displacing 6,707 Tons.. . . . . . . . . . . . . . . . . . . . . . . M-16



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                                                  U.S. Navy Towing Manual


M-4. T-ATF Towing AE 26 Displacing 20,000 Tons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-17
M-5. T-ATF Towing LHA 1 Displacing 40,000 Tons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M-18
M-6. ARS 50 or ATS 1 Towing YRBM Barge Displacing 650 Tons. . . . . . . . . . . . . . . . . . . . M-19
M-7. ARS 50 or ATS 1 Towing FFG 1 Frigate Displacing 3,200 Tons. . . . . . . . . . . . . . . . . . M-20
M-8. ARS 50 or ATS 1 Towing DD 963 Destroyer Displacing 6,707 Tons. . . . . . . . . . . . . . . M-21
M-9. ARS 50 or ATS 1 Towing AE 26 Displacing 20,000 Tons. . . . . . . . . . . . . . . . . . . . . . . M-22
M-10. ARS 50 or ATS 1 Towing LHA 1 Displacing 40,000 Tons. . . . . . . . . . . . . . . . . . . . . . M-23
P-1. System of Metric Measures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-1
P-2. System of English Measures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-2
P-3. Basic English/Metric Equivalents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-3
P-4. Circular or Angular Measure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-4
P-5. Common Pressure Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-4
P-6. Common Density Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-4
P-7. General Conversion Factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-5
P-8. Power Conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-11
P-9. Temperature Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-11
P-10. Common Flow Rate Conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .P-12
Q-1. DDG 67 Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-6
Q-2. Draft at Instability Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-7
Q-3. Draft at Instability Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-8
Q-4. Draft-at-Landing Fore and Aft Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-11
Q-5. Draft-at-Landing Fore and Aft Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Q-11
Q-6. Heavy Lift Ship (HLS) Blue Marlin Characteristics and Motions . . . . . . . . . . . . . . . . . . Q-13




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                                                   sponsibility of the tugs attached to the naval
                Chapter 1                          districts.

      OPERATIONS OVERVIEW                          This arrangement made it difficult to muster a
                                                   large quantity of tugs to respond to large
                                                   groups of casualties that often resulted from
                                                   German U-boat attacks. These casualties in-
1-1    Introduction to Navy Towing                 cluded not only fleet vessels but merchant
                                                   ships, which were logistically critical to the
Modern Navy towing, as we perceive it to-          war effort. The US knew the value of keeping
day, began at the beginning of World War II.       a strong logistical force operating. To help
Prior to WWII, the Navy owned few salvage          rectify these shortfalls and to better utilize the
ships of its own and depended heavily on           available assets, the Navy formed the Navy
contracted assets to perform the duties of         Rescue Towing Service. This service fell un-
towing and salvage. Merritt-Chapman and            der the command of Commander, Eastern Sea
Scott was one of the premier towing and sal-       Frontier, and operated exclusively on the At-
vage contractors of the day and maintained an      lantic Coast. All available tugs were orga-
inventory of assets. They held a contract with     nized under this service, which was headed
the Navy to perform ship salvage on an as-         by Edmond Moran of Moran Towing. Ed-
needed, no cure-no pay basis. As the US            mond Moran understood the towing industry
watched the war in Europe develop, the need        and was enrolled in the Naval Reserve to per-
for specialized vessels and a dedicated ser-       form this duty. This organization for the most
vice became apparent. The Royal Navy of            part alleviated the problems of asset alloca-
Great Britain was forced into performing           tion and allowed the rescue of many tons of
these tasks as German U-boats inflicted dam-       ships and cargo.
age throughout the military and commercial
fleet. Performing towing and salvage services      The Navy operated tugs in support of their lo-
on damaged vessels was most often a faster         gistic requirements for many years, but as the
and cheaper way of putting the necessary ton-      war in Europe wore on, the need became ap-
nage back into service.                            parent for more capable, sea-going tugs. The
                                                   Navy designed a fleet tug (ATF) for the 1939
In October of 1941, Congre ss presse d             shipbuilding program. The NAVAJO (ATF
through legislation that gave the Navy the         64) was the first of a fleet of 3,000 shaft
contracting authority to obtain the salvage re-    horsepower, diesel-electric ocean-going tugs
sources, public or private, necessary to per-      equipped with an automatic tensioning tow-
form operations that were deemed in the best       ing engine. Almost seventy of these vessels
interest of the country. On December 7, 1941,      were constructed before the end of the war.
the Japanese bombed Pearl Harbor and four          These ships would serve the Navy well for
days later the Navy signed a contract with         nearly fifty years. Their towing winches
Merritt-Chapman and Scott establishing the         proved very successful in taking disabled
Navy Salvage Service. This service was re-         ships under tow. Their long range and sea-
sponsible for performing offshore salvage on       worthiness also made them very suitable for
east and west coasts, the Caribbean, Alaska,       combat operations.
and Panama. To do this, it utilized leased
commercial assets including tugs. The re-          The Navy also built many Auxiliary tugs
sponsibility for towing distressed or disabled     (ATA) to be used in support of the fleet tugs.
ships into port, however, did not lie with the     They were also diesel driven (relatively new
Navy Salvage Service. This duty was the re-        technology for tugs) but carried about half the

                                                                                                 1-1
                                     U.S. Navy Towing Manual


horsepower of the fleet tugs. They had con-        an increase in the number of all ships being
siderable endurance and were well suited to        decommissioned. This has placed a high de-
perform operations just outside the combat         mand on the Navy's few towing assets and
zone. They would often relieve a fleet tug of a    has increased the amount of work being sent
disabled vessel and continue the tow into          to commercial firms.
port. This allowed the fleet tugs to return to
                                                   U.S. Navy towing and salvage ships also pro-
the combat zone where they also performed
                                                   vide battle damage control as an adjunct duty
some fire-fighting and salvage assistance.
                                                   to their primary role as towing and salvage
Rescue tugs (ATR) were wooden-hulled, used
                                                   platforms. Fleet battle damage control is ren-
in submarine infested waters, and provided
                                                   dered in the combat zone to a battle-damaged
excellent fire-fighting support. Their range
                                                   ship casualty, often under direct enemy fire.
was limited, and although they were not con-
                                                   The assistance can take the form of off-ship
sidered the best vessels for long distance tow-
                                                   fire-fighting from a salvage tug’s fire-fighting
ing, they were excellent in coastal areas.
                                                   monitors or of a damage control team from
The early designs of the salvage ships were        the salvage ship boarding the casualty.
not particularly well suited for towing. This      Towing can vary from routine, well-planned
changed with the steel-hulled BOLSTER              activities to time-critical emergencies such as
class (ARS 38) which were built as salvage         rescue or salvage towing. Routine Navy tow-
ships but were of similar horsepower to the        ing includes a wide variety of activities such
fleet tugs (ATF). They were originally             as harbor work and offshore or open ocean
equipped with a powered reel for towing, but       towing. Navy emergency towing consists al-
it was soon discovered that they performed         most entirely of escort, rescue, and salvage
towing duties almost interchangeably with          missions. The types of vessels that may re-
the fleet tugs. Automatic towing winches           quire towing include Navy ships ranging
were diverted from the ATA program and in-         from small patrol boats to large aircraft carri-
stalled on these six vessels. These ships were     ers; non-combatant vessels, including tar-
of excellent design and operated until the last    gets, large fleet oilers, and supply ships; and
one was decommissioned in the mid-1990's.          vessels such as barges and floating dry docks.
The primary mission of the Navy's towing           The Navy recognizes several distinct types
and salvage ships today is not very different      of towing:
from the early days of these vessels. They
provide support to distressed or disabled                • Harbor towing
ships in the combat zone. However, during                • Point-to-point towing
peacetime, the daily operation of these ships
differs tremendously. The Navy now operates              • Rescue and emergency towing
only a few open ocean towing ships. At the               • Salvage towing
time of this writing, the Navy had four ARS
                                                         • Special ocean engineering projects
50 class salvage ships and the Military Sealift
Command (MSC) had five T-ATF class ves-                  • Tow-and-be-towed by Naval vessels
sels operating for the Navy. T-ATFs are
manned by civilian crews and do not carry the      1-2      Harbor Towing
extensive salvage equipment of the ARS class
but are extremely capable towing platforms.        Harbor towing is limited to protected waters.
In recent years, efforts to reduce military ex-    Harbor towing and base support includes
penditures have resulted in not only decreases     docking/undocking, standby duty, and safety
in the number of tow ships available but also      escort duty. These services are the province of

1-2
                                    U.S. Navy Towing Manual


yard tugs. These vessels are incapable of sus-    land towing in the United States largely origi-
taining long-distance, open ocean towing due      nated on the Mississippi and Ohio River sys-
to design limitations. Harbor tugs do not have    tems. This type of towing was also done on
the range, crew size, berthing and messing ca-    the Erie Canal and other inland man-made
pacity, and, in some cases, the structural or     navigational systems managed by the Army
hydrodynamic design needed to support open        Corps of Engineers, such as the St. Lawrence
ocean towing. Their moderate horsepower,          Seaway and East Coast Intercoastal Water-
limited seakeeping characteristics, and mini-     way. The Navy does very little inland towing.
mal towing machinery also makes these ves-        1-3.2   Ocean Towing
sels unsuitable for the open sea. The U.S. Na-
vy currently operates three classifications of    Ocean towing is point-to-point towing where
yard tugs: the YTL, the YTM, and the YTB.         there are few, if any, places of refuge enroute.
                                                  Open ocean towing was a natural evolution-
1-3    Point-to-Point Towing                      ary step from harbor towing. The demand for
                                                  open ocean tows led to more advanced tug
                                                  designs that could accommodate more diffi-
Point-to-point towing can be defined as tow-
                                                  cult towing missions.
ing a vessel from one harbor to another.
Point-to-point towing and “open ocean” tow-       After harbor towing, open ocean towing is the
ing are largely synonymous. Open ocean tow-       most widely practiced form of Navy towing.
ing was a natural outgrowth of harbor towing.     Because of the unforgiving conditions faced
If vessels could be moved from one end of a       on the open ocean; it demands the most prep-
harbor to the other end, the next step was to     aration; the most robustly designed and con-
move them from one harbor to another.             structed equipment; and a higher level of op-
                                                  erator knowledge.
Until the 1960’s, Navy ATAs or similar ves-
sels, were generally home-ported in each na-      1-3.3   Defueled Nuclear Powered Ships
val district in the continental United States,    The Navy has devoted a considerable effort in
Alaska, Hawaii, and at selected overseas          developing guidelines for towing Unmanned
bases. The tugs were used for coastwise tow-      Defueled Nuclear Powered Ships. The unique
ing of floating equipment, such as barges,        considerations of towing a nuclear vessel that
pile drivers, and dredges. Since the 1960’s,      is unmanned have led to the development of
the Navy’s towing fleet has declined in num-      specific instructions that deal with this specif-
bers, with whole classes of towing vessels        ic situation. NAVSEA has published a series
being decommissioned. The ATF 76, ARS 6,          of instructions to specifically deal with un-
ATS 1, and all ATAs have been decommis-           manned towing of nuclear vessels (including
sioned, and, in FY94, the last WWII vintage       submarines, cruisers, and moored training
ARS 38 class salvage ships were also retired.     ships). The information contained in those
Consequently, Navy point-to-point towing is       documents will not be repeated in this manu-
currently performed by ARS 50 and the             al.
MSC-operated T-ATF class vessels, with an
increasingly large percentage of tows con-        1-4     Rescue and Emergency Towing
tracted to commercial firms.
1-3.1 Inland Towing
                                                  The mission of rescue towing encompasses
                                                  saving a stricken ship at sea and towing to a
Inland towing is point-to-point towing per-       safe refuge. The vessel may be adrift at sea,
formed on inland waterways such as rivers,        or near a shore or harbor. In the latter case, a
bays, canals, or intercoastal waterways. In-      connection must be made quickly to prevent

                                                                                               1-3
                                      U.S. Navy Towing Manual


the disabled ship from going aground. On            landings, these rescue tugs and salvage ships
high value tows, the Navy may assign a tug to       can be subject to enemy fire.
escort duty, to provide emergency towing ser-
vices without the delay of mobilization.            1-6    Special Ocean Engineering
                                                           Projects
1-4.1   Naval Task Force Standby Duty

Navy salvage ships routinely deploy to the          Navy tugs often become involved with un-
Mediterranean and to the Western Pacific to         usual projects, such as target services, sub-
provide salvage and towing support to the           mersible towing, array movements, deep
5th, 6th, and 7th Fleets. Aside from partici-       ocean search and recovery, and classified op-
pating in salvage exercises with foreign            erations.
navies, the salvage ships perform any salvage       Many of the attributes that make salvage
or towing mission tasked by the Fleet Com-          ships good salvage and towing platforms also
mander for the deployed Carrier Battle              make them good platforms for performing
Group, Surface Action Group, or any auxilia-        these ocean engineering operations. Specifi-
ry ships requiring salvage or towing assis-         cally, tugs that can perform open ocean tows
tance.                                              are often equipped with heavy lift cranes,
                                                    have large expanses of deck area for tempo-
1-5     Salvage Towing                              rary installation of specialized equipment,
                                                    and are designed to keep station or moor over
Salvage towing generally follows immediate-         a site of interest. Navy tugs can also serve as
ly after a salvage operation. Immediately af-       diving platforms and perform a variety of
ter salvage services are rendered, prepara-         deepwater tasks, including the support and re-
tions are made to tow the stricken vessel. The      covery of remotely operated vehicles.
vessel may be towed to a safe haven for tem-
porary repairs, or to a port or facility where      1-7    Tow-and-Be-Towed By Naval
complete industrial-level repairs are possi-               Vessels
ble. The vessel may also be towed to a dis-
posal site for sinking. In either case, tow         Although most towing is performed by ships
preparations usually entail more than the nor-      that have been specifically designed and built
mal tow system installation. The added mea-         for towing, emergency towing is sometimes
sures include reinforcing weakened sections         accomplished by ships other than tugs. This
of the ship, either through shoring or tempo-       concept is referred to as “tow-and-be-towed”
rary structural reinforcement, or possible spe-     or “emergency ship-to-ship towing.” In an
cial rigging to release the tow for sinking in a    emergency, any ship can tow another in its
safe, controlled manner.                            own or similar class, with each ship providing
                                                    half the towline. Ships not specifically
1-5.1   Combat Salvage and Towing                   equipped for towing can fashion a temporary
                                                    towline from anchor chains, wire straps,
Ships involved in combat salvage and towing
                                                    mooring lines, or combinations of these
missions often escort amphibious landing
                                                    items.
forces and battle groups in hostile areas.
Their job is to provide towing and salvage          NAVSEA General Specifications require that
services to ships or landing craft that are dam-    every class of U.S. Navy ship (except aircraft
aged, afire, disabled, or stranded. They are al-    carriers and submarines) be able to tow-and-
so prepared to tow transport and supply ships       be-towed in an emergency. Definitive tech-
laying off the beachhead. During amphibious         nical instructions for U.S. Navy tow-and-be-

1-4
                                    U.S. Navy Towing Manual


towed operations can be found in Naval            tions, NSTM Chapter 582 shall take prece-
Ship’s Technical Manual (NSTM) S9086-             dence. In all other towing matters, with the
TW-STM-010, Chapter 582, Mooring and              exception of nuclear tows, this manual is the
Towing (Ref. A). This topic is also covered in    governing document. Tow-and-be-towed op-
Section 6-6 of this manual. In the event of a     erations for NATO navies are covered in Al-
conflict between NSTM Chapter 582 and this        lied Tactical Publication (ATP)-43(A) (NA-
manual regarding tow-and-be-towed opera-          VY), Ship-to-ship Towing (Ref. B).




                                                                                           1-5
           U.S. Navy Towing Manual




      This Page Intentionally Left Blank




1-6
                                          U.S. Navy Towing Manual


                                                           • Fire protection
                   Chapter 2
                                                        For long ocean tows, the tug can be called up-
                                                        on to provide complete logistic support for
 OVERVIEW OF TOWING SHIPS                               the tow and the riding crew. The tug may also
                                                        be required to serve as a supply base and shop
                                                        for repairs, rigging, and damage control dur-
                       NOTE                             ing rescue salvage towing operations. Addi-
                                                        tionally, the tug may have to supply all the
       Although ocean-going tow ships are
       significantly different from harbor              rigging for the towing system.
       tugs, the term “tug” will be used to de-
       scribe all tow ships in this manual to           2-3 Design Characteristics
       avoid confusion between “tow ship”
       and “towed ship.”                                Most U.S. Navy ships can tow in an emergen-
                                                        cy, but only properly designed and outfitted
                                                        tugs make good towing ships. The specific
2-1 Introduction                                        items to be considered in the design of an
                                                        ocean tug are dependent upon the missions
This chapter contains a brief description of            and services that it will be called upon to per-
some typical design features found on ocean-            form. Characteristically, a tug’s superstruc-
going tugs. It also presents a good overview            ture is set forward, allowing a clear fantail so
of the latest generation of the US Navy’s               the towing point can be close to the ship’s
ocean towing ships. Modern harbor tugs uti-             pivot point. The towline, secured well for-
lize recent advances in propulsion and syn-             ward of the rudder and propellers, is allowed
thetic line, but ocean going tugs remain large-         to sweep the rail without limiting the maneu-
ly unchanged from earlier versions.                     verability of the tug. In addition to a clear
                                                        fantail area, characteristics of a good tug may
2-2 Requirements Placed on Towing                       include the following:
    Ships                                                  •   High horsepower
                                                           •   Slow speed maneuverability
The degree of service that the tug may be re-              •   Large diameter propellers
quired to furnish to its tow depends upon the              •   Large area rudders
circumstances and principal missions of the                •   Towing machinery
Navy at the particular time and can cover a                •   Power capstans
wide spectrum of needs. The primary require-               •   Towing points
ment placed on a tug is to provide power that              •   Bow thrusters
the tow does not have due to its construction,             •   Deck crane
its service condition (i.e., decommissioned), a
                                                        In general, a Navy ocean tug is a very versa-
casualty, or a failure of its main power plant.
                                                        tile ship, but its design involves many com-
Secondary requirements include:
                                                        promises. Appendix K provides data on fea-
   •   Steering for maneuverability                     tures of some commercial tugs. The design of
   •   Navigation                                       a Navy tug will differ from a commercial tug
                                                        because a commercial tug must make a profit.
   •   Communications
                                                        This influences manpower, automation, sec-
   •   Security                                         ondary missions, and a host of other charac-
   •   Damage control                                   teristics.


                                                                                                    2-1
                                    U.S. Navy Towing Manual




                            Figure 2-1. ARS 50 Bulwark Forward Limits.


2-3.1   Stern Arrangement                          2-3.2   Tug Powering and Bollard Pull
The stern of the tug is designed to minimize       The design of the main propulsion plant is a
chafing and damage to both the tug structure       compromise among wide-ranging require-
and the hawser. Caprail radius is generous         ments. The tug must have high free-running
and free from unintended obstructions to the       speeds for reaching the scene of a casualty
hawser’s sweep from side to side as the tug
                                                   quickly. It also needs good economy with
maneuvers in restricted waters. Most tugs
                                                   high towline pull for long-distance tows at
have a system to restrain the tow hawser
                                                   reasonable towing speeds. High bollard pull
sweep, such as vertical stern rollers or Nor-
                                                   is required for holding a distressed ship to
man pins, while towing under steady-state
                                                   prevent it from grounding and for refloating
conditions at sea. To reduce wear on both
                                                   stranded ships. It is also required for along-
the hawser and the tug’s structure, chafing
                                                   side operations (docking, maneuvering) and
gear is often used where the towline crosses
                                                   other high power/near zero speed evolutions.
the stern.
On most Navy towing and salvage vessels,           In the absence of a good automatic towing
the bulwark and the caprail are gently curved      machine or other accurate means of
upward and faired into the deck above the          measuring the towline tension, a knowledge
towing deck (see Figure 2-1.). This ship’s         of the tug’s available towline pull and bollard
structure restricts the tow wire from leading      pull is required for controlling the tension.
forward of the beam at the tug’s tow point         Appendix K presents the methodology for
just aft of the tow winch.                         estimating towline pull and bollard pull.


2-2                                                                                  As of 7/23/02
                                      U.S. Navy Towing Manual


2-3.3 Fenders                                             These take no time to deploy but pro-
Fenders are energy absorbing materials or                 vide little energy absorption beyond
devices that protect both the tug and the tow             simply eliminating steel-to-steel con-
(see Figures 2-2 and 2-3). Modern fendering               tact. Foam and pneumatic fenders re-
is an important part of towing operations when            quire rigging and handling but are far
alongside evolutions are required. Tugs work-             superior in their absorbing capability.
ing alongside submarines should have subsur-
face fenders. Small tugs working with large               Large truck tires are very effective from
ships should have fendering to protect the                the standpoint of energy absorption and
deckhouse from being damaged during large                 are inexpensive. They can be rapidly
rolls (see the wing fenders in Figure 2-2).               deployed and are particularly useful
                                                          during salvage where unusual condi-
Three types of standard fenders are currently             tions may exist. Some tires should be
in use:                                                   kept on board for emergency fendering.
    • Rubber                                           • Size vs. Capacity. Low-pressure and
    • Pneumatic                                          high-pressure pneumatic fenders have
        — High-pressure (5 to 7 psi)                     similar characteristics. Because they
        — Low-pressure (1 psi)                           are filled with air, however, they must
                                                         be larger than foam-filled fenders to
    • Closed-cell foam covered with urethane             absorb the same amount of energy. On
      elastomer                                          the other hand, equal capacity and
2-3.3.1   Features and Characteristics of                quality foam-filled fenders will likely
          Fenders                                        be more expensive and heavier than
The most significant features of fenders are:            pneumatic fenders.
   • Energy absorption                                 • Pneumatic Fender Maintenance. In
   • Durability                                          addition to the larger size of pneumatic
   • Handling characteristics                            fenders, other attending disadvantages
   • Ease of storage aboard ship                         are the extra equipment needed to
                                                         pre ssurize them and to c heck the
   • Ease of maintenance when not in use
                                                         internal pressure. Patch kits and special
   • Required support equipment                          s l i n g s t o s u p p o r t t h e f e n d e r ’s
Other important characteristics include:                 midsection when being deployed and
                                                         retrieved are also necessary for the low
    • Standoff distance                                  pressure types.
    • End fittings
    • Time required for deployment and                 • Pressure Loss. All pneumatic fenders
      recovery                                           have safety valves. When these valves
                                                         relieve under high fender loads, the
    • Capability of being used if damaged.
                                                         fenders lose nearly all their energy ab-
2-3.3.2 Operating Considerations                         sorption capability.
The following items should be considered
                                                       • Fender Displacement. A major oper-
when using fenders:
                                                         ating problem can arise when either the
    • Energy Absorption vs. Deployment.                  tug or the tow has a low freeboard rela-
      Rubber fenders, as seen in Figure 2-2,             tive to the other ship. When the heav-
      are an integral part of a tug’s structure.         ing or rolling motions of the two ships


                                                                                                       2-3
                 U.S. Navy Towing Manual




                             W ing
                  Foam -Filled Fend ers (P /S )
                              B ow
                            Fend ers

      W /L           S ide Fenders                     W /L



                           S ubsurface Fenders




             Figure 2-2. Typical Rubber Fenders.




2-4                                                As of 7/23/02
       U.S. Navy Towing Manual




Figure 2-3. Pneumatic and Foam Fenders.




                                          2-5
                                       U.S. Navy Towing Manual


        get out of step, the fender can be rolled    latest tug designs, but at the same time dimin-
        upward between the two ships and pop         ishes its fleet.
        out onto the deck of the one with the        2-4.1   YTL Class
        lower freeboard.
                                                     Yard tugs having 400 shp and under belong to
      • Friction Damage. When the tug and            the YTL class. The primary mission of the
        the tow have nearly equal freeboard,         YTL class is moving small craft and unload-
        the out-of-step motions of the two ships     ed barges from one berth to another within a
        can create a great deal of frictional        harbor. YTLs can also assist in moving larger
        heating on the surfaces of the fenders.      vessels because they are small enough to
        Spraying seawater onto the rubbing sur-      maneuver in tight, confined areas between
        faces helps lubricate them and keep          the large vessel and obstructions. Characteris-
        them cool.                                   tics of this class are small size, maneuverabil-
      • Ship Shell Plating Damage. Care must         ity, and less robust construction than larger
        be exercised in the fore and aft place-      harbor tugs. The term YTL is used to cover
        ment of the fenders to ensure that they      an entire spectrum of harbor “pusher boats.”
        do not bear against relatively large         Very few of the vessels are constructed to the
        areas of side plating that are not well      same class specifications, and a large number
        supported by internal framing and lon-       are custom built by the naval base using
        gitudinal structural members. This is        them. The YTLs are still in service, but liter-
        especially important in quartering seas      ally hundreds have been retired as their use-
        when swells will cause the two ships to      ful life has ended (most were built during
        pivot about the bow or stern and then        WWII).
        slam the sides together at the other end.    2-4.2   YTB Class
                                                     The largest tugs, with 1,000 to 2,000 shp, be-
2-4 Yard or Harbor Tugs
                                                     long to the YTB class. The larger YTBs cur-
                                                     rently have as much as 2,000 shaft horsepow-
The design of harbor tugs and the equipment
                                                     er and are similar to commercial harbor tugs.
they employ varies. The typical Navy harbor
                                                     YTBs can be used in open-ocean towing, but
tug is a single-screw, deep-draft vessel
                                                     only for short distances under the most opti-
equipped with a capstan aft, H-bitts forward
                                                     mal weather conditions. YTBs are mostly
and aft, towing hawsers, and additional lines
                                                     confined to harbor operations with occasional
for handling ships or barges in restricted wa-
                                                     point-to-point towing performed on inland
ters. Harbor tugs may also be equipped with
                                                     waters or on coastwise towing routes. YTBs
fenders, limited fire-fighting equipment, and
                                                     configured for servicing submarines have a
deck equipment to support harbor operations.
                                                     specially designed fender system. YTBs are
Twin-screw harbor tugs have greater maneu-
                                                     widely used in all Navy ports, especially
verability and ship control.
                                                     overseas bases, but may also face retirement
Harbor tugs are classified by shaft horsepow-        in the near future.
er (shp). The U.S. Navy operates three classes
of harbor tugs that are used primarily in har-       2-5 Ocean Tugs
bors and sheltered waters. As these vessels
age, the Navy relies more and more on con-           The Navy’s ocean tugs are far more versatile
tracted vessels to perform harbor operations.        than harbor tugs in terms of horsepower and
This allows the Navy to take advantage of the        range capabilities, as well as in terms of the


2-6                                                                                     As of 7/23/02
                                      U.S. Navy Towing Manual


services they can provide to their tows. Ocean      The automatic towing machine (ATM) on
tugs and salvage/rescue type ships are the          board the ARS 50 Class is a Series 322 winch
only U.S. Navy ocean-going ships whose pri-         built by Almon A. Johnson, Inc. (see Figure
mary mission includes towing. Thus, they are        L-2). This double-drum ATM stores two
the only types considered to be specifically        3,000-foot, 2¼-inch diameter towing haw-
built for towing and for which towing activi-       sers. The ARS 50 Class also has a Series 400
ties have significantly influenced the design.      traction winch for handling synthetic line up
                                                    to 14 inches in circumference. The traction
Most of the ocean tugs used by the Navy to-         winch is also useful for mooring and ocean
day are carryovers and replacements or suc-         engineering operations. These vessels are ex-
cessors to similar ships used or developed          tremely versatile. They are capable of sup-
during WWII. Some of the differences be-            porting a wide range of missions and are ex-
tween the WWII vintage and the more mod-            cellent towing platforms as well as fully
ern ships lie in increased horsepower and bol-      capable salvage ships.
lard or towline pull, hawser size, provisions
for use of synthetic fiber hawsers, and, of         2-5.2   T-ATF 166 Class
major importance, vastly improved onboard           The T-ATF Class is a multipurpose, long-
equipment and accommodations.                       range, high-horsepower, seaworthy tug (see
                                                    Figure 2-5). It can conduct long-distance tows
In addition to their power, range, and endur-       and, when augmented with additional crew
ance capabilities, the ocean tugs can work          and equipment, operate in support of fire-
and survive in heavy weather independently          fighting, diving, and salvage missions. These
of other auxiliary or support ships. They are       seven vessels were designed for and are oper-
also used in stranding and other salvage oper-      ated by the Military Sealift Command (MSC).
ations. Ocean tugs should have automatic            They carry approximately 20 crew members,
towing machines and load sensing systems to         and 18 transients.
reduce dynamic loads and, if necessary, rap-
idly release the towline.                           This ship was conceived and specified to re-
                                                    place the Auxiliary Ocean Tugs (ATA) and
The following section contains a brief de-          Fleet Tugs (ATF) for routine towing. Because
scription of modern U.S. Navy towing and            it also was designed to serve as a salvage ten-
salvage ships. Due to shrinking budgets,            der, it has a large afterdeck, similar to an off-
some of these vessels are no longer in service.     shore supply vessel. Although not normally
Their characteristics are presented here for        carried, various suites of special equipment
both historic and comparative purposes. For         can be installed on board the T-ATF to sup-
more detailed information, refer to the opera-      port air and mixed gas diving, beach-gear op-
tions manual of each vessel.                        erations, off-ship fire-fighting, search and re-
                                                    covery operations, and oil spill recovery. This
2-5.1 ARS 50 Class                                  class is capable in rescue towing applications,
                                                    but has limited salvage capabilities on its
The four ships of the ARS 50 Class (see Fig-        own. With its large fantail area, it can be aug-
ure 2-4) are replacements for the ARS 38            mented to perform salvage, but carries little
Class. Each ship carries a crew of over 100         of its own equipment.
and equipment sufficient to handle ocean
towing, independent salvage, diving, damage         This 7,200 horsepower tug class carries a
control, and fire-fighting capabilities in times    2,500-foot, 2 1/4-inch wire rope tow hawser.
of war.                                             The T-ATF is equipped with a traction winch


                                                                                                 2-7
                                        U.S. Navy Towing Manual




          These ships replaced the ARS 6 and ARS 38 Class and have modernized salvage
          and towing capability. They are also equipped with off-ship fire-fighting improvements.




      Length (ft):          255                      Shaft Horsepower:        4200

      Beam (ft):            52                       Cruising Range (nm):     8,000 @ 8 kt

      Draft (ft):           17.5                     Fuel Consumption         2 engine: 2100
                                                     (Gal/day):               4 engines: 4200
      Displacement,         3282
      Full Load (LT):                                Complement:              94 crew
                                                                              16 transients
      Propulsion, Main:     4 diesel
                            2 screws                 Towing Machine:          Almon A. Johnson, Inc.
                                                                              Automatic towing ma-
      Maximum Sustained     15                                                chine, Series 322 (double-
      Speed (kts):                                                            drum) 2 1/4-wire, 3000 ft.
                                                                              (will accept 2 1/2-inch
                                                                              wire) and 14-inch traction
                                                                              winch, Series 400

                                                     Bow Thruster:            1 @ 500 HP




                                  Figure 2-4. ARS 50 Class Salvage Ship.




2-8                                                                                             As of 7/23/02
                                         U.S. Navy Towing Manual




          This class of tug has replaced the ATF 76 Class. These ships have a large working
          space aft for VERTREP (replenishment by helicopter) and can be readily outfitted for
          specialized salvage and ocean engineering missions.


Length (ft):            240                          Shaft Horsepower:       7200

Beam (ft):              42                           Cruising Range (nm):    10,000 @ 13 kt

Draft (ft):             15.5                         Fuel Consumption        1 engine: 4149
                                                     (Gal/day):              2 engines: 8300
Displacement,           2260
Full Load (LT):                                      Complement:             16 crew
                                                                             4 Navy communicators
Propulsion, Main:       2 diesel, 2 screws                                   18 transients
                        Controllable,
                        reversible pitch in Kort     Towing Machine:         SMATCO* 2500 ft.
                        nozzles                                              2 1/4-inch wire winch
                                                                             (15-inch Lake Shore,
Maximum Sustained       15                                                   Inc. traction winch)
Speed (kts):
                                                     Bow Thruster:           1 @ 300 HP


                                                      * Some vessels of this class have been refitted
                                                         with Almon A.Johnson automatic machines.




                                 Figure 2-5. T-ATF 166 Class Fleet Tug.




                                                                                                        2-9
                                    U.S. Navy Towing Manual


that can handle synthetic hawsers up to 15        industry (see Figure L-3). Some members of
inches in circumference. The class was origi-     this class have been refitted with an (Almon
nally equipped with a single-drum, diesel         A. Johnson) electrohydraulic automatic tow-
driven, non-automatic SMATCO towing               ing machine (see Figure L-4).
winch relatively common to the offshore oil




2-10                                                                              As of 7/23/02
                                      U.S. Navy Towing Manual


                                                            tion. Use the appropriate safety factors
                Chapter 3                                   for the materials and equipment
                                                            involved, anticipated weather, and
    TOWING SYSTEM DESIGN                                    other conditions of the particular tow-
                                                            ing mission (see Section 3-4.1 and (Ref.
                                                            M).
3-1 Introduction                                    3. Make necessary adjustments.
This chapter provides guidance on the steps             • Recheck the refined calculations against
to take when preparing to perform a tow. It               the tug’s capabilities.
contains information for the planner in choos-          • If calculated requirements for power or
ing a tug and for predicting tow speed.                   towline strength exceed capacities of
                                                          available equipment, another iteration
3-2 Designing a Towing System                             is required. Options may include:

Tow system design is often an iterative pro-                 — Selecting a slower towing speed
cess. Each iteration has three core stages:                  — Using additional or more powerful
                                                               tugs
1. Calculate steady towline tension. Start-
   ing with the ship to be towed:                            — Decreasing resistance by changing
                                                               the tow’s characteristics, routing,
   • Select the desired towing speed and cal-                  and/or schedule.
     culate the steady state tension that the
                                                    3-2.1    Tug and Tow Configuration
     towline will encounter at that speed
     (see Section 3-4 and (Ref. G)).                The design of a towing system is dependent
                                                    o n t h e t y p e o f t ow i n g p e rf o rm e d , t he
   • Select representative tow speeds above
                                                    configuration of the tow, and the number of
     and below the desired speed and calcu-
                                                    vessels being towed. For examples of the
     late the corresponding steady towline
                                                    types of tow configurations used for open
     tensions. The calculated tensions
                                                    ocean towing for single and multiple tows,
     should be either plotted or arranged in a
                                                    see (Ref. I).
     table to allow interpolation later.
   • Repeat this process for representative         3-3 System Design Considerations
     wind/sea combinations anticipated dur-
     ing the tow.                                   When planning a tow and designing the tow
2. Select the tug and design a rig.                 system, important considerations are:

   • Compare the predicted towline tension              • Tow size, type, and condition
     to the capabilities of available tugs and          • Expected or required towing speed
     select the tug best suited for the task.
                                                        • Capabilities of available tugs (bollard
   • Once a tug is selected, design an initial            pull, range, equipment, and crew)
     towing rig. Select the towing connec-
                                                        • Towing hawser system specifications
     tion elements (such as bridles and chain
                                                          (type, diameter, expected maximum
     pendants), determine a recommended
                                                          tension, scope and configuration)
     hawser length, and check the catenary.
     Account for the effects of weather, type           • Towline tension as determined by the
     of towline, and dynamic load mitiga-                 total resistance of the tow and re-


                                                                                                       3-1
                                              U.S. Navy Towing Manual


        spective seakeeping motions of the tug                    • Resistance of the tow hawser (see
        and tow                                                     3-4.1.2)
      • Maximum practical towline length, as                      • Vertical component of wire catenary
        determined by navigational and hydro-                       (which contributes to the total tension
        graphic restrictions on towline catena-                     of the towline itself but not to tug pro-
        ry depth                                                    pulsion requirements) (see 3-4.1.3)
      • Operational considerations                           Dynamic towline tension, on the other hand,
                                                             is caused by the random motions of the sea
      • Proposed season and route
                                                             and the ships and is difficult to predict with
      • Unique characteristics of the anticipat-             absolute precision over time. Statistics, how-
        ed tow                                               ever, allow prediction of dynamic tension ex-
      • Stability characteristics of the tug                 tremes within probability limits set by the in-
                                                             vestigating engineer (see (Ref. M)). Dynamic
These factors are interdependent. For exam-                  tension has two components:
ple, in theory, the desired towing speed would
largely determine the required tow hawser                         • Slow dynamic loads caused by the
                                                                    tow’s yawing, sheering, and surging
size. But, in practice, there is little choice of
                                                                    (see 3-4.1.4)
tow hawser for a given tug class. Hawser
choice is governed by the ship which is avail-                    • More rapid dynamic loads caused by
able for the towing assignment. For large                           the effect of waves on the relative
tows using the full propulsion power of the                         seakeeping motions of tug and tow (see
tug, the tug determines the potential speed of                      3-4.1.4)
the tow. In other cases, tow speed may be                    3-4.1.1 Calculating Steady Resistance of the
limited by weather or by the condition of the                        Towed Vessel
tow. Given the tug and the resulting speed of                Steady resistance (RT) of the towed vessel
the tow, the tow hawser size can be checked                  may be estimated using the following
and an initial towing rig designed.                          approximation:
All of these factors must be considered to de-                          RT = RH + RP + RW + RS
termine which ones will dictate the design of
the system. The system must then be exam-                    where:
ined as a whole to ensure that the best config-
                                                             RH       = Hydrodynamic hull resistance of
uration has been achieved.
                                                                        the tow
3-4 System Design Methodology                                RP       = Hydrodynamic resistance of the
                                                                        tow’s locked propellers
3-4.1    Calculating Total Towline Tension                   RW       = Wind resistance of the tow
To t a l t o w l i n e t e n s i o n h a s t w o m a j o r   RS       = Additional tow resistance due to
components: steady tension and dynamic                                  sea state
tension. Steady (or static) towline tension can
                                                             (Ref. G) provides methods and a convenient
be predicted with a fairly high degree of
                                                             worksheet for predicting each of these com-
accuracy. Static towline tension has three
                                                             ponents. Effort can be saved by computing
components:
                                                             the resistance for two or three different
      • Resistance of the ship to be towed (see              speeds for later comparison to tug capabili-
        3-4.1.1)                                             ties. (Refer to the towing speed limitations

3-2
                                      U.S. Navy Towing Manual



                       Table 3-1. Hydrodynamic Resistance of the Towline.




                                              Added Resistance (lbs)              Added Resistance (lbs)
  Wire      Wire     Chain     Chain            10,000 lb. Tension                  20,000 lb. Tension
  Size      Scope     Size     Scope
  (in)       (ft)     (in)      (ft)        4 kts       8 kts          12 kts   4 kts       8 kts          12 kts

   1 5/8    3000       --        --         1000        4000            7200    1000        3000            5800
   1 5/8    2000       --        --          900        3500            4100     700        2500            3300

    2       2000        --       --         2000        2200            6000    1500        2200            4000
    2       2000      2 1/4      90         2500        5100           12000    1900        3900            7900
    2       2000      2 1/4     270         3100       10000           19300    2000        7200           15000
    2       2000      4 3/4     270         3700       12000           24500    2700        8900           17600

   2 1/4    2000      2 1/4      90         1500        5200           11500    1300        3800            8000
   2 1/4    2000      2 1/4     270         3000        8000           18500    1600        6500           14500
   2 1/4    2000      4 3/4     270         5000       14100           25500    4500       12900           23000

   2 1/4    3000      2 1/4      90         1900        8300           17500    1600        5700           13100
   2 1/4    3000      2 1/4     270         3100       12000           24800    2500        8700           20100
   2 1/4    3000      4 3/4     270         5500       14400           27800    5000       13300           26000



                                              Added Resistance (lbs)              Added Resistance (lbs)
  Wire      Wire     Chain     Chain            40,000 lb. Tension                  60,000 lb. Tension
  Size      Scope     Size     Scope
  (in)       (ft)     (in)      (ft)        4 kts       8 kts          12 kts   4 kts       8 kts          12 kts

   1 5/8    3000       --        --          600        2200            5000     500        1900            4200
   1 5/8    2000       --        --          500        2000            3300     250        1000            2500

    2       2000        --       --         1000        1700            3500     300        1200            3000
    2       2000      2 1/4      90         1200        3200            6500    1000        2500            5100
    2       2000      2 1/4     270         1500        5100           10900    1300        4200            8800
    2       2000      4 3/4     270         2500        6900           14600    2000        6800           13200

   2 1/4    2000      2 1/4      90         1200        3500            6500    1100        3100            5000
   2 1/4    2000      2 1/4     270         1400        5100           11500    1200        3700            8500
   2 1/4    2000      4 3/4     270         3600        9300           18100    2900        5700           13200

   2 1/4    3000      2 1/4      90         1400        4100            9500    1200        2500            5900
   2 1/4    3000      2 1/4     270         1900        6500           15500    1300        4200           10900
   2 1/4    3000      4 3/4     270         3500       10500           21500    2000        7700           17000



USE OF TABLE: Towline resistance can be selected for the case closest to the actual towline
configuration. The figures can be interpolated as required if additional accuracy is desired.
    • For towline scopes less than shown, make a proportional reduction from the scopes
      listed.
    • For tension greater than 60,000 pounds, extrapolate assuming a resistance curve be-
      tween 40,000 and 60,000 pounds in a straight line.




                                                                                                                    3-3
                                      U.S. Navy Towing Manual


cited in Section 6-4.2) Likewise, wind and          tension at the tow, even with as much as 270
sea state resistances should be computed for        feet of chain pendant, shows that the total
best and worst expectations as well as for the      tension is less than at the tug.
most probable conditions of each assumed
                                                    The steady-state towline tension at the stern
tow speed.
                                                    of the tug is expressed by the formula:
3-4.1.2 Calculating Steady Towline
                                                                                    2     2
        Resistance                                              T =   ( R T + R wire ) + T v
In addition to the tensions calculated in Ap-
pendix G, the hydrodynamic resistance of the        where:
towline must be included, and this can be sig-      RT       = Tow resistance (Section 3-4.1.1
nificant for a typical wire hawser tow rig. The                and (Ref. G))
resistance is dependent upon the size, length,
and catenary of the towline, which in turn are      Rwire    = Towline resistance (3-4.1.2 and
dependent upon characteristics of the selected                 Table 3-1)
tug and towing speed. When using a synthetic        TV       = Vertical component of the towline
hawser, the added resistance of the towline is                 tension
negligible and can be ignored in these calcu-
lations.                                            TV is the weight of the towline forward of the
                                                    catenary low point, less the slight upward
If a particular tug has not yet been selected,      component of hydrodynamic drag on the for-
estimate the hawser resistance (Rwire) to be 10     ward half of the catenary. Location of the
percent of the tow resistance (RT). Experi-         catenary low point and the vertical compo-
ence has shown that when Rwire is significant-      nent of the hydrodynamic drag are beyond
ly more than 10 percent of RT, the catenary is      the scope of this manual. Errors will tend to
very deep and tension is, therefore, out of the     cancel out, however, if TV is assumed to be
range of concern for towline strength. If a         the weight (in water) of one-half the scope of
particular towline configuration is being eval-     the wire towline. (See Table B-2 in (Ref. B)
uated, Table 3-1 provides a more refined esti-      for dry weights and methods for calculating
mate of the hydrodynamic resistance.                weights in water.) Do not include the weight
                                                    of any chain pendant at the tow in this
3-4.1.3 Calculating Steady Towline Tension
                                                    computation.
Normal wire rope towline arrangements will
                                                    For example, assume that an ARS 50 is tow-
assume a sag or catenary, as depicted in
                                                    ing a ship that provides a steady tow
Figure 3-1. The total towline stress at any
                                                    resistance of 60,000 pounds at 8 knots. The
point is the vector sum of the horizontal and
                                                    towline consists of 2,000 feet of 2 ¼-inch
vertical components of the stress at that point.
                                                    IWRC tow hawser and 270 feet of 2 ¼-inch
Figure 3-1 includes a vector diagram of the
                                                    chain pendant at the tow. Table 3-2 provides
towline forces acting immediately astern of
                                                    a towline resistance estimate of 3,700 pounds.
the tug. Maximum stress occurs near the stern
                                                    According to Table B-2, 1,000 feet of wire
because the hydrodynamic resistance of the
                                                    towline (one-half of the scope) weighs 8,143
entire towline is added to the resistance of the
                                                    pounds in water. Therefore:
tow, whereas no hydrodynamic resistance is
added at the bow of the tow. Because stress is      T       = Total Towline Tension
highest near the stern of the tug, this is the
                                                    RT      = 60,000 lbs.
point of interest to towing planners.
Computing the total steady-state towline            Rwire = 3,700 lbs.


3-4
                                          U.S. Navy Towing Manual




                              D Horizontal Distance



       /L
                                  C
               Tow                                                                                    Tug


                                                          S




                                                                                            T               TV




                                                                                  RT              R W ire

                                              W - Weight Per
                                                  Unit Length


                        T                                 2           2
                                  R       +   R               +   T
                                      T           w ire               V



                            Figure 3-1. Towline Forces at Stern of Tow.


TV    = 8,143 lbs.                                                        tug must support the weight of the towline, it
                                                                          does not require forward thrust to do this.
Solving the vector diagram provides a maxi-
                                                                          3-4.1.4 Dynamic Loads on the Towline
mum steady-state tension:
                                                                          While towing at constant tug speed in a sea-
                              2               2                           way, the towline tension is not steady, but
     T = ( 60 ,000 + 3 ,700 ) + 8 ,143                                    varies over time (see Figure 3-2). In addition
     T = 64 ,218 lbs                                                      to steady horizontal resistance (T), the tow-
                                                                          line is also subject to stress from yawing
This example supports a rule of thumb that                                movements (Tyaw) and from the wave-in-
total steady-state tension can be estimated by                            duced motions of the tug and tow (Twave), al-
adding 10 percent to the predicted steady tow                             so known as dynamic tensions.
resistance (RT). This method is reasonable
and accounts for both the wire resistance and                             Yawing, also referred to as sheering, is the
the vertical component of the catenary. In the                            slow swinging of the towed vessel from one
previous example, adding 10 percent to pre-                               side of the course line to the other. Tyaw also
dicted steady tow resistance of 60,000 pounds                             includes surge - the slow change of span be-
would yield a total tension of 66,000 pounds,                             tween the tug and towed vessel. Sheering ten-
a conservative estimate when compared to the                              sion fluctuates in such a way that the average
64,218 pounds seen above.                                                 value is zero. Because each swing takes sev-
                                                                          eral minutes, sheering tension also takes sev-
In Figure 3-1, the tug must supply excess                                 eral minutes to vary from its maximum to its
thrust only equal to the horizontal component                             minimum and is sometimes called a “quasi-
of the tension, that is, RT + Rwire. While the                            steady” tension.
                                                                                                                     3-5
                                                 U.S. Navy Towing Manual




                 35.0

                              Extrem e Tension (Te )
                 30.0
                                                                            Wave-Induced (Twave )

                 25.0
 Tension, Kips




                                                                                                           Steady or
                 20.0
                                                                                                           Static (T)


                 15.0

                                                                         Yawing and
                 10.0                                                   Surging (Tyaw )


                  5.0


                  0.0
                        0.0       .50          1.0           1.5             2.0          2.5        3.0

                                                       Tim e (M inutes)

                                           Figure 3-2. Towline Tension vs. Time.


Wave-induced tension is caused by the effect                       duced in the system components after repeat-
waves have on both tug and tow and is a ran-                       ed cyclic loading.
dom process with typical half-periods (the                         Extreme towline tension occurs when yawing
time taken to vary from maximum to mini-                           tension and wave induced tension are addi-
mum) of 1 to 8 seconds. Like sheering ten-                         tive. The total towline tension or extreme ten-
sion, wave-induced tension has an average                          sion (Te), can be expressed as:
value of zero. Dynamic tension is the accu-
mulation of the complex dynamic responses                                      T e = T + T yaw + T wave
of tug, tow, and towline to time-varying forc-                     where:
es. While both the components of dynamic
tension have an average value of zero, at any                      Te        = Extreme Tension
instant in time, dynamic tension can have a                        T         = Steady towline tension
significant, if not catastrophic, effect on the                                (RT + Rwire)
towing system. Specifically, these damaging                        Tyaw      = Time-varying tension due to yaw-
effects occur when the cumulative tensions                                     ing of the towed vessel
from yawing and waves are additive to the
steady-state tension in the towing system.                         Twave = Time-varying tension due to wave
The effects manifest themselves in peak load-                              action on the ship and on the tow.
ing that can destroy the towing system by                          In Figure 3-2, for example, T is 20,000
pure overload or by metallurgical fatigue in-                      pounds. Tyaw varies between -3,000 and


3-6
                                     U.S. Navy Towing Manual


+3,000 pounds. Twave varies between -5,000         compare this number to the new breaking
and +5,000 pounds. Te, in this example, is ap-     strength of the component. Safety factors also
proximately 28,000 pounds.                         account for many other effects such as tow-
                                                   line fatigue, corrosion, and wear.
The example shown in Figure 3-2 assumes
constant speed and could apply to a tow that       This approach is suitable when the operators
is small compared to the size and power of         have a great deal of experience with the tow-
the tug. For large tows, where slow swings         ing system under consideration. Unless it is
can take 10 minutes or more, and for the typi-     known that dynamic loads will increase the
cal situation where the tug’s power setting is     steady-state tensions by more than 100 per-
constant, the tug and tow both slow down to        cent, apply these safety factors to the calcu-
accommodate the increased tow resistance.          lated steady-state tension to determine the re-
This is especially true when the tow sheers off    quired hawser strength. The safety factor
to one side. A poorly behaved tow, therefore,      shall be increased appropriately if the tow is
cannot be expected to attain the speed predict-    unfamiliar or there is significant uncertainty
ed by (Ref. G) without a significant increase      about the degree of dynamic loads or the con-
in tug power and hawser tension, neither of        dition of the hawser. Judgement is required
which may be possible or wise.                     when assessing the situation. If technical as-
A badly sheering tow can apply a significant       sistance is required, contact NAVSEA 00C.
additional tension peak when it “fetches up”       3-4.1.6 Predicting Dynamic Tensions
at the end of each excursion to the side. This     Until recently, the use of safety factors was
may dictate a further, deliberate slowing to       the only way to offset unpredictable dynamic
protect the towing gear, especially if the tow-    loads. Difficulties occur with this approach,
ing machine is not in its automatic mode.          however, when using a new towing system or
Determination of maximum values for the            towing a ship or structure with which there is
three components of towline tension is desir-      no previous experience. Sometimes this
able in the planning or design of a tow, as        happens when the standard design or material
well as in the actual towing operation. During     for a towing system has been changed. The
a tow operation, precise determination of          more effects that are combined into one factor
towline tension requires precision instrumen-      of safety, the greater the uncertainty.
tation. Normally tugs are not equipped with        A new statistical approach is emerging for
instrumentation sufficiently accurate to mea-      predicting the impact of ship motion on tow-
sure Twave. Most tugs, however, are equipped       line tension in a given sea condition. This ap-
with towing machine tension meters suffi-          proach estimates the extreme dynamic ten-
ciently accurate for determining steady and        sion level that the towline is likely to
quasi-steady tension. Twave must be treated as     encounter under certain conditions. This in-
discussed in the following sections.               formation can be used by the tow planner if
3-4.1.5 Factors of Safety                          sea conditions are known. More importantly,
                                                   the data can be used to predict acceptable
Tow planners and operators have traditionally
                                                   risks of extreme dynamic towline loadings,
dealt with unpredictable dynamic tensions by
                                                   rather than relying solely on traditional fac-
applying large safety factors to steady ten-
                                                   tors of safety that are based on steady-state
sions when sizing components. Recommend-
                                                   tensions. (Ref. M) describes this approach in
ed factors of safety for various components
                                                   detail.
are presented in Table 3-2. To use this ap-
proach, multiply the calculated steady-state       When using these new statistical techniques,
tension by the appropriate factor of safety and    multiply the “extreme tension” that is calcu-

                                                                                              3-7
                                        U.S. Navy Towing Manual



                                  Table 3-2. Safety Factors for Good Towing Practice.




                                                                      Minimum Factors of Safety*

                                                                                                            Shackles
                                                           **                                                 and
              Towing Mode for Tug            Wire         Wire        Chain                    Synthetic    Detach-        Bitts,
                                             Rope         Rope       Pendant                     Line         able        Padeyes
                                            Hawser       Pendant     or Bridle    Polyester     (Other)      Links          etc.

                     Note                      1            1            1            2           2, 3          4              5

      Long-Scope Wire Rope
      Hawser
      On automatic tension control         3 (4)***         4            4            -            -            3              3
      On the brake                             5            6            6            -            -            5              5
      On the pawl (dog)                        7            8            8            -            -            7              7
      On the hook (bitt, pad, etc.)            7            8            8            -            -            7              7
      On the hook or brake with chain
      pendant                                  4            5            6            -            -            4              4
      On the hook or brake with syn-
      thetic spring                            4            5            5            6            -            4              4

      Long-Scope Synthetic
      Hawser with Wire Rope Pendant
      On automatic tension control             -            4            4            4            10           3              3
      On the brake                             -            6            6            6            12           4              4
      On the pawl (dog)                        -            8            8            8            12           4              4
      On the hook (bitt, pad, etc.)            -            8            8            8            12           4              4



         NOTES:
         1.     Based on Minimum Breaking Strength.
         2.     Based on Minimum New Dry Breaking Strength. These figures are for 8” circumference and larger. For
                smaller lines, increase safety factor by 2. (See 4-2.2)
         3.     For Nylon: Breaking strength is reduced by 15% when wet.
         4.     Based on minimum breaking strength for links and proof load for shackles.
         5.     Based on Material Yield Strength.
              * “Minimum” applies only to new components, good weather, short duration, or emergency conditions.
                Old components, possible heavy weather, long-duration use, etc., may impose uncertainties which require
                use of safety factors greater than the listed minimum safety factors.
           ** When pendant is used as a deliberate “fuse” (i.e., safety link), use the same factor of safety as for the haw-
              ser but applied to the breaking strength of the pendant.
         *** See 4-3.1 For Details.




3-8
                                      U.S. Navy Towing Manual


lated by a safety factor of 1.5. This does not      To avoid dragging or fouling the towline on
supersede the traditional safety factor ap-         the bottom, while maintaining a sufficient
proach described in the previous section.           catenary depth to absorb changes in tug-tow
Both should be checked, since either may            separation, it is necessary to estimate the cat-
control for a specific set of circumstances.        enary depth of the towline. A number of
The more severe criterion shall be considered       methods have been used for estimating tow-
the limit until significant quantitative experi-    line catenary.
ence is gained with the dynamic theory.
                                                    To estimate catenary depth, it is necessary to
Removal of the uncertainty caused by the dy-        have the following data available:
namic effects may eventually permit reduc-              • Steady tension in the towline
tion in traditional towing system safety fac-
tors. As confidence in the dynamic loading              • Lengths of the towline components
approach develops, the use of factors of safe-          • The weight per unit length in water of
ty may no longer be required.                             each component
3-4.2 Calculating Towline Catenary                  The steady tension in the towline may be esti-
                                                    mated by using the tension meter on the tow-
When wire rope is used as a towing hawser,          ing machine, by the estimating procedure in
the catenary is the primary means of relieving      (Ref. G), or by using the chart in Figure 3-3,
the peak dynamic tensions. The weight of a          which presents the calculated tug pull avail-
wire rope towing hawser, either alone or in         able versus speed through the water. The
combination with a short segment of chain at        composition and total length of the towline
the tow end of the towline, causes a catenary,      should be known. Table B-2 and Tables D-1
or sag, in the towline between the tug and the      through D-9 provide the weight per unit
tow. Variations in the towline tension tend to      length for various towline components. When
smooth out in the catenary. Temporary de-           weight in water of steel components is not
crease of the distance between tug and tow, or      given, multiply weight in air by 0.87 to obtain
a decrease in tension, is absorbed by a deep-       weight in salt water.
ening catenary depth. An increase in the sepa-
ration between tug and tow causes the catena-       An initial estimate of the catenary depth of
ry to decrease in depth and the hawser tension      the towline may be determined using the fol-
to increase. Thus, the wire catenary tends to       lowing formula:
act as a spring, softening the tug-tow interac-                                              2
tion.                                                   C = T ⁄ W – T ⁄ W 1 – ( WS ⁄ 2T )

Due to the hydrodynamic drag of the wire            where:
during the rising of the catenary, the spring       C        = Catenary or sag (ft)
effect is not immediate. For load increases of
a sharp or sudden nature, the catenary cannot       T        = Steady tension (lbs force)
be expected to absorb the accompanying in-          W        = Weight in water per unit length
creases in towline tension completely. Using                   (lbs/ft)
an automatic towing machine or a synthetic
                                                    S        = Total scope (ft) (total of all com-
spring (see Section 4-6.5) in conjunction with
                                                               ponents)
the catenary will help provide an effective re-
lief of changing loads over the full range of       Total weight in water per unit length (W) is
conditions.                                         computed as the sum of the weights of the in-


                                                                                                 3-9
                                       U.S. Navy Towing Manual


dividual towline components divided by the           Likewise, a ship could plot curves of catenary
total towline scope.                                 versus tension for several tension figures to
                                                     provide a graphical representation of the
This formula applies to single component             effect of change in hawser scope. When water
wires hanging under their own weight. For            depth is limited, the ship can start with the
calculating, scope (S) and weight (W), when          required catenary depth and work backwards
the towline includes a bridle, the total weight      to determine the required scope/tension
of the bridle should be used, but the scope is       combination. In addition, some towing
estimated as a single leg of the pendant. This       machine technical manuals include tables or
formula provides an acceptable estimate of           curves to assist in solving scope/catenary/
towline catenary for towline configurations          tension questions.
where the ratio of towline scope (S) to catena-
ry (C) is greater than 8:1. When the ratio is        It should be noted that catenary depth will
less than 8:1, the catenary depth predicted by       change with varying tensions. If catenary
this formula does not provide an accurate es-        depth is a concern (e.g., bottom contact), ex-
timate of the towline catenary.                      pected minimum tensions should be used.
Based on this formula, Figures 3-4 through           3-4.3   Reducing Anticipated Towline
3-12 show the calculated catenary for various                Peak Loads
common compositions and lengths of tow-              Several aspects of the towline design can sig-
line. These curves may be used for towing            nificantly affect the peak towline tensions.
speeds up to 12 knots. To decrease catenary,         Some are adjustable during the tow; some are
towline scope may be shortened or the towing         not, but nonetheless should be considered
speed increased.                                     during the tow planning phase. These mea-
These graphs assume the chain is attached at         sures include:
the bullnose. If additional chain is used after a       • Increasing towline scope
wire pendant (i.e., closer to the middle of the
tow configuration) a deeper catenary will re-           • Increasing length of chain pendant or
sult. These figures provide estimates and care            bridle
should be taken when there is a risk of bottom          • Inserting a synthetic spring into the
contact.                                                  towline system.
To quantify effects of changes in tension, a         The towline scope used during a tow depends
ship can draw its own curve, representing the        primarily on four factors:
scope actually used, and proceed along that
curve to different tensions to find the new cat-        • Type of towing rig employed
enary. For example, a ship using a 2-inch               • Water depth
hawser and no chain would refer to Figure
3-7. For a scope of 1,500 feet, a new curve             • Catenary required to absorb changes in
could be plotted in between the existing                  towline tension
curves for 1,000 and 2,000 feet. This new
                                                        • Scope required to keep the tug and tow
curve would show that increasing tension
                                                          “in step”
from 20,000 pounds to 30,000 pounds would
decrease the catenary depth from about 100           To estimate the towline scope required, it is
feet to about 65 feet. Slowing down to a ten-        first necessary to estimate the steady towline
sion of 10,000 pounds will almost double the         tension required to maintain the desired tow-
catenary to about 190 feet.                          ing speed. Calculating total tow resistance is


3-10
                                U.S. Navy Towing Manual




Figure 3-3. Available Tension vs. Ship’s Speed for U.S. Navy Towing Ships.




                                                                             3-11
                                           U.S. Navy Towing Manual




       C atenary (feet)




                                                                                S cope (ft)




                                                   Tension (pounds)




                          Figure 3-4. Catenary vs. Tension; 1 5/8-Inch Wire, No Chain.




3-12
                                             U.S. Navy Towing Manual




C atenary (feet)




                                                                                   S cope (ft)




                                                     Tension (pounds)




                   Figure 3-5. Catenary vs. Tension; 1 5/8-Inch Wire, 90 Feet of 2 1/4-Inch Chain.




                                                                                                     3-13
                                                     U.S. Navy Towing Manual




       C atenary (feet)




                                                                                         S cope (ft)




                                                         Tension (pounds)




                          Figure 3-6. Catenary vs. Tension; 1 5/8-Inch Wire, 270 Feet of 2 1/4-Inch Chain.




3-14
                                          U.S. Navy Towing Manual




C atenary (feet)




                                                                     S cope (ft)




                                           Tension (pounds)




                   Figure 3-7. Catenary vs. Tension; 2-Inch Wire, No Chain.




                                                                                   3-15
                                          U.S. Navy Towing Manual




       C atenary (feet)




                                                                                      S cope (ft)




                                                        Tension (pounds)




                          Figure 3-8. Catenary vs. Tension; 2-Inch Wire, 90 Feet of 2 1/4-Inch Chain.




3-16
                                                          U.S. Navy Towing Manual




C a te n a ry (fe e t)




                                                                                      S co p e (ft)




                                                         Te n sio n (po u n d s)




                         Figure 3-9. Catenary vs. Tension; 2-Inch Wire, 270 Feet of 2 1/4-Inch Chain.




                                                                                                        3-17
                                           U.S. Navy Towing Manual




       C atenary (feet)




                                                                               S cope (ft)




                                                  Tension (pounds)




                          Figure 3-10. Catenary vs. Tension; 2 1/4-Inch Wire, No Chain.




3-18
                                                  U.S. Navy Towing Manual




C atenary (feet)




                                                                             S cope (ft)




                                              Tension (pounds)




               Figure 3-11. Catenary vs. Tension; 2 1/4-Inch Wire, 90 Feet of 2 1/4-Inch Chain.




                                                                                                  3-19
                                             U.S. Navy Towing Manual




       C atenary (feet)




                                                                                               S cope (ft)




                                                           Tension (pounds)




                          Figure 3-12. Catenary vs. Tension; 2 1/4-Inch Wire, 270 Feet of 2 1/4-Inch Chain.




3-20
                                     U.S. Navy Towing Manual


described in detail in Section 3-4 and (Ref.       instance, from an initial tension of 20,000
G).                                                pounds, the 1,000-foot hawser can absorb
                                                   about 19 feet of additional separation be-
Having an estimate of the total towline resis-
                                                   tween the tug and tow before it reaches
tance, it is then possible to compute the cate-
                                                   200,000 pounds tension; the 1,800-foot haw-
nary that will be associated with a chosen
                                                   ser will not reach that tension until separation
towline scope and towline rig. Section 3-4.2
                                                   is increased by almost 36 feet. Similarly, a 20
presents a simple formula for estimating the
                                                   foot stretch of the 1,800-foot hawser increas-
catenary. For hawser scopes greater than or
                                                   es its tension to only about 75,000 pounds.
equal to 1000 feet, Figures 3-4 through 3-12
                                                   The longer hawser significantly reduces the
will provide catenary depth directly, given
                                                   peak tensions caused by the same ship move-
hawser tension.
                                                   ments. A similar trend would be seen with
The following explores the ability of a wire       IWRC wire. Ships with different hawsers can
catenary to absorb ship movements by includ-       prepare a family of curves showing the
ing “stretch” of the wire.                         change in tension as the separation between
If the effects of hydrodynamic drag are ig-        the ships changes.
nored, catenary theory estimates the separa-       The quantitative data shown in Figure 3-13
tion between tug and tow as:                       are based on slow changes in distance or ten-
          D = S ( 1 – WC ⁄ 3T )                    sion. Classic catenary is limited in its ability
                                                   to absorb tug and tow motions, even where
where:                                             there is a relatively modest average hawser
D        = Horizontal distance between the         tension. Effectiveness of the classic catenary
           tug stern and the bow of the tow        in reducing dynamic loads has limitations.
           (ft)                                    This is because the hydrodynamic resistance
                                                   normal to the tow wire significantly impedes
S        = Total scope of the hawser (ft)          it’s rise and fall at typical frequencies of dy-
W        = Weight in water per unit length of      namic seakeeping loads. Therefore, the wire
           the hawser (lbs/ft)                     towline does not always have time to fully re-
                                                   sume its former deep catenary when the next
C        = Catenary or sag (ft)                    surge in tension occurs. So, Figure 3-13
T        = Steady tension in the towline (lbs      should be used for qualitative comparisons of
           force)                                  different towline configurations acting under
                                                   dynamic loading.
See Figure 3-1 for a graphical representation
of these values.                                   A similar analysis of the advantages of add-
                                                   ing chain to the towline can be prepared using
To quantify the effect on the hawser tension
                                                   the methodology shown in Table 3-4. Plot
for a given change in distance between tug in
                                                   curves showing the effect of adding one or
tow, it is necessary to develop a table or
                                                   two shots of chain pendant to a given hawser
curve of distance (D) vs. tension (T) for
                                                   length. The calculation process is identical,
various hawser scopes. The computation is
                                                   except that the comparison will be between
fairly direct if tension (T) is assumed for a
                                                   hawsers of the same length but with different
given scope (S) of hawser; catenary depth
                                                   total length and unit weights, because the
(C) is computed, then horizontal distance (D)
                                                   weight of the chain is distributed throughout
of the catenary.
                                                   the hawser length. The analysis will demon-
Figure 3-13 shows a comparison between an          strate that adding only one shot of 2¼-inch
1800 foot hawser and a 1000 foot hawser. For       chain provides a considerably softer system

                                                                                              3-21
                                                 U.S. Navy Towing Manual



                                       Table 3-3. Section Modulus for Wire Rope.




                                                                                 Wire Diameter

             Wire Type            Load Percentage          1 5/8 inches              2 inches            2 1/4 inches

                                       0 - 20%                   16.4                  24.8                  31.4
            6 x 37 IWRC
                                      21 - 65%                   18.2                  27.6                  34.9

                                                                            multiply all values by 106




                  Table 3-4. Elongation of 1,500 Feet of 6x37, 2 ¼-Inch IWRC EIPS Wire Rope.




                                     Section                                         Catenary        Distance
                   Tension                               Se2            Scope3
                                    Modulus1                                           (ft)4           (ft)5

                          0         31.4 x 106              0            1500             --              --
                     10,000         31.4 x 106            0.5           1500.5         257.9          1395.5
                     25,000         31.4 x 106            1.2           1501.2         184.3          1471.2
                     50,000         31.4 x 106            2.4           1502.4          43.1          1498.8
                     75,000         31.4 x 106            3.6           1503.6          31.3          1501.9
                     88,000         31.4 x 106            4.2           1504.2          26.3          1503.0
                    100,000         34.9 x 106            4.3           1504.3          22.0          1503.4
                    125,000         34.9 x 106            5.4           1505.4          18.5          1504.8
                    150,000         34.9 x 106            6.4           1506.4          15.5          1505.9
                    175,000         34.9 x 106            7.5           1507.5          13.2          1507.2
                    200,000         34.9 x 106            8.6           1508.6          11.6          1508.4
                    250,000         34.9 x 106           10.7           1510.7           9.3          1510.5
                    275,000         34.9 x 106           11.8           1511.8           8.5          1511.6
                    288,000         34.9 x 106           12.4           1512.4           8.1          1512.3

       Note: Assume constructional stretch has been accomplished through previous loadings.

       1.    Section Modulus (Area x Modulus of Elasticity) for 2 1/4-inch IWRC hawser is 31.4 x 106 through 20% strength
             of the wire; 34.9 x 106 over 20% load.
       2.    Change in scope due to wire elasticity. (ft)
       3.    Total scope of hawser after stretch. (ft)
       4.    Catenary depth per formula:
                                                   2
               C = T ⁄ W – T ⁄ W 1 – ( WS ⁄ 2T )
       5.    Distance between tug and tow per formula:             Source: Wire Rope Users Manual, 3rd Edition, Table 17
               D = S ( 1 – WC ⁄ 3T )




3-22
                                                              U.S. Navy Towing Manual


that develops lower peak tensions for the
same change in separation between tug and                                                25000 × 1500
                                                                                                                        -
                                                                                         -------------------------------- = 1.2 ft.
                                                                                                                    6
tow.                                                                                         31.4 × 10
Two components of wire “stretch” must also                                     This formula assumes that constructional
be included when determining the distance                                      stretch has already occurred. Table 3-4 has
between tug and tow: constructional stretch                                    been developed for a 1,500-foot, 2 1/4-inch
and elastic stretch. The Wire Rope Users                                       IWRC extra improved plow steel (EIPS) wire
Manual (Ref. C) estimates constructional                                       hawser.
stretch as 0.5 to 0.75 percent for 6-strand, fi-
ber-core (FC) wire and 0.25 to 0.5 percent for                                 3-4.3.1 Using an Automatic Towing Machine
6-strand, independent wire rope core (IWRC)                                    The automatic payout and reclaim feature of
wire. Constructional stretch is caused by a                                    the towing machines installed in most tugs is
virgin rope’s helical strands constricting the                                 a very effective means of reducing peak tow-
core during initial loading. The constricted                                   line tensions. Table 3-5 provides the range of
core is compressed and lengthened by the                                       automatic settings available on various tugs.
pressure exerted by the helical strands. For fi-                               Generally used when water depth precluded
ber core ropes, constructional stretch is pro-                                 an adequate catenary, the towing machine
nounced due to the high compressibility of fi-                                 was often taken off “automatic” after suffi-
ber when compared to an IWRC. Construc-                                        cient towline catenary had been established in
tional stretch properties fade from wire rope                                  deeper water. Now, however, with questions
early in its life, especially for IWRC ropes.                                  concerning real effectiveness of a wire cate-
Shortly after a wire rope has been repeatedly                                  nary in reducing peak tensions, it appears that
loaded, the constructional stretch characteris-                                the automatic feature is equally as important
tic is no longer exhibited. Fiber core ropes,                                  in deep water. Operation in the automatic
however, will retain this property longer, es-                                 mode is generally preferred; this, however, is
pecially if subjected to only light loads.                                     not intended to conflict with the manufac-
                                                                               turer’s approved operating procedures. For
The elastic stretch of hawsers likewise varies
                                                                               example, in calm seas, the manufacturer’s
with load. For convenience, elasticity is as-
                                                                               recommended standard operating mode will
sumed to be constant through 20 percent
                                                                               probably be manual.
loading, with a different figure applying be-
yond 20 percent loading. For common Navy                                       Additional information on towing machines
hawsers, the figures in Table 3-3 can be used                                  and winches appears in 4-5.1 and in (Ref. L).
where Section Modulus (which incorporates                                      3-4.3.2 Using Synthetic Towlines
compactness factors and variance of elastic
modulus) is expressed as the effective area of                                 Using synthetic towlines is one of the best
the steel in the wire multiplied by the modu-                                  means of absorbing dynamic towing loads.
lus of elasticity of the steel (Section Modulus                                The characteristic elasticity of synthetics has
(lb) = area (in2) x Elasticity (pounds per                                     many advantages over the other means of re-
square inch)).                                                                 ducing dynamic loads. Those advantages in-
                                                                               clude, but are not limited to, the following:
For example, a 1,500-foot, 2 1/4-inch IWRC
wire with a 25,000-pound load will elastically                                    • Speed of response. When compared to
stretch:                                                                            an automatic towing machine (ATM),
                                                                                    synthetic lines are capable of instanta-
                         load ( lb ) × length (ft)
 Change in length (ft) = ---------------------------------------------------
                                                                           -        neous response. If the dynamic load has
                         Section Modulus (lb)
                                                                                    a low acceleration, both the ATM and


                                                                                                                                      3-23
                                      U.S. Navy Towing Manual


        synthetic line absorb the dynamic load      tug must have many other special attributes.
        comparably. If the load has a high ac-      It must be staffed with competent personnel
        celeration, however, the ATM may not        and have adequate power for the tow, proper
        be able to respond fast enough before a     towing gear to connect the tow to the tug, and
        tension spike impacts the towing sys-       sufficient endurance to complete the tow.
        tem.
                                                    The principal measure of a tug’s power is its
   • Passive system. Once deployed, the             ability to exert tensile force on the towline.
     synthetic line requires no operator and        The maximum force a tug can exert on the
     is non-mechanical. Its shock-absorbing         towline is defined as the tug’s maximum pro-
     action requires no active input or main-       pulsion power delivered at zero tug speed. In
     tenance by the operator.                       the jargon of the towing industry, this maxi-
   • Maintenance. An ATM is a complex               mum tug power/zero tug speed condition de-
     machine and while it is not a common           livers a force referred to as “bollard pull.”
     occurrence, is subject to mechanical           The tug’s available propulsion power and hy-
     failure. Proper care and diligent mainte-      drodynamic properties of the tug and tow de-
     nance is necessary to ensure depend-           termine the speed of the tow and, therefore,
     able operation. Synthetic line is affect-      steady forces on the towline. Generally, a
     ed by other factors including abrasion,        tug’s power plant and propeller are designed
     heat, and UV light. By limiting expo-          to deliver maximum power and optimum effi-
     sure to these factors, a synthetic tow         ciency at a designated towing speed. The
     hawser should have a long service life.        greatest thrust (bollard pull) is produced at
                                                    zero speed, with the towline pulling force di-
Synthetic towlines take two forms: a com-           minishing as the towing speed increases.
plete synthetic tow hawser and a “spring”           When the tug reaches its maximum free route
synthetic line inserted in the towing system.       speed, all its horsepower is used in propelling
The synthetic spring consists of a length of        it. At this point, the available towline pulling
synthetic line placed in the towing system be-      force is essentially zero.
tween the steel towing hawser and the chafing
chain extending from the tow. A spring works        Each class of ship should have its own
in combination with the catenary produced by        unique set of available tow tension curves
the heavier steel components. It can assist in      that depend upon engine power setting, ship
absorbing rapid acceleration peak loads while       speed, propeller rpm, and propeller pitch (for
the catenary adjusts to loads applied more          ships with controllable pitch propeller [CPP]
slowly. A synthetic spring should be sized to       systems). For tow planning, the maximum
a comparable breaking strength of the re-           available tow speed is the figure of interest.
mainder of the towing system. Important re-         Figure 3-3 shows the available tow tension
strictions on the use of synthetic tow hawsers      versus ship’s speed for U.S. Navy ocean
and springs found in Section 4-3.2, Section         tugs. Comparing a curve to a horizontal re-
4-6.5, and (Ref. C). Consult and incorporate        sistance value (as calculated in 3-4.1.3)
these restrictions in any towing system design      provides the approximate maximum tow
that involves synthetic line.                       speed for an assumed condition. If the maxi-
3-4.4    Tug and Equipment Selection                mum speed available does not coincide with
                                                    the assumed tow speed conditions, additional
3-4.4.1 Tug Selection
                                                    resistance computations should be performed
Much too often tug assignments have been            to achieve a balance between tension re-
based almost completely on availability. A          quired and tension available. A more direct


3-24
                                        U.S. Navy Towing Manual



  2” FC IP S W ire
  18 00 ’ S cop e
                                                                                             30 0 K




                                                                                                       Tension (pounds)
                                                                                             20 0 K




                                                                                             10 0 K




                                                                                             0
17 80        17 90          18 00         18 10          18 20          18 30        18 40
 -20          -10            0*           +10            +20            +30          +40




   2” FC IP S W ire
   10 00 ’ S cop e
                                                                                              30 0 K




                                                                                                       Tension (pounds)
                                                                                              20 0 K




                                                                                              10 0 K




                                                                                              0
 98 0         99 0           10 00         10 10          10 20          10 30       10 40
 -20          -10             0*           +10            +20            +30         +40
                                    Separation Of Tug A nd Tow (ft)
                               *N ote: Zero re pre sents stea dy-sta te cond ition

                                                      NOTE

                              These curves plot the hawser tension vs. tug/
                              tow separation. They demonstrate the much
                              “softer” nature of the longer scope for the same
                              change in separation.


                     Figure 3-13. Distance Between Vessels vs. Hawser Tension for
                                      1,000 and 1,800 Feet of 6x37 FC Wire.



                                                                                                       3-25
                                     U.S. Navy Towing Manual



        Table 3-5. Operating Range for Automatic Towing Machines of Various Types of Ships.


  Types of Ships                                    Operating Range (lbs.)


  Salvage Ship (ARS 50)                             20,000 - 110,000


  Fleet Ocean Tug (T-ATF)                           30,000 - 110,000

method is to plot a curve of horizontal resis-      quired to provide the required towing
tance directly onto a copy of Figure 3-3. The       capability.
tug and tow curves will intersect at the maxi-
                                                    For emergency or unplanned towing require-
mum speed attainable with each tug for              ments, the tow will be initiated by the first
assumed tow conditions.                             available tug. Procedures outlined herein are
                                                    useful in determining whether additional tow-
If the available tow speed exceeds the amount
                                                    ing assets should be diverted to escort or take
needed (usually for small or non-ship tows),
                                                    over the tow. (Ref. K) contains data useful in
the tug will require less than maximum con-
                                                    estimating the power of commercial tugs that
tinuous engine power. In this situation, a less
                                                    may be needed in an emergency.
powerful tug can be considered. Conversely,
if available tow speed is less than required, a     3-4.4.2 Towing Gear Selection Factors
more powerful tug or multiple tugs must be          Once the tow vessel has been optimally sized
selected. In the latter case, there will be two     to fit operational requirements, towing
or more towlines, so towline hydrodynamic           hardware, including the “jewelry” connecting
resistance must be calculated appropriately.        sys te m c o mp o ne n ts , sha l l b e si z ed to
Otherwise, the available towline tension of         accommodate the anticipated forces for those
the tugs is additive.                               operational requirements. The mechanical
                                                    properties of typical components of towing
When the available tug is underpowered for          systems are covered in the Appendices of this
the desired tow speed, the most important           manual. Examples include wire rope and wire
consideration is whether it has sufficient          rope terminations ((Ref. B)), synthetic fiber
power to keep the tow out of danger under           lines ((Ref. C)), chain, shackles and links
the most severe wind and sea conditions that        ((Ref. D)), and line stoppers ((Ref. E)).
can be reasonably expected. For instance, it        Engineering design factors of safety for all
may be acceptable that a given tug is unable        system components are dis-cussed in 3-4.1.5.
to make headway over the ground, while              Towing gear is discussed at length in (Ref. 4).
towing a large ship in a sudden gale in the
open sea. The same tug, however, may be             Hawser size is generally fixed for a given tug.
considered inadequate for towing the same           If a specific size hawser is required by the
ship under the same conditions near a lee           type of tow, that fact, rather than the avail-
shore. In the case of a planned tow of a large      ability of tugs, may determine tug selection.
ship, adjustment to tow dates and careful           The calculated steady towline tension values
weather routing are essential. For more se-         are multiplied by the safety factor to obtain
vere cases, adjustment of the assignments           the required minimum breaking strength of
and schedules of other tugs also may be re-         the wire rope hawser. With the minimum


3-26
                                    U.S. Navy Towing Manual


breaking strength known, (Ref. B) may be          be reversed to find the maximum allowable
used to evaluate the wire hawsers carried by      steady tension.
candidate tugs. If there is no good match, the
assumed tow speed can be adjusted until a
match between required hawser strength and
available tugs is achieved. For a particular
tug, with a specific hawser, the problem may




                                                                                       3-27
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       This Page Intentionally Left Blank




3-28
                                     U.S. Navy Towing Manual


                                                      • Strength (static loads, dynamic loads,
                Chapter 4                               fatigue)

          TOWLINE SYSTEM                              • Ability to nondestructively inspect
           COMPONENTS                                 • Elasticity (stretch vs. load over a full
                                                        range of loads and over the lifetime of
                                                        material, set or permanent stretch)
4-1 Introduction
                                                      • Predictability (strength and compliance)
This chapter presents guidance on the use of
the wide variety of components available for          • External abrasion resistance
use in towing. Tow planners and tow ships             • Internal abrasion resistance (related to
should carefully consider relative advantages           fatigue life)
and disadvantages of each component when
designing a tow rig. Consideration should be          • Weight and specific gravity
given to durability, availability, ease of han-
dling, and other pertinent factors.                   • Survivability in a specific environment
                                                        (effects of corrosion, ultraviolet light,
4-2 Towline System Components                           sea water, acids, temperature, moisture)

                                                      • Ease of handling (surface characteris-
The towline system is made up of many com-
                                                        tics: slippery, sticky, pliable, minimum
ponents. A tow hawser often called the tow-
                                                        bend radius)
line or towline connection, is only one com-
ponent of the towline system. Figure 4-1              • Stowage (volume shrinkage upon dry-
illustrates a complete towline system. A tow-           ing, flexibility)
line system includes attachment points, rope
terminations, and tension components such as          • Adaptability to fittings and terminations
chain pendants, wire rope pendants, and
spring pendants. These elements are joined            • Compatibility of fittings and terminations
by shackles, links, or other connecting hard-      In various towing applications, one or more
ware.                                              of these factors may have a predominant in-
A towline system is the tension-carrying link      fluence on the choice of material. Chain, for
between tug and tow and must be able to            example, often is selected as a chafing pen-
withstand steady loads, as well as dynamic         dant or bridle because of its abrasion resis-
peak loads, often called shock loads. The pri-     tance and survivability. When used as a lead-
mary materials used in tension members are         er chain (see Figure 4-1), provides elasticity
wire rope, synthetic fiber lines, and chain.       through catenary action rather than through
                                                   material stretching. Likewise, polyester may
All items must be sized for the towing loads
                                                   be suitable for a tow hawser or spring, but
with an appropriate factor of safety (see Table
                                                   would not be selected as a chafing pendant.
3-2). Size and compatibility are key consider-
                                                   Wire rope is generally favored for use as a
ations.
                                                   tow hawser on ocean tugs because of its
The following is a list of factors that influ-     strength and reasonably high abrasion resis-
ence selection of the components of a towing       tance, with its flexibility, stowability, and
system:                                            ease of handling also being important.


                                                                                              4-1
                                               U.S. Navy Towing Manual




                To w           C h afin g     L ead C h ain   O p tio n al      C h afin g                       To Tu g
           A ttach m ent         C h ain         o r W ire    Syn thetic          G ear                       A ttach m ent
               Po in t         o r B rid le     Pen d ant      Sp ring                                            Po in t




                           D eck Ed g e       C o n nection        Tu g’s To w in g     Stern R oller
                            F airlead           Jew elry              H aw ser          an d C ap rail   H -B itts




                             Figure 4-1. Typical Towline Connection Components.


4-3 Main Towing Hawser                                            lines. At first, elasticity loss was countered by
                                                                  using long spans of hawser, where the weight
The tow hawser is the primary tension ele-                        of the wire rope formed a catenary in the wire
ment of the towline system. Tow hawsers are                       and provided a measure of effective elastici-
normally wire rope or a synthetic line. The                       ty. Later, tow ships often used manila spring
end of the hawser that extends to the tow is                      pendants, or “springs,” in conjunction with
usually equipped with an end fitting such as a                    wire rope to provide the needed elasticity.
socket, thimble, or spliced eye; if the tug                       Today, synthetic fiber springs perform this
doesn’t have a towing machine or winch,                           function and are common in commercial
both ends of the hawser may have fittings.                        practice.
When the tow hawser is part of a tug’s equip-
ment, it is stowed on the drum of the towing                      For wire rope in new or very good condition
machine, or in the case of synthetic line, in a                   and used in conjunction with an automatic
bin below deck. When the tow hawser is part                       towing machine, a minimum safety factor of
of the towed vessel’s equipment, it may be                        3 is appropriate for routine ocean tows in good
stowed on a storage drum, reel, or brackets,                      weather (see Table 3-2). To be on the conser-
or faked down in a tub, ready for use.                            vative side and allow for unforeseen occur-
4-3.1   Wire Rope Hawser                                          rences, a safety factor of 4 is recommended
                                                                  for routine tows. Other conditions require
Before the development of wire rope in the                        higher factors, as noted in the table.
19th century, the primary material used for
tow hawsers was natural fiber line made from                      4-3.2      Synthetic Hawser
manila, sisal, and hemp. As ships became
larger, the diameter of natural fiber lines in-                   When synthetic fiber line was developed for
creased to the point where handling and stor-                     commercial applications, it began to replace
age became difficult. Because of its superior                     manila rope for towing springs and hawsers
abrasion resistance and strength-to-weight                        on sma ll tu gs. Sy nt he ti cs al so g a ine d
and strength-to-size ratios, wire rope rapidly                    acceptance as open-ocean towing hawsers,
replaced natural fiber lines for towing haw-                      often replacing wire rope. The elasticity of
sers. Wire rope was accepted for towing de-                       synthetic hawsers easily absorbs tension
spite being far less elastic than natural fiber                   caused by motion.


4-2
                                               U.S. Navy Towing Manual


One of the first synthetic materials to be used              are provided in (Ref. C). Existing nylon tow-
in towing was nylon. The Navy began to                       ing hawsers shall be replaced with the ap-
experience problems, however, when using                     proved polyester lines on a size-for-size ba-
nylon line as the peak load mitigation system.               sis. Synthetic springs are discussed in Section
Some of the problems were due to nylon                       4-6.5.
being weaker when wet than when dry.
Additionally, the safe working load (SWL)                    Table 3-2 provides factors of safety for syn-
and factors of safety for nylon in the marine                thetic lines being used as a main towing haw-
e nv iro nm en t h a d n ot be e n a d eq ua te l y          ser. The steady towline tension value calcu-
defined.                                                     lated in Section 3-4 shall therefore be
                                                             multiplied by the safety factor listed in this ta-
A better understanding of the strength, ser-                 ble to obtain a required minimum strength.
vice life, degradation, and elasticity of syn-               Note that the safety factor depends on the tug
thetic line has led to limitations in nylon’s use            attachment point and the degree of tension
as the main towing hawser. Nylon line is only                control. (Ref. C) provides data for use in eval-
approved for:                                                uating one or more candidate synthetic lines.
    • Open-ocean towing of craft with less                   S m a ll e r l i ne s ( l e ss th a n 8 i n c h e s c i r-
      than 600 long tons displacement or in                  cumference), with a greater portion of their
      to w -a n d - b e -t o w e d o r e m e rg e n c y      fibers exposed to abrasion and the effects of
      towing operations                                      ultraviolet light and other chemical attack,
    • Unique or special tows approved by                     require higher factors of safety. Increase the
      NAVSEA on a case-by-case basis.                        factors listed in Table 3-2 by adding a value
                                                             of 2.
                        NOTE
                                                             4-4 Secondary Towline
       Appendix C provides the breaking
       strength values for synthetic line.
       Ma n u fac tu re r ’s ta b le s u su a lly            A secondary towline shall be rigged on all
       quote values for dry nylon. Break-                    tows. The secondary towline is intended for
       ing strength for wet nylon line is                    emergency, short-term use. It may be of less-
       about 15 percent less than for dry
       line and, thus, the manufacturer’s
                                                             er strength than the primary towline (although
       v a lu e s g e n e ra ll y m u s t b e d e -          it does not need to be) and is often made up
       creased by 15 percent for towing or                   with synthetic line. Rigging methods will
       other “wet” uses. Wet strength re-                    vary, depending on whether the tow is
       ductions do not apply to synthetics                   manned or unmanned. A secondary hawser is
       other than nylon.                                     placed on the tow and is generally led down
                                                             one side of the deck edge, rigged with a
A material that is better suited for towing ap-              heavy messenger led outboard of the ship’s
plications is polyester. NAVSEA’s continu-                   structure, and terminated by a lighter floating
ing investigation into using improved and                    pendant with a marker buoy trailing astern of
composite designs of synthetic hawsers has                   the tow (see Figure 4-2). This system is
led to the approval for general use of single                rigged so that the tug merely recovers a trail-
and double braided polyester lines in all rou-               ing messenger and heaves aboard the second-
tine and emergency towing applications, ex-                  ary towline for connection to the hawser. A
cept where otherwise dictated. The                           secondary tow system can be rigged to tow
specifications of the approved polyester lines               from either the bow or stern.


                                                                                                                   4-3
            U.S. Navy Towing Manual




      Figure 4-2. Secondary Towline System.



4-4
                                     U.S. Navy Towing Manual


For small tows a primary pendant is rigged         4-5 Attachment Points
using the ship’s anchor chain a secondary
pendant is rigged from a stern tow pad using       This section discusses various types of attach-
the ship’s emergency towing hawser with a          ment points on tows and describes the loading
200-foot floating messenger and small trail-       various types of attachments may be subject-
ing buoy. When rigging an emergency tow            ed to. Every possible effort should be made to
hawser aft, chafing chain should be connect-       ensure that an attachment point is subjected to
ed to the tow pad with a safety shackle. Peli-     only one type of load in a known direction.
can hooks should not be used. For large            Horizontal and vertical padeyes, for example,
tows or ships that will not tow well from the      should be subjected to a force only perpendic-
stern, a secondary tow hawser should be            ular to the axis of the pin. See Section 4-5.3
rigged from the bow and fairled down the           for more information.
sides, stopped off in bights, to the messenger.
                                                   The attachment points on tugs and tows trans-
                                                   mit the towing load from the towline to the
                  CAUTION
                                                   vessel. Attaching the towline system is of vi-
      Bights of wire hanging can be dam-           tal importance and must be given careful con-
      aged or loosened if the tow goes             sideration with regard to seamanship, rigging,
      alongside a tug or a dock.                   and basic engineering mechanics.

Secondary wire can be secured with a piece         Towline attachment points on U.S. Navy tugs
of 3/8-inch round bar. The round bar is tack       are the towing machine or traction winch.
welded to the deck edge and bent up and            Attachment to the tow may be at a hard point
around the wire. This allows the wire to be        specifically intended for towing, such as a
pulled out easily if needed. These clips           deck padeye, chain stopper, or specialized
should be spaced about 5 feet apart. A similar     towing bracket, although many ships do not
method can be employed when securing               have an attachment point specifically de-
chain. (see Figure 4-3) .                          signed and fitted for towing. Some commer-
                                                   cial ships are not designed to be towed, or the
All stops should be strong enough to hold in       tow attachment is located somewhere other
heavy weather but accessible to allow cutting      than originally designed. Often attachments
and light enough to be broken without dam-         require use of fittings or gear intended for
age to the towing pendant or tow. It must be       other purposes, such as single point mooring
rigged outboard of all existing structure, in-     (SPM) fittings, bitts, anchor chain holding fit-
cluding bitts and handrails, and should fall       tings, or the tow’s anchor chain. Sometimes,
free without turns that will cause kinking as      for planned tows, a new attachment point will
they pull out.                                     be installed. The attachment point shall be in-
                                                   spected for planned tows. Non destinctive
In all cases, a secondary towline will already     testing (NDT) shall include visual inspection
be connected to an appropriate hard point on       of the attachment point and surrounding area
the tow and provided with necessary chafing        and a dye-penetrant test of the padeye or bitt
protection. As a minimum for vessels above         attachment points is recommended. If there is
600 L-ton displacement, the secondary              any doubt about the strength of the padeye or
towing pendant should be 1 5/8-inch wire           attachment point, further testing and repairs
rope with the necessary chafing gear.              are required.


                                                                                               4-5
                                                                U.S. Navy Towing Manual




                                                                                                                            T YP
                                                                                                                    1/4
                                                 S econd ary W ire
                                                                                                                     1/8” x 1/8” (M ax) Tack W eld Together
      D e ck                                                                                                         A fter Installation on Every T hird C lip

                                                 R ou nd B ar                                                             A pprox. 1” R adius
                                                                                         3 1/2”
                                                                                                                                     3/8” D ia. R ound B ar (O S S)
                                                                                                                                     A ttachm ent C lip

                                                 S ide S hell                                                             D iam eter E quals W ire D ia. P lus 1/8”

                                                                                                                                 S econdary Tow P endant W ire Rope
                                                                                                                E xisting Deck / B ulwark Structure


               S e e R ig ht for m ore de tail                                    Typical S eco ndary Tow P endant W ire R ope A ttachm ent C lip




                                                                                                                          T YP
                                                                                                           3/16
                                      N O TE
                                                                                                                           1 1/4” R AD (TY P )

                      1. D im ensions of chain attachm ent
                         clips are to be determ ined by
                         allow ing 1/16” clearance all
                         around the chain                                     NOTE 1
                         (i.e., W idth plus 1/8”)

                      2. C hain attachm ent clips are to                                                                           S econdary Tow
                         be spaced approxim ately 3                                                                                P endant C hain Link
                         feet apart.
                                                                                                                               3/8” D IA. R oundbar (O S S )
                                                                                                                               A ttachm ent C lip

                                                                                                       NOTE 1


                                                            Typical S eco ndary Tow P endant C hain C lip




                                         S econdary Tow P endant W ire Rope
                                                                                       3/8” D ia. R ound B ar (O S S)
                                                                                                    A ttachm ent C lip




                                                                              E xist D eck / B ulw ark S tructure
                                                                                   A BT 60” (T Y P )


                                                     Typical S eco ndary Tow P endant S ecurem ent
                                             Typical S eco ndary Tow P endant M essenger Lin e Securem ent




                                                   Figure 4-3. Secondary Towline System.



4-6
                                      U.S. Navy Towing Manual


For an emergency tow, a makeshift connec-           were a natural evolution, providing the in-
tion, such as a heavy chain wrapped around a        haul and storage features for wire rope haw-
strong foundation, may be used. In every            sers, while eliminating the use of bitts and
case, the material condition of the fittings and    hooks.
structures should be carefully inspected.
                                                    All U.S. Navy tugs have automatic towing
For deck fittings designed specifically for         machines, except for the MSC-operated
towing, operators may assume that the appro-        T-ATFs, which use SMATCO winches. MSC
priate engineering was performed, if these fit-     has backfitted automatic towing machines on
tings pass the NDT inspection. If the attach-       selected T-ATFs. Each T-ATF requires a ship
ment point is inadequate or does not exist, it      check for applicability.
must be designed, fabricated, and installed.
                                                    The principal functions of towing machines
Activity preparing a tow must arrange for en-
                                                    are:
gineering analysis to ensure a safe connec-
tion.                                                  • Acts as a hard point or attachment point
                                                         for securing the towline to the tug.
An important factor when locating and in-
stalling an attachment point is the need for an        • Pays out and heaves in the towline dur-
integrated attachment point and fairlead sys-            ing towing operations.
tem. A fairlead ensures that the tow load is
                                                       • Transports or stows the towline as it is
applied in the designed direction, i.e., no side
                                                         heaved in.
loading. Therefore, attention should be paid
to both the attachment point and the fairlead.         • Acts as a quick-release device for dis-
A common failure of the attachment system                connecting a towline if necessary dur-
involves gross structural failure of either the          ing an emergency.
attachment point or fairlead. This problem is
                                                       • Acts as an automatic tension control de-
especially relevant when towing minecraft,
                                                         vice to limit or relieve peak dynamic
non-oceangoing craft, and wooden, alumi-
                                                         loads in a towline system, thereby en-
num, or fiberglass vessels. Fairleads on these
                                                         hancing the life and utility of the equip-
types of vessels may not be strong enough to
                                                         ment, increasing maximum speed, and
withstand towing loads.
                                                         increasing safety.
Safety factors for attachment points should be
                                                       • Monitors and displays tow hawser con-
designed and built in accordance with the                ditions such as tension and scope.
General Specifications for Overhaul of Su-
face (GSO) Ships, U.S. Navy, Sections 582           A towing machine has a power-driven drum
and 077, Naval Sea Systems Command,                 that serves as an attachment point and stores
S9AA0-AB-GOS-010/GSO (Ref. D). The                  unused portions of the wire rope towing haw-
cri-terion generally applied is that the break-     ser. The powered drum is used to control the
ing strength of the line should not exceed          length of wire towline. Most U.S. Navy tow-
35% of the padeye’s bitt, or cleats yield           ing machines have an automatic control sys-
strength.                                           tem that automatically pays out line when
                                                    tension exceeds a set value. More sophisticat-
4-5.1 Winches and Towing Machines
                                                    ed machines also have an automatic reclaim
Although wire rope is somewhat easier to            capacity, which hauls back the hawser when
handle than wet manila line of equal strength,      tension decreases. Towing machines have a
it cannot be faked out on deck when hauled          free-spooling feature that serves as a quick
in. Powered winches and towing machines             disconnect system for the towing hawser.


                                                                                               4-7
                                        U.S. Navy Towing Manual


As synthetic fiber line towing hawsers were           The term bollard is occasionally applied to a
being introduced in Navy towing, the multi-           bitt, but more commonly is applied to a de-
sheave traction winch was developed (see              vice on a pier for securing mooring lines.
Figure L-1). In addition to providing a hard          Bitts on U.S. Navy ships are designed to
point for attachment, the winch has payout            withstand a load equal to at least three times
and heave-in features for adjusting the tow-          the breaking strength of the line they were
line scope. Because reel-type storage is not          designed to hold. See Section 6-2.6.2 for the
practical for synthetic line, the hawser is           sa fe workin g loads of sp ecific design
fairled into a stowage bin located below              strengths of U.S. Navy bitts.
decks as it comes off the traction winch.
Some traction winches are now equipped                Towing or H-bitts are heavy steel castings or
with automatic controls, that pay out hawser          weldments secured to the ship’s structure (see
to relieve high towline tensions. This control        Figure 4-4) . Generally located near the tug’s
generally does not provide automatic reclaim          pivot point, they provide the hard point that
                                                      sustains athwartship loads imposed by a tow-
on the traction winches. Periodic heave-in,
                                                      line when it sweeps the fantail. In tugs fitted
under manual control, may be required to
                                                      with towing machines, the H-bitts are used to
maintain the desired towline scope. Traction
                                                      fairlead the main tow hawser to the drum to
winches for wire hawsers are often found on
                                                      prevent transverse strain on the level wind
larger commercial ships.
                                                      mechanism and are used to stop off the tow
Most towing machines and winches have a               wire when necessary. On the ARS 50, the
                                                      function of the H-bitts is integrated into the
“dog” system that positively holds the drum
                                                      deckhouse structure.
against towing loads. A dog is a pawl or
ratchet type system that cannot be released           Under normal towing conditions, using the
against tension. When towing “on the dog,” a          H-bitts for holding the hawser is not recom-
towing machine must be started up, engaged            mended; such use is usually restricted to de-
and the hawser heaved in slightly to release          beaching operations or other instances when
the dog. Therefore, when towing on the dog,           isolating the towing machinery from hawser
there is no quick-release capability.                 tension is necessary.
                                                      4-5.3   Padeyes
Refer to (Ref. L) for a more complete discus-
sion of U.S. Navy towing machines and                 The most frequent means of attaching a tow-
winches.                                              line to the towed vessel is by means of a pad-
                                                      eye. Three distinct types of devices collec-
4-5.2   Bitts                                         tively are referred to as padeyes. Personnel
                                                      rigging the connection must understand de-
A bitt is a strong post used for belaying, fas-       sign features. The three types of padeyes
tening, and working ropes, hawsers and                found in towing are:
mooring lines. Bitts usually appear in pairs
                                                         • Horizontal padeye
and are named according to their use.
                                                         • Vertical free-standing padeye
                      NOTE                               • Towing bracket

        Unless specifically designed, bitts           Figure 4-5 shows two different styles of hor-
        are generally not suited as towing            izontal padeyes. Their distinctive feature is
        attachment points and are not in              that the pin has a vertical axis. The towline,
        the proper position to be used as             therefore, is free to sweep in the horizontal
        towing fairleads.                             plane, while constrained in the vertical plane.
4-8
                                    U.S. Navy Towing Manual




    To w W ire P assages                                                         S ynthetic L ine
                                                                                    Fairlead

                                                                         To w W ire C arpenter
                           S ynthetic S topper                             S top per P adeye
                                 P adeye

                W ire R ope P late Fairleads



                  A ft E nd of A R S 50 Tow ing M ach inery R oom Lo oking Fo rw ard




                                                                                       T-AT F

             A R S 38




                                                   ATS




Figure 4-4. Aft End of ARS 50 Towing Machinery Room and Typical Towing Fairleads /Bitts .




                                                                                                    4-9
                                  U.S. Navy Towing Manual




             Deck Plate




              Full
       Penetration                                                       Stiffener
            W elds




             Longitudinal
                                      Integral-Pin Padeye




            Deck Plate




                                                                         Stiffener




                                                       Full
                                                    Penetration
                                                      W elds
                Longitudinal
                                       Chain Stopper P adeye

                                               CAUTIO N
                                 C hain stoppers are designe d to bear
                                 only 60% of the breaking strength of
                                 the chain. C hain stopper pad eyes
                                 should, therefore, not be used as a
                                 single attachm ent point for pendants
                                 or bridles. They should be used only
                                 as attachm ents for chain stoppers.



                               Figure 4-5. Horizontal Padeyes.




4-10
                                     U.S. Navy Towing Manual



There are two types of horizontal padeyes in             them as components in a towing sys-
use today.                                               tem.

   • The integral-pin type comes with its                             CAUTION
     own pin, with the female threads locat-
                                                         Chain stoppers are designed to
     ed in the base plate of the padeye (see
                                                         bear only 60% of the breaking
     Figure 4-5, upper sketch). A locking                strength of the chain. Chain stop-
     device prevents pin rotation. This style            per padeyes should, therefore, not
     padeye has a lower profile, so the mo-              be used as a single attachment
     ment arm of the towing load is corre-               point for pendants or bridles. They
                                                         should be used only as attach-
     spondingly lower to the deck. This al-              ments for chain stoppers.
     lows for lower loading moments and
     eases the design of the structure. Addi-
     tionally, the integral-pin padeye allows      The vertical free-standing padeye comes in
     the open or end link of a chafing chain       two basic designs as shown in Figure 4-6 .
     to be pinned directly to the padeye, re-      The difference is in the shape of the eyehole.
     quiring no additional connecting jewel-       The eye of a shackle-pin type padeye is a cy-
     ry.                                           lindrical hole through the plate designed to
                                                   accept the pin of a connecting shackle. In the
   • The shackle-style padeye is located on
                                                   dipped-shackle type padeye, the hole is elon-
     the forecastle of most U.S. Navy ves-         gated and the bearing area of the hole is
     sels (see Figure 4-5, lower sketch). It is    rounded so that the bow of the shackle can
     the standard fitting for the attachment       properly bear against the end of the slot. In
     of chain stoppers to the forecastle deck.     this case, the shackle’s pin is presented to the
     When using horizontal padeyes, there is       chafing pendant.
     often insufficient space to accommo-
     date the bolt of a safety shackle due to      The vertical free-standing padeye is less resis-
     the padeye’s low profile. Therefore,          tant to lateral loads than the horizontal pad-
     U.S. Navy chain stoppers are provided         eye. The free-standing padeye must be used
     with a specially forged, screw pin            with a towing fairlead strong enough to with-
     shackle that is appropriate for use in a      stand the lateral loads of the towline, to mini-
     towing rig. Chain stoppers and their as-      mize the risk of tripping the padeye. The
     sociated padeyes are nominally de-            width of the shackle-pin type padeye plate
     signed for only 60 percent of the anchor      should occupy 75 to 80 percent of the jaw
     chain’s breaking strength. The strength       width of the shackle, to prevent it from rack-
     of chain stoppers and their associated        ing and creating loads that tend to open the
     padeyes must be considered when using         jaw of the shackle.




                                                                                               4-11
                                U.S. Navy Towing Manual




                                                  F u ll P e n etratio n W eld s
       Ten sio n

                                                                                      L o n g itu d in al

                   G u s se t




                                                                     D eck P la te


                                   S tiffen e r



                                  D ipped-S hackle Type




                                                  F u ll P e n etratio n W eld s
       Ten sio n

                                                                                      L o n g itu d in al
                   G u s se t




                                                                      D eck P la te



                                   S tiffen e r




                                    Sh ackle-P in Typ e




                   Figure 4-6. Vertical Free-Standing Padeyes.




4-12
                                       U.S. Navy Towing Manual


The vertical free-standing padeye may have a         c. To find the minimum hole diameter (d),
higher attachment point than the horizontal             draw an imaginary line from the intersec-
padeye. This makes for larger loading mo-               tion point straight up to the top of the
ments on the structure itself and on the attach-        chart. The diameter measurements are
ment system to the ship deck or frame struc-            displayed across the very top of the chart.
ture. This in itself is not a disadvantage if the
                                                     d. To find the minimum length from the hole
design is proper and those who rig the system
                                                        to the edge of the plate, find the point
understand it.
                                                        where the diameter measurement (d) in-
                                                        tersects with the broken diagonal line that
                                                        appears in the upper left-hand portion of
                   CAUTION                              the chart. Look on the right-hand side of
                                                        the chart to find the minimum distance to
       If time and the situation permit, a              the edge (L). This minimum distance ap-
       detailed analysis of the padeye and              plies in all directions around the hole, in-
       connection should be made to
                                                        cluding above and below.
       avoid unexpected failure of either.
                                                     e. To determine the minimum length for the
4-5.4 Padeye Design                                     padeye, choose a particular thickness for
                                                        the continuous fillet weld (T). Each thick-
Figure 4-7 provides an acceptable padeye de-            ness is represented by a dashed diagonal
sign for situations where no suitable connec-           line. These four lines are labeled on the
tion point exists. Given the predicted towline          bottom of the chart on the right-hand side.
tension and a specific plate thickness, the             Find the point where the thickness (T) in-
chart provides the minimum hole diameter                tersects with the predicted load (F). To
and the minimum distance from the hole to               find the padeye length (l), draw an imagi-
the edge of the plate. Given the same predict-          nary line from the intersection point
                                                        straight up to the top of the chart. The
ed tension and a specific thickness for the
                                                        length measurements are displayed across
continuous fillet weld, the chart also deter-           the very top of the chart and are expressed
mines how long the padeye must be.                      in inches.
To design a padeye using this chart, follow          For example you are tasked with planning a
these steps:                                         tow using an automatic tow machine with a
a. Estimate the towline tension that the pad-        new towing hawser. Per calculations you esti-
   eye will meet. In this case, use the ap-          mate the towline tension (F) to be 80,000
   proximate towline tension as determined           pounds and the maximum plate thickness
   from the results of the calculations in Ap-       available to fabricate a towing padeye is 1 1/2
   pendix G. In Figure 4-7, this number is           inches thick (t). Determine the diameter of
   called the load or force (F). Locate this         the hole (d) required and the distance (L) the
   level using the numbers on the far right-         hole must be from the leading edge of the
   hand side of the chart.                           plate. By using Figure 4-7 we can determine
                                                     the diameter of the hole (d) required. Entering
b. Choose a particular plate thickness (t).          the left side of Figure 4-7 find the line corre-
   Each thickness is represented by a solid          sponding to the plate thickness (t). Trace the
   diagonal line. These lines are labeled in         line upward until it intersects with the esti-
   the lower left-hand side of the chart. Find       mated towline tension of 80,000 pounds from
   the point where the plate thickness (t) in-       the right side of Figure 4-7. Draw a line verti-
   tersects with the predicted load (F).             cally from this intersection to the top of Fig-


                                                                                                4-13
                                                                       U.S. Navy Towing Manual




                                                                                                                         t                                       L
                                                                                                                                                                          d
                                                  M inim um Hole
                                                Diam eter,d (Inches)                                                                         F
                                         5     4      3      2     1    0




                                                                                 D istance to Edge, L (Inches)
                                                                            8
                                                                                                                     T                                           l
                                                                                                                         d = Diam eter of Hole, in Inches
                                                                                                                         F = Load, in 1000 Pounds
                                                                            6                                            L = M inim um Distance to Edge, in Inches
                                                                                                                         t = Plate Thickness, in Inches
                                                                                                                         T = Thickness of C ontinuous Fillet W eld,
                                                                            4                                                 in Inches
                                                                                                                         l = Length, in Inches
                                                                            2
                                                                                                                                    Length, l (Inches)
                                                                            0                                10     20       30        40        50         60       70       80         90
                                                                                                                                                                                              0


                                                                                                                                                                                              20


                                                                                                                                                                                              40
                                   1/2
   Plate Thickness, t (Inches)




                                   5/8                                                                                                                                                        60




                                                                                                                                                                                                    Load, F (1000 Pounds)
                                   3/4
                                                                                                                                                                                              80
                                   7/8

                                     1                                                                                                                                                        100


                                 1-1/4                                                                                                                                                        120


                                 1-1/2                                                                                                                                                        140


                                                                                                                                                                                              160
                                 1-3/4

                                                                                                                                                                                              180
                                    2

                                                                                                                                                                                              200
                                             2-1/4
                                             2-1/2
                                             2-3/4




                                                                                                                              5/8




                                                                                                                                                      3/8
                                                                                                                                       1/2




                                                                                                                                                                                   1/4




                                                                                W eld Thickness, T (Inches)




                                                                                                                  NOTE

                                                                   Padeye material should be ASTM-
                                                                   36, ABS Grade A, or similar.




                                                          Figure 4-7. Minimum Padeye Design Requirements.



4-14
                                        U.S. Navy Towing Manual


ure 4-7. Where this line intersects the top of        mum distance to the edge of the padeye (L)
Figure 4-7 determines the minimum hole di-            required by the load and plate thickness.
ameter (d). In this case, a minimum hole di-
ameter (d) of 2 3/4 inches is required. We can                             CAUTION
also determine the minimum distance from
                                                              This method yields a design with a
the leading edge of the hole to the edge of the               minimum factor of safety of 3 for all
plate (L) by using the vertical line previously               failure modes. For a stronger pad-
drawn and determining where it intersects                     eye, use a higher assumed load.
with the dashed diagonal line crossing the top                For instance, if a padeye with a fail-
of Figure 4-7. Going right from this intersec-                ure load of 300,000 pounds is de-
                                                              sired, use 100,000 pounds as the
tion to the right of Figure 4-7 determines the                design load. The below-deck struc-
minimum distance to the edge of the plate                     ture must be checked or altered to
(L). In this case the minimum distance (L) to                 transmit towing stresses to the
the edge of the plate is 4 inches. Assuming                   ship’s structural members. Simply
the fillet welds are 1/2 inch thick (T), what                 welding the padeye to the deck
                                                              plating is not enough.
length (I) padeye is required? We can deter-
mine the length of the padeye by finding the
                                                      4-5.5    Deck Structure
line on the bottom of Figure 4-7 that corre-
sponds to the weld thickness (T). Trace this          When designing and locating padeyes, it is
line upward to the left until it intersects with      extremely important to examine the below-
the estimated towline tension from the right          deck structure. Towing padeyes produce
of the figure. Draw a vertical line from the in-      large local loads that cannot be supported by
tersection to the top of Figure 4-7. Where this       deck plating alone. It is necessary to locate
vertical line intersects the top of the figure de-    padeyes atop both longitudinal and trans-
termines the minimum distance to the edge of          verse members to adequately distribute load-
the plate (I). In this case the minimum dis-          ing to the surrounding structure, particularly
tance to the edge of the plate (I) is 16 inches.      if the padeye is likely to be subject to side
                                                      loads. The longitudinal member should be
The example is satisfactory for 80,000                aligned directly under the main plate of the
pounds of tension. To verify that the hole is         padeye. The transverse member location is
of sufficient size, check the size of the shack-      somewhat less critical but should be located
le required. If an automatic tow machine is           as close to the padeye as possible, preferably
used, Table 3-2 shows a factor of safety of 3         directly underneath.
is required or a 240,000-pound proof-load             4-5.6    Smit Towing Bracket
shackle (2 1/4-inch Grade B shackle). The pin         The Smit Towing Bracket consists of two
for this shackle is 2 1/2-inches thick and will       vertical plates, similar to a pair of free-stand-
just fit the hole in the padeye. If the tow were      ing padeyes, with an elliptical pin fitted be-
to be performed without an automatic towing           tween them (see Figure 4-8) . The pin is fit-
machine, but with a chain pendant, Table 3-2          ted with a keeper key or locking pin and can
would require a shackle factor of safety of 4.        be released in an emergency. The principal
(Ref. D) and Tables D-7 through D-9 show              advantage of the Smit Towing Bracket is the
that the minimum required Grade B shackle             ease of breaking the tow connection, even un-
size is 3 inches, with a 3 1/4-inch pin. The 1        der significant load. This is accomplished by
1/2-inch available plate can be used with a           removing the locking pin and driving the
larger hole, taking care to maintain the mini-        striking bar to port with a sledge, allowing the


                                                                                                       4-15
                                      U.S. Navy Towing Manual




       S lo t W eld s
                        A




                                                                       P in




                        A
                                                               A -A           S ID E V IE W
                 TO P V IE W


                                                           C o n n ecting link,
                                                            as app ropriate
                               S trikin g
                                  B ar
                  Lo ckin g
                    P in


                                                                        F u ll P en e tra tio n W eld s




                                            F R O N T V IE W




                                              CAUTION

                                Chain smaller than about 3 1/4” will
                                require a pear-shaped link or an
                                anchor shackle to connect to the
                                standard Smit bracket. Check di-
                                mensions carefully.



                                 Figure 4-8. Smit Towing Bracket.



4-16
                                      U.S. Navy Towing Manual


main pin to slide out of the pear-shaped link.      4-5.8   Chocks
The design uses no shackle. This style of tow-
                                                    Most tows make the towline connection on
ing attachment, like the vertical free-standing
                                                    deck. Whether using a bridle arrangement or
padeye, is susceptible to tripping loads and is
                                                    a single point connection, the selection of the
dependent upon the fairlead chock.
                                                    point where the towline (or bridle legs)
The standard Smit Bracket design is manu-           crosses the deck edge is critical to protect
factured in two sizes. The larger size will ac-     bo th the tow line and t he towe d sh ip’s
cept the standard end link of a 3-inch chain.       structure. These robust points include
Smaller chains will require a large safety an-      bullnoses, closed chocks, and roller chocks
chor shackle or a pear-shape link. This link        with a generous radius (see Figure 4-9).
may possibly be found aboard the ship outfit-       Planned tows often will involve installation
ted with such a towing bracket.                     of a special fairlead, because the radii of
                                                    chocks and other fittings designed for
                                                    mooring are generally not sufficient for
                  CAUTION                           towing. Emergency tows generally must
                                                    make do with whatever is available,
      The large and small Smit Brackets             remembering that towline chafing and
      are designed to accept the stan-              structural damage to the tow are probable. In
      dard end link of 3-inch and 2-inch
      chains, respectively. They will di-
                                                    this case, the towline component crossing the
      rectly accept the common link of              deck edge will usually be a chain, heavier in
      considerably larger chains. Check             size than otherwise would be required for
      dimensions carefully in designing             strength alone.
      the tow connection.
                                                    4-5.9   Fairleads
The smaller standard size Smit Bracket is de-       Fairleads are used to lead mooring lines
signed to accept the end link of 2-inch chain,      around obstructions and align them properly
or the common link of 2 3/4-inch chain.             with winches or capstans. Fairleads are locat-
Sometimes the Smit Bracket design is adapt-         ed to accommodate lines from both sides of
ed to other dimensions. In all cases, the di-       the ship. Fairleads usually have rollers to re-
mensions must be checked carefully to ensure        duce line wear.
that properly sized jewelry is available to
make the connection.                                4-6 Connecting Hardware (Jewelry)
4-5.7 Towing Hooks
                                                    Connecting hardware or towing jewelry used
Towing hooks rarely are seen in the United          to rig the tow system include a variety of
States, but may be found on foreign tugs, es-       shackles, chain detachable links, special fit-
pecially European tugs. They are heavy steel        tings such as flounder plates, splices and end
hooks mounted on vertical pins that allow           terminations for wire and synthetic line. This
them to swing. Each hook is shock-mounted           hardware is used to connect the various por-
by using a heavy compression spring and fit-        tions of the towline system to each other and
ted with a quick-release device that trips the      to the tow (see Figures 4-10 through 4-14).
hook, much like a chain stopper. The com-           Components of different sizes are connected
pression spring provides a small amount of          by using offset plate shackles and pear-
dynamic load relief for the towline system.         shaped detachable links.



                                                                                              4-17
                                       U.S. Navy Towing Manual




                   P lain Closed C hock                             N avy Standard
              (A m erican M arine S tandard)                        C losed C hock




                    P lain O pen C hock                          P lain O pen C hock
                   W ithout K eeper B ar                          W ith K eeper B ar
              (A m erican M arine S tandard)               (A m erican M arine S tandard)



                                     Figure 4-9. Types of Chocks.


4-6.1   Shackles                                      egories, only four can be used as towline con-
General purpose Navy shackles are described           nectors. These are as follows:
in detail in RR-C-271D, Amendment 1, Fed-                • Type I Anchor Shackles
eral Specification, Chain and Attachments,                 Grade A - Regular
Welded and Weldless (Ref. E) and in (Ref.                  Class 3 - Safety Bolt and Nut
D). There are two types, two grades, and
three classes of shackles. Of these twelve cat-




4-18
                                     U.S. Navy Towing Manual


   • Type I Anchor Shackles                        Although screw-pin shackles are a commonly
     Grade B - High Strength                       used type of marine shackle and afford a
     Class 3 - Safety Bolt and Nut                 quick and simple means of connecting and
                                                   disconnecting, the screw pin shackle should
   • Type II Chain Shackles                        not be used for connections in a towing rig.
     Grade A - Regular                             Due to the cyclic loading associated with
     Class 3 - Safety Bolt and Nut                 towing, it is possible that the pin could back
   • Type II Chain Shackles                        out. Excessive vibration or alternate athwart-
     Grade B - High Strength                       ship movement coupled with the surging of
     Class 3 - Safety Bolt and Nut                 the towline may cause screw pin shackles to
                                                   come undone.
Examples of chain and anchor shackles are
shown in Figure 4-10.                                                CAUTION

                  CAUTION                                Shackles and other fittings fre-
                                                         quently come with cotter keys or
      Special forged shackles, when                      pins. Cotter keys are not used in
      used with chain stoppers and car-                  towing. Replace cotter keys with
      penter stoppers, use carefully ma-                 locking bolts with two jam nuts. The
      chined screw pins and are permis-                  head of the locking bolt and the jam
      sible in towing. Such pins must                    nuts shall be appropiately sized to
      remain accessible for inspection                   ensure the head of the locking
      and service while in use.                          bolts and the Jam nuts are in con-
                                                         tact with the nut of the safety
                                                         shackle.The locking bolt can be
Navy shackles are permanently and legibly                peened over if desired.
marked in raised or indented lettering on the
shackle’s body identifying the manufacturer’s
name, trademark, shackle size, and
recommended Safe Working Load (SWL).
SWL of both Grade A and Grade B Navy                                  CAUTION
safety shackles is suitable for sizing hardware
for lifting purposes. However, SWL cannot                 Never weld on forged steel shack-
be used in towing. Proof loads for a shackle              les. The welding process can
                                                          weaken the shackle.
must be used vice SWL. Recommended
factors of safety listed in Table 3-2 and
Section D-14 describe appropriate methods          Navy shackles are made of forged steel;
for sizing shackles for towing.                    welding to forged steel shackles can reduce
                                                   the strength of the shackle by as much as 30
                                                   percent. Shackles should never be welded on,
                                                   nor should pins be secured by welding. The
                  CAUTION                          nuts on safety shackle pins are secured by a
                                                   small locking bolt, with two jam nuts to se-
      Screw-pin shackles, other than the           cure the pin nut. The locking bolt can be
      special forged shackles for stop-            peened over if desired. This belt shall be ap-
      pers, must never be used for con-
      nections in towing rigs. The pin
                                                   propriately sized to ensure the head of the
      could back out due to the constant           locking bolt and the jam nuts are in contact
      vibration set up by the hydrody-             with the nut of the safety shackle. This belt
      namic actions on the towline.                should not exhibit looseness of play. If
                                                   change out or breaking the shackle connec-
                                                                                                4-19
       U.S. Navy Towing Manual




        Figure 4-10. Shackles.




4-20
                                    U.S. Navy Towing Manual


tion are anticipated during the tow it is good
practice to procure additional properly sized
                                                                        NOTE
locking bolts prior to getting underway. Cot-
ter keys should not be used in towing.                    When inspecting chain, inspect the
                                                          detachable links to determine
4-6.2 Other Connecting Links
                                                          whether they have been properly
It is often difficult to pass a safety shackle            assembled. The key slot must be in
through the opening in a link of chain. Alter-            the proper place and the match
                                                          marks must be identical and
native connecting devices when rigging chain
                                                          matched. This is necessary be-
and wire pendants, bridles, include:                      cause detachable links are hand-
   • Plate shackles (see Figure 4-10)                     fitted to ensure proper assembly
                                                          and full strength. All assembled
   • Detachable links (Navy and Kenter
                                                          links should be visually inspected
     type)                                                and sounded.
   • Detachable anchor connecting links
     (pear-shaped or detachable end link)         The practice of welding detachable links
Plate shackles shown in (Ref. I) (Figure I-16     closed to assure security of the towing rig is
through Figure I-18) are commonly used to         one that continually plagues towing com-
make connections to the flounder plate and        mands. This practice should never be
to connect chain and wire pendants.               permitted. It is much safer and more cost-ef-
                                                  fective to use a hairpin to secure the tapered
Detachable links are similar in shape to chain
                                                  pin in the link. This ensures that the link will
links, but can be disassembled into several
                                                  not come apart and simplifies the eventual
pieces (see Figures D-1 through D-3). This
                                                  disassembly and re-use of the link. Details for
allows the link to be used as a connection be-
                                                  modifying detachable links for use with hair-
tween chain and other components. Pear-
                                                  pins are contained in (Ref. D).
shaped links have one end that is smaller than
the other; they are used to attach components     Detachable links should not be used in in-
of different sizes.                               stances where they might be subjected to
                                                  bending or twisting.
                  CAUTION
                                                  4-6.3    Wire Rope Terminations
      Never weld detachable links. The
      welding process can weaken the              Three types of wire rope terminations are
      links.                                      normally used in Navy towing applications:
                                                  swaged, spliced, and socketed (see Figure
                                                  4-11)
                                                  The wire rope swaging process attaches fit-
                                                  tings to wire rope by means of cold plastic
                                                  flow of metal under extremely high pressures.
                                                  The process uses hydraulic presses in con-




                                                                                               4-21
                                     U.S. Navy Towing Manual




                           Figure 4-11. Types of Wire Rope Terminations.


junction with suitable dies. The swaged fit-        joys continued popularity because of field re-
tings are usually made of special alloy steels.     pair capability.
An advantage of this process is low cost and
high efficiency.                                    A subset of a wire splice is the use of wire
                                                    clips. This is the preferred over the hand-
Swaged eyes are more common than spliced            splice because it can withstand 80 percent of
eyes. Existing swaging technology is so high-       the wire’s breaking strength if completed
ly advanced that virtually all types of wire        properly. Both the hand-splice and the wire
rope terminations can be made. Properly             clip termination have less strength than the
made swaged eyes develop 85 percent of the          breaking strength of the wire and should be
strength of the wire. Swaged terminations are       used only in an emergency (such as damage
                                                    to or loss of the normal end fitting). See Table
applied only to wire rope with wire rope
                                                    4-1 and Naval Ship’s Technical Manual
cores. A fiber rope core wire can be swaged
                                                    (NSTM) S9086-UU-STM-010, Chapter 613,
by replacing the fiber core at the termination      Wire and Fiber Rope and Rigging (Ref. F) for
with a strand of wire.                              the proper placement and number of wire
                                                    clips.
The second type of wire rope termination, the
hand-spliced eye, has less strength than the        The third type of termination, the poured zinc
breaking strength of the wire. For instance, 1      or Spelter socket, is very common and is pre-
5/8-inch to 2-inch hand-spliced eyes have 75        pared in accordance with NSTM, Chapter
percent of the breaking strength of the wire,       613 (Ref. F). This termination will withstand
while 2 1/4-inch and larger wires have an ef-       100% of the rope’s breaking strength if pre-
ficiency of 70 percent. (See Table B-3 for          pared properly. The end of the rope is seized
more details.) Nonetheless, hand-splicing en-       and the strands are unlayed all the way to the


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                                         U.S. Navy Towing Manual




                                        Lock W ith M achine Bolt
                                      Secured W ith Two Jam N uts
                                           (Not Cotter Pins)




                           Open Socket                              Closed Socket




                                     Figure 4-12. Towline Termination.


individual wires. This broomed end is insert-            connect the standard hawser termination to a
ed into the socket and secured in place with             wide range of chain sizes.
the poured zinc. Epoxy-type poured sockets               4-6.4      Synthetic Line Terminations
are not suitable for towing purposes.
                                                         In general, the same methods are used for
                                                         splicing synthetic lines as for natural fiber
                                                         line. When splicing a synthetic fiber line,
                    CAUTION                              however, exercise care to maintain the strand-
                                                         ed form. If this is not done, the strand will
       Whenever a poured socket is in-                   collapse and form a bundle of tangled yarns.
       stalled on a wire rope, the condition
                                                         Also, since the felting action (tendency to mat
       of the lubricant in the portion of the
       rope near the socket should be                    together) of synthetic fiber is considerably
       checked and new lubricant applied                 less than that of natural fibers, more tucks are
       to dry areas.                                     needed to produce a safe splice. This is gener-
                                                         ally true for lines of plaited construction. For
                                                         guidance in splicing single or double braided
Sockets are of two types, open and closed
                                                         lines, consult the manufacturer’s recommen-
(see Figure B-8). The open socket is fitted
                                                         dation or contact NAVSEA 00C.
with a locking bolt and secured by a locking
bolt with two jam nuts. Frequently used. on              The traditional standard end fitting for manila
towing hawsers, the closed socket forms an               was a tear drop wire rope thimble. With the
eye with a solid bail (see Figure 4-12). Figure          advent of high-strength synthetics, however,
4-13 demonstrates using a safety shackle and             the eye of the line could stretch sufficiently to
three standard pear-shaped detachables to                allow the thimble to capsize out of the eye. In




                                                                                                     4-23
                U.S. Navy Towing Manual




                             NOTE

              Three different pear-shaped de-
              tachable links will satisfy all normal
              chain connection requirements.




       Figure 4-13. Pear-Shaped Detachable Links.




4-24
                                       U.S. Navy Towing Manual


addition, the higher strength of the synthetic       as strong as twice the original breaking
line caused thimbles to crush and fail. To re-       strength. For this reason, the line used for the
solve these problems, a variety of solid thim-       grommet must have a basic breaking strength
bles have been developed and have become             equal to at least 5/9 of a single line spring in
the standard end fittings used on synthetic          order to have the same total strength when
line. Figure 4-14 shows the approved Navy            fabricated into a grommet.
standard thimbles.
                                                     An alternative to the grommet arrangement
4-6.5 Synthetic Spring                               is the synthetic spring consisting of a length
A spring is a line made of material exhibiting       of line with a standard eye splice in each end.
elastic behavior. In towing, a spring absorbs        Each of these eyes will normally employ a
shocks due to dynamic loading of the towing          thimble. Since the spring is not doubled, the
system; this is one reason that the ocean tow-       line diameter must be greater than that used
                                                     in the grommet, but should be easier to han-
ing industry first became interested in nylon
and other synthetic fiber lines. Nylon re-           dle.
placed manila in hawsers and spring pendants         At present, there is no agreement on the
because of its superior elasticity and because       method to calculate the proper length of a
it is smaller, lighter, and easier to handle than    synthetic towing spring. Commercial opera-
manila of similar strength. Polyester has re-        tors generally use a spring of 200 to 400 feet
placed nylon (see Section 4-3.2) in most syn-        in length. For additional guidance on sizing a
thetic line towing applications. (Ref. C) con-       synthetic spring, contact NAVSEA 00C.
tains more information on synthetic springs
and specifications for lines made of polyester       For a 2-inch IPS fiber core hawser towing on
fiber that are approved for use as tow hawsers       the brake a grommet made from 10-inch
and springs.                                         circumference double-braided polyester
                                                     would be required. This would be determined
A synthetic spring is sometimes inserted be-         by applying the required factor of safety for
tween the towing pendant and the tug’s haw-          wire from table 3-2 which is 4, when used
ser for dynamic load mitigation. Seen most           with synthetic spring. Therefore the
frequently in commercial towing, the spring          maximum steady working load is 288,000 ÷ 4
usually is a length of synthetic fiber rope,         or 72,000 pounds. For a synthetic spring
spliced together, arranged into a grommet            (polyester), the factor of safety is 6. Thus a
(see Figure 4-15).                                   single polyester spring, capable of handling
A grommet is fabricated by splicing a line to        72,000 x 6 or 432,000 pounds is required.
form one continuous loop. The two sides of           Use in a grommet configuration will require a
the loop are pulled together around two thim-        strength of 432,000 x 5/9 or 240,000 pounds.
bles, and seized with small stuff to form the        So, a 10-inch circumference double-braid
grommet or strap shown in Figure 4-15. The           polyester line, with a specified breaking
line used to make the grommet must be sized          strength of 277,000 pounds, will be required
so the assembled grommet will have a total           for this grommet. The grommet’s weight per
safe working load that is equal to or greater        foot in air is approximately equal to that of
                                                     the wire (6.74 vs. 6.72 lbs/ft), however, the
than the design load for the towing system.
                                                     grommet will be far more bulky
Although the line is doubled in the grommet,
                                                     4-6.6   Bridles
its strength is not twice that of a single line.
There are losses in strength in the splices, so      If the tow has a configurational, operational,
that the assembled grommet is only 0.9 times         or directional stability problem that makes a


                                                                                                4-25
                            U.S. Navy Towing Manual




       Thim ble w ith End Link                         Closed Thim ble


               Lock W ith M achine Bolt Secured
               W ith Tw o Jam Nuts (Not Cotter
               Pins)




        Synthetic Rope Thim ble                                 Nylite Thim ble

                    Figure 4-14. Synthetic Line End Fittings.




4-26
                                      U.S. Navy Towing Manual




                                Figure 4-15. Synthetic Line Grommet.




                                      Figure 4-16. Towing Rigs.

single pendant inadequate, a bridle should be        for larger ships, where the 2 1/4-inch beach
rigged (see Figure 4-16 and Figure 4-18).            gear chain carried by Navy towing/salvage
Barges with square bows are rigged with              ships is appropriately sized for the power and
bridles because of the stabilizing effect            hawser size of these tugs.
produced by pulling from both legs of the
                                                     The flounder plate, or fish plate, is a
bridle. Some barges have a hull form and/or
                                                     component of a typical towing bridle. A
appendages that increase the directional
                                                     flounder plate is designed to distribute the
stability of the barge; these barges may be
                                                     towing force of a tug’s hawser to the separate
rigged with a pendant, rather than a bridle,
                                                     legs of a bridle. The deployment of flounder
attached on centerline. Chain is the preferred
                                                     plates on typical towing rigs is described in
material for bridles in deep ocean towing and
                                                     detail in (Ref. I). Flounder plate design is
often complements or substitutes for the wire
                                                     detailed in Figures I-15.
pendant. Chain’s advantage over wire comes
from its greater weight per foot, which              For service craft up to 500 tons, the bridle
deepens the catenary, and from its superior          must be equal in size to the ship’s anchor
resistance to chafing. As a rule of thumb, the       chain, but not less than 1 1/4-inch. For craft
size of the chain to use for bridles and             greater than 500 tons, a minimum of 1 5/8-
pendants should be at least equal to the size of     inch chain shall be used. Ships do not need
chain used to anchor the tow. An exception is        chain larger than 2 1/4 inches when towed by


                                                                                              4-27
                                                U.S. Navy Towing Manual




 B rid le Le g                              R e trievin g W ire                                    R e trievin g W ire


                                             B rid le Le g        B rid le Le g                         B rid le Le g




                                                                                                                   P late
                                                    F lo un de r P la te                                           S ha ckle
   S afe ty
   S ha ckle




       D e ta ch ab le Link




                           P en da n t                                              P en da n t
                               or                                                       or
                         L ea d C h ain                                           L ea d C h ain




                                                             NOTE

                                          See (Ref. A) for additional examples
                                          of bridle sizes of components.




                   Figure 4-17. Chain Bridles Using Plate Shackles and Safety Shackles.




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                                      U.S. Navy Towing Manual


U.S. Navy tugs. More powerful commercial            to a breaking strength well in excess of the
tugs will require larger chain bridles. Non-        other components.
magnetic chain and attaching hardware shall
                                                    On some ships with large bows (e.g., CV, AD,
not be used for towing bridles. The length of
                                                    AOR, or AFS) it may be necessary to rig a
each leg of the bridle from the towing
                                                    one- or two-shot chain pendant between the
attachment point to the flounder plate after        bridle flounder plate and the towing hawser.
rigging is completed must be equal to or            Both bridle legs should be the same size and
greater than the horizontal distance between        length and should be checked by counting the
the attachment points. The bridle apex angle,       links when rigging is complete. All detachable
defined as the angle between the two bridle         links in the bridle legs and chain pendant must
legs as measured from the flounder plate            be locked with a hairpin (see Section 4-6.2).
vertex, shall be less than 100 degrees, with an
optimal angle between 30 and 60 degrees (see        Because chain and wire bridles and pendants
Figures I-1, I-2, I-3 for an illustration).         are often subjected to wear and abrasion dur-
                                                    ing towing, it is recommended practice to
All towing bridles, when rigged correctly,          “over design” to allow for wear, particularly
must have a backup securing system. This is         for long tows. Tables in (Ref. B) and (Ref. D)
normally accomplished by using wire rope of         provide the breaking strength and weight per
appropriate size (able to lace through chain        foot of various types of wire and chain. These
links) and taking sufficient bights of wire from    tables can be used together with the calculat-
a second securing point (bitts, heavy cleats,       ed towline tensions and factors of safety ob-
etc.) and lacing the wire rope through the after    tained from Table 3-2 to determine whether
end of links in the chain bridle (no less than      the available selected wire or chain is suffi-
four bights). Size and number of bights of wire     ciently strong.
should equal the strength of the chain used in      Sometimes a heavy wire is used as a bridle
the bridle. If a towing pad is used to connect      for short tows or emergency situations, but
the bridle to the tow, the backup wires must be     special care is required to minimize chafing
laced through the portion of the chain that is      of the wire and damage to the structure from
forward of the towing pads. The securing point      the wire’s extremely hard material. If the hard
should be aft of the towing pad to prevent          point is a considerable distance from the fair-
snap-loading. If a set of mooring bitts is used     lead, a fairly short length of chain, sufficient
as a securing point for the bridle on the tow,      to ride in the fairlead, may be used to save
the wire should be laced thorough the chain         weight and sometimes to simplify the final
links that remain astern of the bitts after the     connection to the tow, such as when using
three or more “figure eights” are secured on        bitts as the hard point.
the bitts. There must be a sufficient number of
                                                    4-6.7   Pendants
wire clips (see Table 4-1) on each bitter end of
the backup wire, aligned in the same direction      A pendant is often used between the tow and
(See (Ref. I) and (Ref. J) for tow rig design       towing hawser to facilitate the rigging problem
plans.)                                             of connecting heavy components. This is
                                                    called a “lead” or “reaching” pendant. The
It may not always be possible or practical to       lead pendant usually is wire rope and should
rig a backup system (i.e., submarine towing).       have the same breaking strength as the main
In these cases, additional analysis of the main     hawser unless it is intended to be the safety
towing attachment may reduce the risk. Where        link, in which case it will be of lower strength
possible, the attachment should be designed         (see Section 4-7 for information on the “safety


                                                                                               4-29
                                      U.S. Navy Towing Manual


link” concept). The pendant may be up to 300        be exposed to towing loads. Examples of re-
feet long to permit connection/disconnection        trieval pendant rigging are shown in Figures
on board the tug, while maintaining a safe          4-16 and 4-18 and throughout (Ref. I).
standoff under heavy weather conditions. Of-
                                                    To size a bridle retrieval pendant, use a 4:1
ten, the arrangement of the tension member
                                                    safety factor for lifting bridle weight but no
that extends outboard of the bow of the tow is
                                                    less than 5/8-inch wire rope.
such that chafing could occur. If a wire rope
pendant is used in this case, it must be care-      4-6.9   Chain Stoppers, Carpenter Stop-
fully protected from chafing, or a portion of it            pers, and Pelican Hooks
must be replaced with chain to provide chafing      The term “stopper,” as used in seamanship,
protection.                                         describes a device or rigging arrangement
                                                    that is used to temporarily hold a part of run-
Chain pendants frequently are employed
                                                    ning rigging or ground tackle that may come
when using a single-leg attachment between
                                                    under tension. There are many types of stop-
the hawser and tow. This attachment general-
                                                    pers and methods of attaching them to the
ly runs through a centerline bullnose, chock,
                                                    tension members. Most stoppers cannot be re-
or fairlead near the tow’s centerline. A chain
                                                    leased under load and require the held line to
extending forward from the apex of a towing
                                                    be heaved in to slack the stopper and allow its
bridle is also called a lead chain. The purpose
                                                    removal. Some stoppers, however, such as
of the lead chain is to add weight to the end of
                                                    the pelican hook and carpenter stopper, can
the towline system. This improves the spring
                                                    be released when under load.
in the system by increasing the towline’s cat-
enary. Sometimes the chafing/lead chain is          In towing applications, the stopper is usually
the tow’s anchor chain, which can be veered         connected to the deck pad by means of chain
to the desired total length. During emergency       shackles. It is used to hold a towing pendant
tows of merchant ships, the tow’s anchor            on deck during the hookup and breaking of a
chain is frequently used as a tow pendant.          tow. A chain stopper is sometimes employed
                                                    as a quick-release device (see Figure 4-18).
A wire lead or reaching pendant is frequently       Stoppers are nominally rated to hold a mini-
used with a chain pendant to simplify con-          mum of 60 percent of the breaking strength of
necting up the tow.                                 the chain or wire for which they have been
4-6.8   Retrieval Pendant                           designed. This must be considered in their
                                                    use. It is important not to confuse chain stop-
A retrieval pendant is a wire rope leading          pers with pelican hooks. Pelican hooks are
from the deck of the tow to the end of the          significantly weaker than chain stoppers of
towing pendant or flounder plate. The retriev-      the same nominal size and are unable to grasp
al pendant facilitates bringing the tow gear        the chain in the desired manner.
back onto the foredeck of the tow so it will
not drag the seafloor or foul the ship’s ap-        Pelican hooks can be used to grasp chain
pendages when the tow is disconnected. The          through a long link attached to the bitter end
retrieval pendant often is handled on the deck      of a chain. They cannot grasp a chain in the
of the tow by a hand-powered winch or a             middle like a chain stopper can (see Figure
deck capstan. It must be capable of being           4-18). They can be used as quick release de-
handled by the riding crew or by a boarding         vices, although they do not have the holding
party put aboard the tow. The wire must be          strength of chain stoppers.
strong enough to lift the flounder plate, bri-      Carpenter stoppers are used when it is neces-
dle, and/or pendant, but it is not intended to      sary to develop a grip on a wire rope and hold


4-30
                                          U.S. Navy Towing Manual



                                           Table 4-1. U-Bolt Clips.




Recommended Method of Applying U-Bolt Clips to Get Maximum Holding Power of the Clip. The following
is based on the use of proper size U-Bolt clips on new rope.
1.   Refer to the Table 4-1 (part 2) in following these instructions. Turn back specified amount of rope from
     thimble or loop. Apply first clip one base width from dead end of rope. Apply U-Bolt over dead end of
     wire rope with live end resting in saddle. Tighten nuts evenly, alternating form one nut to the other until
     reaching the recommended torque.
2.   When two clips are required, apply the second clip as near the loop or thimble as possible. Tighten nuts
     evenly, alternating until reaching the recommended torque. When more than two clips are required, apply
     the second clip as near the loop or thimble as possible, turn nuts on second clip firmly, but do not tighten.
     Proceed to Step 3.
3.   When three or more clips are required, space additional clips equally between first two - take up rope
     slack - tighten nuts on each U-Bolt evenly, alternating form one nut to the other until reaching recom-
     mended torque.
4.   Prior to use, apply a load to test the assembly. This load should be of equal or greater weight than loads
     expected in use. Next, check and retighten nuts to recommended torque.
In accordance with good rigging and maintenance practices, the wire rope and termination should be inspected
periodically for wear, abuse, and general adequacy.
Inspect periodically and retighten to recommended torque.
A termination made in accordance with the above instructions, and using the number of clips shown in part 2
of this table, has an approximate 80% efficiency rating. This rating is based upon the nominal strength of wire
rope. If a pulley is used in place of a thimble for turning back the rope, add one additional clip.
The number of clips shown in part 2 of this table is based upon using right regular or lang lay wire rope, 6 x 19
classification or 6 x 37 classification, fiber core or IWRC, IPS or EIPS. If Seale construction or similar large
outer wire type construction in the 6 x 19 classification is to be used for sizes 1 inch and larger, add one addi-
tional clip.




                                                                                                                4-31
                                         U.S. Navy Towing Manual



                                    Table 4-2. Applying U-Bolt Clips.




         Clip Size             Minimum                Amount of                  Torque            Weight
                               Number of               Rope to                   Ft./Lbs.         (Lbs. per
                                 Clips                Turn Back                                     100)
                                                       (Inches)

       1/8                    2                      3 1/4                  4.5                 6
       3/16                   2                      3 3/4                  7.5                 10
       1/4                    2                      4 3/4                  15                  20
       5/16                   2                      5 1/4                  30                  30

       3/8                    2                      6 1/2                  45                  47
       7/16                   2                      7                      65                  76
       1/2                    3                      11 1/2                 65                  80
       9/16                   3                      12                     95                  104

       5/8                    3                      12                     95                  106
       3/4                    4                      18                     130                 150
       7/8                    4                      19                     225                 212
       1                      5                      26                     225                 260

       1 1/8                  6                      34                     225                 290
       1 1/4                  7                      44                     360                 430
       1 3/8                  7                      44                     360                 460
       1 1/2                  8                      54                     360                 540

       1 5/8                  8                      58                     430                 700
       1 3/4                  8                      61                     590                 925
       2                      8                      71                     750                 1300
       2 1/4                  8                      73                     750                 1600

       2 1/2                  9                      84                     750                 1900
       2 3/4                  10                     100                    750                 2300
       3                      10                     106                    1200                3100
       3 1/2                  12                     149                    1200                4000

       If a pulley (sheave) is used for turning back the wire rope, add one additional clip.
       If a greater number of clips are used than shown in the table, the amount of turnback should be increased
       proportionally.
       The tightening torque values shown are based upon the threads being clean, dry, and free of lubrica-
       tion.
       Above values do not apply to plastic coated wire rope.




4-32
                                                 U.S. Navy Towing Manual



it to the breaking strength of the wire (see Fig-              worming, parcelling, roundings, and serving
ure 4-19). Advantages of the carpenter stopper                 (see Figure 4-20). Material specifically man-
include its quick application and release, abili-              ufactured for chafing gear is also available
ty to develop full tension without damage to                   and works very well. These materials lessen
the wire, and low maintenance requirements.                    or prevent towline chafing and are applied at
                                                               the point where the towline crosses the stern
                     WARNING                                   rail or other structure.
      Old-style carpenter stoppers                             Another method to control chafing is to peri-
      with smooth covers are con-                              odically adjust the scope of the wire to reduce
      demned and should not be used.
      These old models are made of
                                                               the wear on any one point. The amount of
      cast metal and are subject to ex-                        time between adjustments will depend on the
      plosive brittle fracture upon im-                        behavior of the tow and the sea state. This is
      pact. Serious injury to personnel                        called “nipping” the wire or “freshening the
      may result from flying frag-                             nip.”
      ments.

                                                               4-7 Fuse or Safety Link Concept

                       CAUTION
                                                               A safety link, sometimes called a fuse pen-
                                                               dant, is the point or component in every tow-
       A carpenter stopper should not be                       ing rig most likely to fail at a predicted load.
       used unless it is specially designed                    A safety link is analogous to a safety valve or
       f o r t h e l a y, h e l i x , n u m b e r o f
                                                               a circuit breaker. The safety link’s primary
       strands, and diameter of the specif-
       ic wire rope. The stopper and the                       characteristic is its predictability; it ensures a
       wire should both be clean and free                      known location and mode of towing system
       from sand or other abrasives.                           failure in event of an overload. The safety
                                                               link should not fail under the anticipated ten-
Three types of carpenter stoppers have been                    sions of a planned tow. Tow preparing activi-
used in the U.S. Navy:                                         ties should identify the safety link of the sys-
    • The “old WWII” style                                     tem and provide that information to the
    • The “improved 1948” style                                officer responsible for the tow so that design
    • The “modified-improved 1968” style                       limits are not exceeded.

Only the last style listed is approved. It can be              Since every rig will have a weakest point, it is
identified by four heavy ribs on the hinged                    often prudent to intentionally incorporate a
cover and will have a Boston Naval Shipyard                    safety link to protect a critical portion of the
test date of 1968 to 1973 or be manufactured                   tow system, usually the hawser, from a possi-
by Baldt after 1973.                                           ble overload. A wire rope pendant is usually
                                                               selected as the safety link, and is sized to
Refer to (Ref. E) for more information on the                  have a 10 to 15 percent lower breaking
use of stoppers.                                               strength than the main tow hawser. If a tow-
4-6.10 Chafing Gear                                            ing system overload occurs, the failure will
                                                               not damage the tow hawser and it can be re-
Chafing gear is usually used to reduce wear
                                                               connected.
on both the hawser and the tug’s structure.
Chafing gear includes materials such as mats,                  The breaking strength considers the hydrody-
battens, strips of leather, canvas, grease,                    namic resistance of the towline, which creates


                                                                                                            4-33
                U.S. Navy Towing Manual




                Detachable
                Link




                  P elican H o ok




                   C h ain S top per




                             WARNING

                 Do not confuse pelican
                 hook with chain stopper.




       Figure 4-18. Pelican Hook and Chain Stopper.




4-34
   U.S. Navy Towing Manual




Figure 4-19. Carpenter Stopper.




                                  4-35
                                           U.S. Navy Towing Manual


a higher tension at the tug end of the hawser.           stalled in several configurations. They can be
Such pendants should never be subjected to               fabricated from pipe or plate. On newer tugs,
chafing or other unusual service.                        they have large radius surfaces contoured to
                                                         the tug’s deck layout. It is important to keep
                                                         the caprail smooth and free of nicks and burrs
                     CAUTION                             that damage both synthetic and wire hawsers.
                                                         In current design practice, the bearing surface
        Since the safely link is the weakest
                                                         of the caprail is hardened to a minimum
        point in the tow system, this will de-
        termine the safe working load. Tow               Rockwell C hardness of 40 to 50.
        planners must ensure the safety
        link is capable of withstanding all              4-8.2   Towing Bows
        expected loads.
                                                         Towing bows are transversely installed
                                                         beams or pipe that bridge the caprails on the
4-8 Line Handling Devices                                afterdeck of the tug (see Figure 4-22). Their
                                                         function is to keep the towline clear of all
                                                         deck fittings and to furnish a protected area
                     WARNING
                                                         below the sweeping tow hawser where per-
        Motions of the tug and tow can                   sonnel can pass safely.
        cause the towline to change po-
        sitions rapidly and without warn-                4-8.3   Horizontal Stern Rollers
        ing. Personnel must be aware of
        the potential danger of a sweep-                 Horizontal stern rollers minimize chafing
        ing towline and remain clear of                  during heave-in and payout (see Figure 4-20).
        all areas that may be within this                A stern roller is a large-diameter roller, set in
        sweep.                                           the stern bulwarks on the centerline and
                                                         faired to the caprail. The roller rotates with
                                                         the movement of the wire, constantly chang-
Towing requires extensive manipulation of
                                                         ing the contact point. This movement spreads
line. Virtually all of the line used in towing
                                                         the wear from the wire. Because it is also
operations is far too heavy to be handled by
                                                         hardened, it resists scoring and thus provides
anything other than machines and unique de-
                                                         a smooth surface on which the wire rides. The
vices that have evolved in towing practice.
                                                         ARS 50 Class is not equipped with horizontal
The following sections detail the function of
                                                         stern rollers. Instead they have a large-radius,
line handling devices used in towing.
                                                         hardened steel transom that minimizes wear
4-8.1    Caprails                                        on the hawser.

                     CAUTION                             Chafing gear should be used even if stern
                                                         rollers are available. When towing with a
        Whenever the surface of a caprail                constant towing scope, chafing comes from
        becomes rough, steps should be                   port/starboard movement of the wire. Hori-
        taken to repair or replace it to pro-
        tect the hawser. Caprails should be
                                                         zontal stern rollers do not reduce chafing in
        kept free of any nicks or burrs.                 this manner.
                                                         4-8.4   Capstans and Gypsy Heads
The caprail is the riding surface on top of the
bulwark (see Figure 4-21). The tow hawser                Capstans rotate on vertical shafts and are used
bears on the stern caprail as it passes astern of        for line handling, but not as towing machines
the tug and enters the water. Caprails are in-           (see Figure 4-23). The prime mover of a cap-

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                                  U.S. Navy Towing Manual




           H a w se r                        Vertica l
                           C h afing         R o llers
                            G e ar


                                                                 C a pra il

H o rizo n ta l
   S tern
  R o ller




                                       B acksta ys




                        Figure 4-20. Chafing Gear and Stern Rollers.




                                                                              4-37
                                   U.S. Navy Towing Manual




            Large-Radius Caprail                               Half-Pipe
                (Hardened)                                      Caprail



                                               Deck




                                     Figure 4-21. Caprails.




       Tow ing Bow s




                                   Figure 4-22. Towing Bows.



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                                       U.S. Navy Towing Manual


stan is often located below deck. This permits       ers drop when the side force at mid-barrel
the capstan to be mounted so that the line           height exceeds 50,000 pounds.
travels relatively close to deck level. A gypsy
head, which is similar to a capstan, rotates on
a horizontal shaft and is usually powered as                           CAUTION

an auxiliary of a winch. Gypsy heads, like                Using vertical rollers may put the
capstans, are used for line handling, but not             tug “in irons,” seriously limiting the
                                                          tug’s maneuverability.
for towing.

4-9 Sweep Limiting Devices                           The presence of the towline in the stern roll-
                                                     ers limits the maneuverability of the tug be-
Sweep limiting devices restrict the horizontal       cause it moves the tow point from the H-bitts
                                                     back to the caprail.
sweeping of the wire across the fantail.

                                                                          NOTE

                                                           Stern rollers should be properly
                                                           maintained and lubricated to en-
                  WARNING                                  sure rotation and smooth surface
                                                           conditions. Rollers can become fro-
      The vertical stern rollers and                       zen and their surface areas
      Norman pins onboard the ARS                          grooved and scored from towline
      50 Class ships will drop when a                      wear. Such conditions directly con-
      load of 50,000 pounds or more is                     tribute to the abnormal wear of the
      applied to mid-barrel height. The                    towline.
      resulting uncontrolled sweeping
      of the towline may injure person-
      nel or damage equipment.                       Vertical stern rollers act as a fairlead for the
                                                     towing machine. The long distance between
                                                     the stern rollers and the towing machine en-
4-9.1 Vertical Stern Rollers                         ables the tow hawser to naturally reel itself on
                                                     to the drum and the level wind performs only
Vertical stern rollers tend the towline during       light duty. The stern rollers are normally used
heave-in and payout, and during long-dis-            to capture the hawser and to assist when pick-
tance straight towing by preventing the wire         ing up or disconnecting a tow. The vertical
from sweeping across the deck and rail. On           rollers may limit the amount of lateral move-
newer ships, the stern rollers or pins are nor-      ment that the tow hawser receives as the tow
                                                     yaws from port to starboard.
mally operated hydraulically from a remote
location (see Figures 4-24 and 4-25). On-            Vertical stern rollers are designed only as a
board the T-ATFs, the hook-shaped items on           fairlead device and cannot structurally with-
either side (just outboard of each vertical roll-    stand loads of the magnitude of which the H-
                                                     bitts is capable. Strong side loads commonly
er) are hydraulically operated “capture
                                                     seen in towing situations could very easily
hooks,” often used instead of the vertical roll-     carry the assembly away. On the ARS Class,
ers to provide lateral restraint for the towline.    the rollers will fold down to their stowed po-
On the ARS 50 Class, the vertical stern roll-        sition at the lateral load of 50,000 pounds ap-


                                                                                                   4-39
                                 U.S. Navy Towing Manual




                                                             W inch

                                                                       G ypsy Head




            Capstan
       (W ith M achinery
         Below Deck)




                                                                  Capstan
                                                             (W ith M achinery
                                                               Above Deck)




                           Figure 4-23. Capstans and Gypsy Head.




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               U.S. Navy Towing Manual




                         W ire
                     Capture Hook




 Nylon Line
Capture Hook




         Figure 4-24. Stern of T-ATF 166 Class.




                                                  4-41
                                        U.S. Navy Towing Manual




                                                      Vertical
                                                     Hydraulic
                           Low ered                 Pins Raised
                          Position of
                           Vertical
                          Hydraulic
                             Pin

                                                                              Davit
                                                                             Sockets




                              Figure 4-25. Stern Rollers (ARS 50 Class).


plied at mid-roller height. The towline is usu-       4-9.2   Norman Pins
ally restrained in a stern roller assembly only       The primary function of Norman pins is to
under light sea conditions. The vertical stern        limit the arc of sweep across the stern (see
rollers should always be dropped when ma-             Figures 4-26 and 4-27). Norman pins also
neuvering in restricted waters or rough seas.         help keep the hawser out of the propellers
                                                      during slack wire conditions. Ocean tugs gen-
When the towline rides against a vertical             erally are provided with sockets along their
stern roller, it is being bent over a small radi-     aft bulwarks into which Norman pins are fit-
us. This causes towline fatigue and possible          ted. Some tugs have two sets of Norman pins,
failure at a lower load. Chafing gear is re-          with one set that may be inserted into the
                                                      stern caprail.
quired on the towline when it is scheduled for
long periods in the stern roller. Slacking off a      Retractable or movable Norman pins have
few inches, or “freshening the nip” regularly,        various designs, ranging from simple, remov-
                                                      able round stock or pipe to remote controlled,
is a good practice to reduce wear on the wire.
                                                      hydraulically operated devices. On older
Wire grease is often used to reduce chafing at        ships, the round pins could be removed from
these hard points. This is especially true when       any socket and moved to another location.
using a capture hook (as on a T-ATF) because          This necessitated personnel moving about on
there is little room for chafing gear.                the fantail and thus being subject to hazards;


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           U.S. Navy Towing Manual




                              Typical Norm an Pin




                       Rem otely O perated Norm an Pin




Forw ard                Aft




           Figure 4-26. Norman Pins.




                                                         4-43
                                          U.S. Navy Towing Manual


with remote control, the procedures are now             Because the hogging strap transfers the tow
safer. Newer tugs and salvage ships, such as            point aft from the H-bitt, it can cause reduced
the ARS 50 class, have remote controlled, hy-           maneuverability.
draulically operated Norman pins in fixed lo-           4-9.4    Lateral Control Wire
cations. On board the ARS 50 class, the Nor-
                                                        A lateral control wire is similar in
man pins are set to drop when the lateral force
                                                        configuration to the hogging strap, but it has
at mid-barrel height exceeds 50,000 pounds.
                                                        the added feature of variable scope. Instead of
The hazard potential is formidable. When the            a fixed-length strap holding the towline to the
pins start inclining toward the horizontal, the         deck, a snatch block is secured to the deck
wire (with 25 tons force propelling it) can             and the lateral control wire is led through it to
jump the pin and sweep forward.                         a deck winch, lateral control winch, or
                                                        capstan. In this manner, the line can be fully
Current design practice requires that the wire          slacked to let the towline sweep free or can be
bearing surface of the Norman pins be hard-             taken in to give either partial or full snugging
ened to a minimum Rockwell C hardness of                like a hogging strap. The lateral control wire
40 to 50.                                               is helpful in keeping the towline out of the
                                                        propellers during slack wire conditions.
4-9.3    Hogging Strap
                                                        A dedicated lateral control winch, limited to
                     CAUTION
                                                        approximately 2,000 pounds straight line
                                                        pull, is available on the ARS 50 Class ships.
        A hogging strap may be necessary
        to prevent the towline from jumping             Like the hogging strap, the lateral control
        the stern rollers when towing a                 wire moves the tow point aft and can limit
        high-bowed ship at short stay. A                maneuverability.
        hogging strap may be subject to
        excessive vertical loads. Care                  4-10 Cutting Gear
        should be taken not to part the
        strap. Failure of a hogging strap
        may result in the loss of tug control           Most Naval ships are equipped with oxy-acet-
        or ranging up by the tow.                       ylene cutting equipment. Additionally, some
                                                        tugs and most salvage ships are equipped
                                                        with hydraulic cutters.
The hogging strap limits the relative move-
ment between the towline and the stern in
both vertical and horizontal planes (see Fig-                                WARNING
ure 4-28). Movement in the vertical plane is
caused by the stern of the tug dropping faster                  Wire rope stretches far less un-
                                                                der load then most natural and
than the towline or by a tow ranging up. A                      synthetic fiber lines. If it fails un-
hogging strap can be attached to the towline                    der high loads, wire rope has a
with a shackle or a special saddle-like fitting.                smaller zone of danger to by-
The limitation of the shackle is the high con-                  standers if loose ends “snap
                                                                back.” The elongation under
centration of load it imposes on the hawser to                  load is sufficient, nonetheless,
which it is attached. Saddle-like fittings are                  to be dangerous. The recoil can
preferred because they have larger radii; this                  be extremely violent and all per-
increases the area of contact and distributes                   sonnel should stay well away
                                                                from any potential recoil path.
the load over a wider arc.


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                                     U.S. Navy Towing Manual




                                        To w S hould N ever B e
                                       A llo w ed to P ass A beam




                                                                                H -B itts
        M axim um Lim it
 of Haw ser An gle w ith    45                                                              P ivot P o int
  A ft N orm an P ins U p




       S tern
      R ollers                    Free S w eep A rea of H aw ser




                                                 N orm an P ins




                                  Figure 4-27. Norman Pin Use.


For cutting chain, the oxyacetylene or exo-            the safety factor. Securing the cutting torch to
thermic cutting equipment is most suitable;            a boat hook is a good practice. Seizing a wire
hydraulic cable cutters may be better for cut-         hawser on both sides of the intended cut is al-
ting wire. Personnel safety is paramount               so a prudent measure. Cutting a synthetic line
when cutting any member of the tow assem-
                                                       with an axe is hazardous and should not be
bly; therefore, every effort should be made to
reduce the tension. This is particularly true          done under tension. The use of stoppers to
when cutting wire and synthetic lines. The             control snap-back decreases the hazards in-
greater the distance between the person doing          volved when cutting any chain, wire or syn-
the cutting and the cutting point, the greater         thetic line.




                                                                                                     4-45
                                      U.S. Navy Towing Manual




                     S hackle o r S add le-Type Fitting
                                                                 To w Haw ser



                                                                                         R estrains
                                                                                         tow haw ser
                                                                                         up and dow n.

                   Yaw
              P osition




         Tie-dow n point needs to be w ell                    Lim ited Lateral S w eep
         aft on th e fantail to b e effective
         in restrainin g the haw ser.
                                                                            To w Haw ser




                                                                             C ap rail




       S afety S hackles




                                          H ogg ing/G rom m et S trap




        Figure 4-28. Rigging of Hogging Strap on Ships without Horizontal Stern Rollers.




4-46
                                        U.S. Navy Towing Manual


                                                      analysis for towing operations, avoid the fol-
                 Chapter 5                            lowing situations when possible:

        TOW PLANNING AND                                 • Keeping tugs waiting while tows are
                                                           being prepared or disposed of after the
          PREPARATION                                      mission has been accomplished. In this
                                                           connection, when the draft of the tow-
                                                           ing tug is too great for the depth of the
5-1 Introduction                                           water at either terminal, advance ar-
                                                           rangements should be made to deliver
Because each tow is unique, the planning,                  or to take over the tow before the arriv-
preparation, and execution have to be careful-             al of the deep sea tug.
ly worked out each time. Tow preparations
must be meticulous, uncompromising, and                  • Employing large tugs to do work that
farsighted. Fleet Admiral Nimitz provided a                available smaller and less powerful or
valuable guide for any ship operation when                 less seaworthy tugs can do.
he said:
                                                         • Employing small tugs to undertake
 “The time for taking all measures for a ship's            work beyond their capacity.
 safety is while still able to do so. Nothing is
 more dangerous than for a seaman to be                  • Employing tugs designed or especially
 grudging in taking precautions lest they turn             suited for combat zone duty in rear ar-
 out to be unnecessary. Safety at sea for two              eas. Large salvage tugs are well suited
 thousand years has depended on exactly the                for combat towing and for emergency
 opposite philosophy.”                                     salvage or fire fighting in combat areas.
Incidents involving loss of tows have demon-             • Employing tugs that cannot survive
strated an absolute need for a thoroughly                  moderate damage in forward combat
professional approach to towing. Tows have                 areas. Survival factors include stability,
been damaged and lost by inattention to the                reserve buoyancy, and subdivision, as
basic principles of proper planning and prepa-             well as being armed to ward off attacks
ration. The plan must cover all aspects of                 by enemy planes.
the tow and anticipate worst case scenarios.
Planning a tow includes training personnel,              • Routing tugs with large tows over areas
practicing basic procedures, and devising safe             where the water is too shallow for the
evolutions.                                                hawser ’s catenary. Arrangements
                                                           should be provided for shortening the
This chapter discusses tow planning and pre-
                                                           towline where necessary. Tows are fre-
paration in general terms.
                                                           quently lost or involved in difficulties
                                                           due to the towline fouling on sub-
5-2 Lessons Learned                                        merged objects.
As Naval towing has evolved, several obvi-               • Unnecessarily employing tugs for
ous lessons have been learned. A list of these             standby duty on salvage or rescue oper-
lessons first appeared in the first Navy towing            ations. Tugs should not be ordered to
document, COMINCH P-03, and are as valid                   stand by unless there is a definite possi-
and meaningful today as they were in 1944                  bility that their services may be needed
when the document was published. The docu-                 and they are capable of rendering the
ment noted that in the planning and task                   service likely to be required.


                                                                                                 5-1
                                      U.S. Navy Towing Manual


      • Diverting rescue tugs from areas where      This reserves the fleet salvage tug for opera-
        tugs equipped with rescue facilities,       tions for which it is best suited.
        such as salvage or fire fighting, may be
        required.                                   5-3.2   Operational Considerations
                                                    5-3.2.1 Support
      • Employing tugs for tows that could be
        undertaken by other craft scheduled to      The staff planner must determine what sup-
        make the same passage, or by a ship         port will be required for the tow at the point
        that could be more easily made avail-       of origin, en route, and at the point of debar-
        able than a tug.                            kation. Included in support considerations are
                                                    industrial support required for preparing the
                                                    towing rig, temporary berthing and messing
5-3 Staff Planning
                                                    for riding crews, refueling, provisioning, re-
                                                    turn of any special issue equipment, tasking
The underlying issue of staff planning is se-       orders, and the logistics of tug assist for get-
quencing all the required aspects of preparing      ting underway and disconnecting. Many of
the tow. Orchestrating preliminary, opera-          these functions may be passed to the towing
tional, and post mission requirements is a          ship Commanding Officer.
fleet or group staff planner’s mission. Care
                                                    5-3.2.2 Manned Tows
must be taken in planning a tow to select the
proper gear and deciding on a route and de-         The tow sponsor is responsible for providing
parture date. In doing so, a tug may avoid ad-      riding crews for the towed vessel. While di-
verse weather conditions that might subject         rect financial support for riding crew trans-
the towing systems to loads that exceed its         portation, messing, and berthing also resides
safe working load.                                  with the tow sponsor, there are aspects of a
                                                    riding crew that have to be integrated into the
5-3.1    Towing Ship Selection                      planning process for the towing ship. Staff
                                                    planners will determine:
In an ideal world, the staff planner would be          • If there is a need for joint training of the
able to match tug characteristics to the type of         riding crew with the towing ship’s crew
tow to perform the tow in a cost effective               for a special tow
manner. Because the fleet has been reduced
in size, planning appears to be easier because         • If the riding crew will be berthed on the
the potential combinations of choices is fewer           towing vessel
with fewer towing assets. In reality, planning         • How long before the tow departure the
is more difficult because the staff planner of-          riding crew will have to be temporarily
ten cannot properly match the size and resis-            assigned to the towing ship.
tance properties of the towed vessel to the
                                                    5-3.2.3 Tug Selection
horsepower and bollard pull of the towing
vessel. Fleet planners are sometimes forced to      When selecting a tow ship and support crew,
improperly size the ocean tug to the tow.           consideration must be given to any anticipat-
Consequently, the only vessel available to          ed complications of the tow. For instance, if
tow a small, low resistance hull may be the         damage control or salvage may be required,
largest and most powerful ocean going tug in        (towing a rescued vessel) experienced sal-
the fleet. In many cases, routine tows not re-      vage personnel are essential. A fleet salvage
quiring the capabilities and manning of a fleet     vessel should be selected or a similar vessel
salvage tug can be contracted through MSC.          supplemented with a salvage crew. Commer-


5-2
                                      U.S. Navy Towing Manual


cial tow ships have limited manning, and al-        accordingly. Anticipated heavy weather
though capable, may be insufficient in num-         c ould require sele cting a large r, m ore
ber to perform all required tasks without           powerful towing asset. The towing command
additional personnel.                               will use the Optimum Track Ship Routing
5-3.2.4 Unsuitable Tows                             System (OTSR) to predict the weather along
                                                    the planned navigational track and make any
Many ships are unsuitable to be towed in the        changes to the track that adverse weather
open ocean. Table 5-1 lists vessels that are        dictates. A longer course on a favorable
not recommended for open ocean towing,              weather track should be selected in favor of a
along with supporting rationale as to why           shorter one with unfavorable weather. Little
they do not qualify. Of course, any vessel can      time is gained by taking a shorter track
be towed, but the vessels listed in Table 5-1       through bad weather.
cannot be towed without serious risk. These
craft can be made safer by correcting the dis-      Once the navigation track has been selected,
qualifying condition, but in their normal op-       calculate total distance and estimate fuel re-
erational configuration, they should not be         quired for the type of tow. If refueling is re-
towed in the open ocean.                            quired either at the tow termination or en
                                                    route, contingencies must be formulated early
5-3.3 Selecting the Navigation Track                in the planning process.
The transit course should be determined using
pilot charts as an aid. Locations along or near     It should be clearly understood in advance by
the track where a lee can be found should be        any vessels taking the towed vessel into its
noted. These can be utilized, when practica-        berth how close to shore the ocean tug ex-
ble, to effect inspection, repair the tow, or       pects to remain connected. The towing vessel
take shelter in heavy weather. Routine navi-        must hand off the tow to harbor tugs and pi-
gational issues must be reviewed in the con-        lots and pilot’s vessels at some point before
text of having a vessel in tow. Pilot charts,       mooring. If there is confusion, an accident
navigational charts and Fleet guides must be        may occur. Conversely, it should be under-
consulted for any restrictions for towing in        stood how far from shore the harbor tugs are
general, as well as the particular tow. The         prepared to retain charge of the tow. Both
Navigator shall be familiar with charts of all      parties should advise of any weather and sea
areas to be crossed, including potential safe       condition limitations on their abilities. If pos-
havens. He shall account for geographic fea-        sible, a meeting of all vessel captains in-
tures such as lees of headlands, effects of riv-    volved (tow ships and harbor tugs) should be
er outflows, and tidal currents to determine        conducted prior to getting underway. All
the relative safety of a particular haven.          transfer procedures and special requirements
When entering a safe haven, the Navigator           can be worked out at this time.
shall be aware of water depths where the tow
wire may snag, and stand ready to recom-            5-4 Towing Responsibilities
mend shortening the towline as required.
An early consideration in selecting the             Primary commands involved in a towing op-
navigation track is the predicted weather en        eration are the tow sponsor and the towing
route. Frequent contact should be made with         command. Frequently the sponsor will task
the Naval Meteorogical and Oceanographical          and fund an assisting command to perform
Center (NMOC) to maintain an up-to-date             some of the tow preparations. This section
weather picture, and adjusted track                 details the definitions, interrelationships, and


                                                                                                 5-3
                                  U.S. Navy Towing Manual



              Table 5-1. U.S. Navy Craft Not Recommended for Open-Ocean Tows.


                 CRAFT                                   REASONS FOR
             CLASSIFICATION                          NON-RECOMMENDATION

  LCU, YCU Landing Craft                       Low freeboard. Light construction of bow
  LCM8, LCM6 Landing Craft                     door locking mechanism. Structure can be
  YFU, UFB, LWT                                strengthened to reduce risk.

  YFNG, YFNX Lighter,                          All have deck-mounted equipment which
  YNG Gate Craft,                              requires installing special protection before
  YSR Sludge Removal Craft                     towing.

  YPD Pile Driver,                             All tend to be top-heavy (have high center
  YD Crane                                     of gravity) and may also have poor water-
  LSMR                                         tight integrity. Topside high weights may
                                               require removal and stowage prior to open-
                                               ocean towing to attain adequate stability.
                                               All require special preparations.
  YSD (formerly                                Low freeboard and high weights reduce
  Seaplane Derrick),                           sea-keeping ability. Weights may require
  YM Dredge                                    removal and stowage to improve stability at
                                               sea.
  YTL Small Harbor Tug                         Hulls considered too small for open-ocean
  PTF Patrol Boat                              tows. Should be transported as deck cargo.


  Mini-ATC, LCPL                               Low freeboard. Light construction with
  MK2, MK3, MK4, MK5                           poor watertight compartment and weak to
  personnel boats                              no attachment points for towing.

  MK1, MK2 65’ utility                         Low freeboard. Deck mounted equipment.
  boats, MK4 50’ utility
  boat, MK3, MK4 40’
  utility boats




5-4
                                     U.S. Navy Towing Manual


specific responsibilities of the parties in-          • Maintaining and protecting the tow dur-
volved in a towing operation.                           ing transit
                                                      • Delivering the tow and obtaining a re-
5-4.1 Sponsoring Command Responsi-
                                                        ceipt from a receiving activity
      bilities
The sponsoring command is the command              5-4.3   Assisting Command
requiring a tow, and is responsible for prepar-            Responsibilities
ing the tow for sea. Basic responsibilities of     An assisting command is often a naval ship-
the sponsoring command include:                    yard, private shipyard (through the cognizant
   • Reviewing applicable Type Command-            Supervisor of Shipbuilding, Conversion &
     er and Fleet CINC numbered instruc-           Repair, SUPSHIP) or Naval Station. Assist-
     tions and operational orders                  ing command responsibilities may include:

   • Preparing the tow                                • Designing a towing hawser system

   • Assembling towing rig                            • Installing temporary towing hard points
                                                        on the tow
   • Completing Certificate of Seaworthiness
                                                      • Installing temporary alarms or electri-
     (see Appendix H)
                                                        cal systems on the tow
   • Determining when there is a riding               • Supplying a riding crew
     crew requirement
                                                      • Providing temporary messing and ber-
   • Designating a receiving activity                   thing for a riding crew at ports of em-
   • Returning all towing equipment, in-                barkation and debarkation
     cluding towing bridle, to preparing ac-
     tivity or tow originator once the tow has     5-5 Review Instructions and
     been completed.                                   Operational Orders

   • Towing machine/towing winch certifi-          An important preliminary step in any tow is a
     cation                                        review of pertinent instructions that govern
                                                   the type of towing to be performed. Fleet and
   • Tow hawser certification                      Type Commander instructions are provided
   • Commercial vessels (U.S. Coast Guard          for general towing procedures and periodic
     Inspected) - Master’s Towing Certifi-         reporting procedures to be followed during
     cate                                          the tow. Specific guidance may also be pro-
                                                   vided for a particular type of tow, such as
5-4.2 Towing Command Responsibilities              NAVSEAINST 4740.9 (Series) for towing
                                                   defueled, nuclear powered submarines. Oper-
The towing command is the command that             ational tasking, such as a Letter of Instruction
performs the tow. The Commanding Officer           sent via naval message, will also be provided
or Master is responsible for:                      to any vessel performing a tow. All governing
   • Determining sailing date and time             instructions should be reviewed for applica-
                                                   bility to the unique tow being performed.
   • Determining the transit route
                                                   5-6 Riding Crew Requirements
   • Selecting towing rig and determining
     trim conditions
                                                   A riding crew can add immeasurably to the
   • Inspecting and accepting the tow              general safety of the tow. The Navy com-
                                                                                               5-5
                                          U.S. Navy Towing Manual


mand requesting a tow should make a recom-              5-7 Preparing the Tow
mendation after considering personnel safety
and the value of the tow. Safety consider-              A tow’s hull design may require taking nu-
ations must be based on the crew’s influence            merous steps in preparing to tow. Examples
on tow safety rather than the tow’s influence           include cranes, pile drivers, dredges, dump
on crew convenience. Value of the tow can be            scows or other equipment designed for oper-
either its replacement cost; value of its safe          ation in sheltered waters. Preparing the tow
and timely delivery; or cost of consequences            may include removing high weights, secur-
of loss from a tactical, strategic, or public re-       ing booms, dredge ladders, and other deck
lations standpoint.                                     structures; adding or removing ballast or ad-
                                                        justing trim; stiffening the hull and perform-
Rescue tows should have personnel on                    ing other functions. Heavy welded brackets
board, if possible, to make the tow connec-             must be used to secure heavy movable ob-
tion and to respond to changes in the tow’s             jects and a tow should always be secured for
material condition. A riding crew can also re-          the worst sea conditions. Expect large angles
spond to emergencies such as fire, flooding,            of roll and pitch and secure all heavy objects
hawser or bridle chafing, and towline loss.             accordingly.
The tow must be adequately supplied and
equipped to support a riding crew. Crew ac-             Hulls not considered seaworthy for open-
commodations should include berthing,                   ocean tows should be transported as deck car-
messing, and sanitary facilities, all of which          go or on board a floating dry dock, semi-sub-
must be properly ventilated.                            mersible vessel, or LSD type ships.

Under normal conditions, most planned                   5-7.1   Installing Flooding Alarms, Draft
point-to-point tows are undertaken without a                    Indicators, and Other Alarms
riding crew. All tows can be unmanned if
                                                        All unmanned tows shall be equipped with
properly planned.
                                                        flooding alarms. Flooding alarms indicate to
Riding crews shall be limited to personnel re-          the towing ship that there is a problem with
quired for maintenance and security during              the tow, allowing corrective action to be tak-
the voyage.                                             en before the tow sinks. The tow preparing
                                                        activity is responsible for installing flooding
Approval to assign a riding crew must come              alarms. Unmanned tows must be equipped
fr o m t h e F l e e t Co m ma n de rs- i n -C hi e f   with high and low alarms rigged with multi-
(CINCs) in accordance with existing direc-              ple bulbs, and independently wired flooding
tives. Factors governing a decision to assign a         alarm lights in all major compartments clos-
riding crew include:                                    est to the keel.
      • Safety of the riding crew                       A schematic diagram of an acceptable flood-
                                                        ing alarm is shown in Figure 5-1. No attempt
      • Reduction of risk of towed ship loss by         has been made to provide detailed specifica-
        assigning a riding crew                         tions or installation instructions because these
                                                        vary with the type and size of the tow. The
      • Material condition of the tow                   number and location of the electrode blocks
      • Flooding alarms and other monitoring            or alarm switches to be installed in an un-
        devices installed on board                      manned tow are determined by the activity
                                                        preparing the tow and agreed to by the towing
See section 5-7.1 for more considerations.              command. Installation should be sufficient to


5-6
                                   U.S. Navy Towing Manual




             Flashing Light
             Assem bly




                                                                   Flasher Lights
                                                                   Road Construction Type
                                                                   Visible 360°

Flooding Alarm
Contact M arker                                                            Sm ashproof W ire
Assem bly                                                                  STK No. G -6145-191-1962


                      3’
                                   1’




                                                NO TE

                           T his sa m p le dia gra m is no t inte n de d
                           to re strict o r lim it th e n um b e r o f a larm s
                           co n side red ne ce ssa ry to p rovid e
                           a de qu ate pro te ction to all im p o rta nt
                           w a te rtig ht sp aces.




                  Figure 5-1. Example of a Flooding Alarm Schematic.




                                                                                                      5-7
                                      U.S. Navy Towing Manual


provide coverage of major hull subdivisions.           • When placing alarms in a wide flat-bot-
Alarms should be securely rigged and proper-             tomed compartment. It may be benefi-
ly serviced to ensure performance and reli-              cial to place an alarm both port and
ability.                                                 starboard.
5-7.1.1 Flooding Alarm Sensor Mounting              5-7.1.2 Wiring and Power Supply
        Requirements                                        Requirements

There are a variety of alarm types. An elec-           • Wire the flood alarms so that any low
trical contact alarm that closes its circuit             level alarm will activate the low level
when water makes contact is a workable                   lights and that any high level alarm will
alarm most of the time. If used in the engine            activate the high level lights. Existing
room, however, oil in the bilge may coat the             ships wiring may be used to support
wires as flooding progresses and render the              this installation.
alarm useless. Carefully consider the practi-          • Batteries should be sized to support all
cality of each proposed alarm location. Inno-            flood alarms for continuous 24-hour
vation is advisable.                                     operation. Sufficient electrical power
Refer to the following guidelines when in-               shall be provided for all lights and
stalling flooding alarm sensors:                         alarms for the duration of the tow so
                                                         there is no need to board the towed ship
      • Installing flooding alarms may require
                                                         to change power supplies. Power for
        piercing watertight decks and bulk-
                                                         the flooding alarm system should be
        heads. Penetrations should be as high as
                                                         separate from the power source for the
        possible. Every attempt should be made
                                                         navigation lights.
        to use watertight penetrations, or to
        minimize the size of the penetration.          • Secure and protect all wiring from any
                                                         chafing, and protect all topside wiring
      • Low level alarm sensors shall be in-
                                                         from weather damage.
        stalled one foot from the lowest point
        in the compartment, assuming that the          • Where practical, install a wiring board
        ship is in a bow up position while               to act as a compartment indicator. It
        waterborne.                                      must be wired so that a low level alarm
                                                         in a given compartment will activate an
      • High level alarm sensors shall be
                                                         indicator that identifies the flooding
        located three feet above the low level
                                                         compartment.
        alarm sensors.
                                                       • NAVSEA has developed a towing
      • Float type switches are recommended.
                                                         alarm system that utilizes a radio link
        However, if using sensing probes for
                                                         from the towed ship to a console on the
        flooding alarm sensors, they shall be
                                                         tug. This system allows the tow ship to
        securely mounted on a suitable noncon-
                                                         determine the location of the flooding
        ductive, nonporous material such as
                                                         without boarding the tow. Indications
        Melamine. Plywood is not a suitable
                                                         of low battery power and ground faults
        material; C-clamps are not suitable se-
                                                         from alarm wires are also indicated.
        curing devices.
                                                         This system has been packaged for at
      • Areas where flooding alarms are to be            sea use and includes all power sources,
        installed shall be certified gas-free to         lights, alarms, and wiring. This system
        prevent explosion and fire from electri-         is available for issue from the ESSM
        cal contact sparking.                            warehouse.


5-8
                                     U.S. Navy Towing Manual


5-7.1.3 Alarm Lighting Requirements                ed below the waterline, but they must give the
   • At least two high alarm and two low           tow ship a clear indication of a change in the
     alarm lights shall be installed. The high     tow’s trim. See Figure 5-2 for samples of wa-
     level alarm lights shall be positioned        terline marks.
     four feet above the low level alarm           5-7.1.7 Towed Vessel Propeller Preparation
     lights. The alarm lights shall be mount-
     ed topside to be visible from the ships       A towed vessel’s propellers can be a valuable
     in company.                                   tool or an unpleasant obstacle during a tow.
                                                   In either case, they require special attention.
   • The high level alarm strobe lights shall      Tow planners must decide whether to remove
     have an amber lens.                           propellers, lock them in place, or allow them
   • The low level alarm strobe lights shall       to free-wheel. The procedure of free-wheel-
     have a white lens.                            ing propellers is not recommended, but can-
                                                   not always be avoided.
   • Flooding alarm lights should be checked
                                                   5-7.1.8 Removing Propellers
     to ensure their visibility during daylight
     hours. The lights shall be visible from       For long-distance tows, fixed-pitch propellers
     360° and at a minimum distance of             may be removed to decrease towing resis-
     2,000 yards during bright daylight.           tance. For some hull forms, however, the add-
5-7.1.4 Audible Alarm
                                                   ed drag of locked propellers may be desirable
                                                   for better directional stability. Tow planners
An audible alarm can be used to provide noti-      must also consider the economic feasibility of
fication during fog or heavy rain. This alarm      removing the propellers.
must be loud enough to be heard by ship’s
personnel while underway. Items such as fog        It is helpful to consider the vessel’s future
horns can be a considerable power drain. It is     when determining disposition of it’s propel-
recommended that these be rigged for inter-        lers. If the vessel is being transferred, but not
mittent operation (a few seconds of sound;         decommissioned or drydocked, it will proba-
every few minutes) to avoid needing exces-         bly be best not to remove the propellers, as
sive batteries.                                    there would likely be considerable cost for
                                                   re-installation. This high cost may offset any
5-7.1.5 Requirements for Other Alarms              fuel savings gained by the reduced resis-
Depending upon the tow, its equipment and          tance.
cargo, other alarms such as fire, radiological,    If the vessel is being prepared for tow at a site
or combustible gas may also be required.           that may find use for a propeller destined for
Specifications will be provided to the activi-     scrap (spares for sister vessels) it may be ben-
ty preparing the tow. Wiring and powering          eficial to remove propeller prior to towing.
requirements should apply to all additional
alarms.                                            A propeller creates considerable resistance in
                                                   either a locked or freewheel configuration.
5-7.1.6 Draft Indicator Requirements
                                                   This resistance adds a large contribution to
The tow should have large, special waterline       the directional stability of the tow, particular-
marks to allow a towing ship to check trim of      ly in the absence of rudder control. If the pro-
the tow visually by day and by searchlights at     peller is removed, the directional stability of
night. Marks should be painted on the bow,         the tow should be examined. A water brake or
stern, and midships on both sides, in highly       similar device may be added if stability is ex-
visible paint. These marks need not be paint-      pected to be a problem.


                                                                                                5-9
                                            U.S. Navy Towing Manual


Controllable pitch propellers may be left in-             Shifting ballast, fuel, cargo, or equipment on
stalled if set in “maximum forward” pitch,                board can bring about desired trim.
where they offer the least resistance to tow-
                                                          Follow these guidelines when adjusting the
ing. They may also be set in a “zero pitch”
                                                          tow’s trim:
condition for added drag if desired.
                                                             • Trimming by the stern has proven to be
5-7.1.9 Locking Propellers
                                                               a stable and directionally true towed
When propellers remain in place and are not                    ship load condition. A trim of one foot
allowed to free-wheel, lock the shafts by an                   by the stern for each 100 feet of the
installed shaft-locking device or by another                   tow’s length has proven a good trim-
suitable method as illustrated in Figure 5-3.                  ming rule; deep draft tows use some-
5-7.1.10 Allowing Propellers to Free-Wheel                     what less than one foot per 100 feet.
If any type of propeller must be allowed to                  • Completely fill all tanks or leave them
free-wheel due to the condition of the towed                   empty to ensure there is no adverse
vessel’s propulsion train, propulsion machin-                  free-surface effect.
ery must be disconnected from the shafts or                  • Ensure all normally dry compartments
adequate lubrication provided.                                 are dry to avoid adverse stability effects
                                                               of free surface areas and to provide
                     CAUTION                                   greater reserve buoyancy.
        Do not allow main reduction gears                    • Ensure bilges are free of oil and water
        to rotate unless they are properly                     to ensure that bilge flooding alarms are
        lubricated. This requires full lube oil                not tripped by sloshing water. Oil in the
        pressure.
                                                               bilge is a fire hazard and could foul
                                                               alarm electrical contacts.
A means for lubricating the shaft bearings
must be provided. The stern gland on the                     • Close all sluice valves to prevent
shaft will normally be water-lubricated. Pro-                  liquids from flowing between ad-
vision for this must be made while at the                      joining tanks.
same time ensuring that the water does not                   • Ballast landing craft or craft with blunt
flood the space.                                               or raked bows to prevent heavy pound-
5-7.1.11 Stern Tube                                            ing. Pounding can be very destructive
                                                               to the vessel’s bottom and other struc-
There should be no leakoff at the stern tube.
                                                               tural members. Preventing or reducing
Equip the tow with extra packing for the
                                                               pounding also reduces shock loads on
stern gland to allow emergency repair during
                                                               the towing rig.
transit. The gland should be tightened so
there is no leakage with at least two inches of              • Ensure the tow has zero list.
room before its tightest position. Use lock-
nuts to prevent backing off.                              5-7.3   Ballasting for Proper Stability
                                                          Stability of the tow, in the case of an unmodi-
5-7.2    Ballasting or Loading for Proper                 fied or undamaged Navy commissioned ship,
         Trim
                                                          can be determined by reviewing Chapter II(a)
Proper trim is important because it can affect            of the ship’s Damage Control Book. Similar
stability, towing characteristics, and speed.             information for commercial ships should be



5-10
                     U.S. Navy Towing Manual




                                 30”
                                                 6"

      M idship




                                                        3"




                                 20”

                                                   6"




12"




                                  36"
                              Bow & Stern




                 Figure 5-2. Special Draft Markings.




                                                             5-11
                  U.S. Navy Towing Manual




                                                             B olt

                                                             S teel S ecu ring S trap
                                                   S haft
                           Fillet W eld
                                                                     W elded or B olted


                                          Flange             B olt

                                                             S teel S ecu ring S trap
                                                   S haft

                                                                     W elded or B olted

                                          Flange            B olt


                                                            S teel S ecu ring S trap
                                                   S haft

                                                                W elded or B olted

                                          Flange




       A. Heavy plate, 3/4″ to 1″ cut to
          accommodate two (2) of the
          coupling bolts.
       B. Intermediate, horizontal plate
          cut to accommodate and weld-
          ed to "A" and "C" with full fillet.
       C. Deep channel or angle beam,
          welded to the nearest hull
          frames with full penetration fillet
          weld.




          Figure 5-3. Securing the Propeller Shaft.




5-12
                                       U.S. Navy Towing Manual


available in the ship’s Trim and Stability           This book also contains stability characteris-
Booklet, as well as in the Deadweight Survey.        tics for various loading conditions that meet
When formal documentation of the ship’s sta-         the Navy’s stability criteria.
bility is not available, stability may be ap-
                                                     For small craft and barges that do not have a
proximated by timing the ship’s roll period.
                                                     Damage Control Book, follow a few general
This method is reasonably accurate and is
                                                     guidelines when attempting to improve
used by the U.S. Navy, U.S. Coast Guard, and
                                                     stability:
regulatory bodies to confirm the accuracy of
inclining experiments and other similar sta-            • Completely fill any slack tanks
bility determinations. For small craft, timing          • Lower and secure or off-load high
roll period is the approved method for deter-             weights
mining stability.
                                                        • Secure any large hanging weights and
The roll period can be estimated accurately               add ballast.
enough even in fairly calm water by watching
the masthead. Time several successive rolls          In addition to improving stability, completely
(from extreme port to starboard back to ex-          filling tanks or adding ballast will decrease
treme port is one period), then divide the total     freeboard.
time by the number of rolls observed to ob-
                                                     5-7.4   Two Valve Protection
tain a good estimate. To determine the ade-
quacy of the roll stability, compare the time        Tows of inactivated Navy vessels imply that
period with the value calculated from the fol-       the preparing activity has met the require-
lowing formula:                                      ments of Naval Ship’s Technical Manual
                                                     (NSTM) S9086-BS-STM-010, Chapter 50,
              T = 2 Beam ( ft )                      Readiness and Care of Inactive Ships (Ref.
where:                                               G). This NSTM calls for installing hull
                                                     blanks for all sea chests that don’t provide
              T=Time in seconds.                     two valve protection to the ship’s interior.
For adequate stability, the time in seconds for      However, tow inspectors should still be at-
a ship to roll from port to starboard and back       tuned to potential flooding conditions on in-
to port must be equal to or less than the calcu-     activated vessels.
lated time (T) in seconds. For example, for a        For unmanned tows of other vessels, a towing
ship with a beam of 100 feet, the time ob-           vessel’s tow inspection team should pay add-
served for the ship to complete a roll period        ed attention to machinery room or low lying
must be less than the 20 seconds calculated. If      spaces for potential flooding conditions such
the observed time is longer than the calculat-       as single valve protection. Two valve protec-
ed value, stability generally is considered in-      tion consists of either two valves wired shut
adequate. Equally important is frequent              or one valve and a blank flange. Sea valves
checking for a change in the tow’s roll period.      must be wired shut with steel wire to protect
Even if overall criteria are satisfactory, inves-    all sea openings from the sea.
tigate promptly any significant increase in pe-
                                                     Attention should be paid to any loose connec-
riod, since this suggests flooding and/or addi-
                                                     tions or badly deteriorated spots in the drain
tional free surface.
                                                     piping which originates above the waterline
Each commissioned ship in the U.S. Navy has          and terminates within 20-feet of the water-
a Damage Control Book containing specific            line. If this piping shows excessive rust or
measures for improving a ship’s stability.           other damage, this may represent a potential


                                                                                              5-13
                                        U.S. Navy Towing Manual


flooding path in the case of severe weather.          barges. Bottom plate thickness in the forward
These pipes should be repaired to ensure any          one-fifth of the vessel must meet these mini-
drainage flows overboard.                             mum values for safe towing.

5-7.5    Inspecting the Tow for Structural            These values are the minimum thicknesses re-
         Damage                                       quired to meet 1991 American Bureau of
                                                      Shipbuilding (ABS) 10, Rules for Building
Every tow should be inspected to ensure that          and Classing Steel Barges (Ref. H). If actual
its structure is capable of withstanding the ef-      thickness is less than 75% of these values,
fects of towing. If there is any question about       consider reinforcement. The values in Table
the vessel’s structural integrity or if the struc-    5-2 are for the forward section; thicknesses in
ture shows signs of extensive deterioration or        the mid-section can be seen in Table 5-3.
damage, a qualified structural engineer               Again, a 75% criteria should be applied. Re-
should be consulted.                                  inforcement should be considered if there are
In emergencies, such as salvage and rescue            any signs of serious corrosion or excessive
towing, structural reinforcement and load dis-        out of plane damage (buckling, frame trip-
tribution may be accomplished with addition-          ping, etc.)
al structure or shoring. See Figure 5-4 for typ-
                                                      To avoid special dry docking before towing,
ical timber framing practice. Protection
                                                      barges, cranes and other service craft should
against slamming damage may be effected by
                                                      be thoroughly examined during routine main-
pressing up the bow section of the hull with
                                                      tenance. Plate thickness and weld inspections
water. This action may require counter-flood-
                                                      should be conducted regularly during sched-
ing or shifting of cargo. If the tow is to be
                                                      uled dry docking, or by ultrasonic inspection
rigged for towing by the stern (secondary or
                                                      in water.
emergency rigging), these areas should re-
ceive similar attention.
                                                      5-7.6   Locking the Rudder
Inspection may reveal damage or deteriora-
tion of frames, bottom or weld seams. Partic-                            CAUTION
ularly when this occurs in the forward one-                   Do not use temporary lashings or
fifth of the vessel’s length, the vessel should               other makeshift measures to lock
be dry-docked or ultrasonically tested, and                   the rudder of a towed ship. Lock
necessary repairs made. While in dry dock,                    the rudder amidships for towing.
check bottom, side, decks and inner bottom.
All defective welds and plating should be re-         Because a drifting rudder will cause the tow
paired or replaced.                                   to behave erratically, the rudder should be
5-7.5.1 Barge Hull Thickness                          generally locked amidships. The method used
                                                      to secure a rudder depends upon the tow’s
                                                      steering gear.
                    CAUTION
                                                         • Yoke or tiller arm steering gear.
        Many barges and barge-like ves-
        sels tend to be more susceptible to
                                                           Structural steel can be welded across
        damage and deterioration than                      the tiller arm to suitable ship’s structure
        conventional ship type vessels.                    on either side. (An independent engi-
        They should therefore be inspected                 neering evaluation is required to ensure
        for hull strength prior to towing.                 that both securing device and ship’s
                                                           structure are adequate). Figure 5-5 de-
Table 5-2 lists minimum thicknesses based on               picts an example of such an arrange-
barge length and frame spacing, for typical                ment.
5-14
                                     U.S. Navy Towing Manual



          Table 5-2. Minimum Plate Thickness for Forward One-Fifth of Barge Bottom.




                                            Frame Spacing (inches)

Barge          18            21           24         27          30        33          36
Length

100 ft.       0.250        0.271         0.292      0.313      0.334     0.355        0.376

120 ft.       0.261        0.282         0.303      0.324      0.345     0.366        0.387

140 ft.       0.272        0.293         0.314      0.335      0.356     0.377        0.398

160 ft.       0.282        0.303         0.324      0.345      0.366     0.387        0.408

180 ft.       0.293        0.314         0.335      0.356      0.377     0.398        0.419

200 ft.       0.304        0.325         0.346      0.367      0.388     0.409        0.430

220 ft.       0.315        0.336         0.357      0.378      0.399     0.420        0.441

240 ft.       0.326        0.347         0.368      0.389      0.410     0.431        0.452

260 ft.       0.336        0.357         0.378      0.399      0.420     0.441        0.462

280 ft.       0.347        0.368         0.389      0.410      0.431     0.452        0.473

300 ft.       0.358        0.379         0.400      0.421      0.442     0.463        0.484

320 ft.       0.369        0.390         0.411      0.432      0.453     0.474        0.495

340 ft.       0.380        0.401         0.422      0.443      0.464     0.485        0.506




All thickness dimensions are given in inches.




                                                                                            5-15
                                       U.S. Navy Towing Manual



                        Table 5-3. Minimum Plate Thickness for Mid-Section.




                                              Frame Spacing (inches)

  Barge          18            21           24         27          30          33      36
  Length

   100 ft.      0.286        0.316         0.346      0.376      0.406        0.436   0.466

   120 ft.      0.299        0.329         0.359      0.389      0.419        0.449   0.479

   140 ft.      0.312        0.342         0.372      0.402      0.432        0.462   0.492

   160 ft.      0.326        0.356         0.386      0.416      0.446        0.476   0.506

   180 ft.      0.339        0.369         0.399      0.429      0.459        0.489   0.519

   200 ft.      0.352        0.382         0.412      0.442      0.472        0.502   0.532

   220 ft.      0.365        0.395         0.425      0.455      0.485        0.515   0.545

   240 ft.      0.378        0.408         0.438      0.468      0.498        0.528   0.558

   260 ft.      0.392        0.422         0.452      0.482      0.512        0.542   0.572

   280 ft.      0.405        0.435         0.465      0.495      0.525        0.55    0.585

   300 ft.      0.418        0.448         0.478      0.508      0.538        0.568   0.598

   320 ft.      0.431        0.461         0.491      0.521      0.551        0.551   0.611

   340 ft.      0.444        0.474         0.504      0.534      0.564        0.594   0.624




  All thickness dimensions are given in inches.




5-16
                                 U.S. Navy Towing Manual




                                        Angle Strongback

                            W edge                     W edge
   Bottom
Longitudinals



          Tim ber Packing                                    Bottom Plating


                                                          Watertight
                                                          Bulkhead
                                                          Strongback

                                                          Truss

                                                          Strongback

                                                          Truss

                                                          Strongback

                     Bottom Longitudinal

                Longitudinal Spacing 22-24 Inches
                    Truss Spacing 8-10 Feet                                NO TES

                                                           1 . W eld stro ng b ack to
                                                               lon gitud in a l fla n ge s.
                    Strongback
                                                           2 . D o n ot se t w ed ge s w ith a
      Bottom Longitudinal                                      h am m er he a vier tha n 2 lb s.

                                                           3 . N ail w ed ge s to p acking so
                                  W edge                       tha t w e d ge s w ill no t w o rk
                                                               loo se .
                     Packing                               4 . A lte rna te dire ctio n o f
                                                               w ed g es to se cu re pa ckin g
                Bottom Plating                                 tim be rs.




                      Figure 5-4. Reinforcing Bottom Plating in Barges.



                                                                                                    5-17
                                     U.S. Navy Towing Manual


   • Vane type steering gear. Extend an            For a manned tow, if the steering machinery
     emergency wrench (or wrenches) with           is operable and reliable, a decision may be
     a heavy channel or beam to reach a            made to steer the tow.
     strong ship structure. Use full penetra-
     tion welds on both the wrench and the         5-7.7   Installing Navigational Lights
     ship structure (see Figure 5-6).
                                                   The preparing activity must ensure a tow is
   •Hydraulic steering gear. The rams can          equipped with proper navigational lights.
     be secured by positioning the rudder          Specific requirements concerning the correct
     amidships and securing the hydraulic          positioning, number and color of lights are
     system in an attempt to maintain a hy-        contained in Code of Federal Regulations
     draulic lock. Sheet rubber is wrapped         (CFR) Part 81-72 COLREGS, Implementing
     around the piston and split pipe is cut to    rules (Ref. I).
     the proper length so the ends bear
     against the cylinders and/or yoke. The        Navigational lights should have a solar switch
     split pipe should be secured in place         built in to increase battery life and meet COL-
     with bands. Both rams should be se-           REG requirements. Alternately, a single solar
     cured in this fashion. Welding a plate or     switch can be added to the system. Towing
     structural member to the yoke and to          lights generally have a 10 foot leader wire for
     the foundation or ship’s structure adds       attachment to batteries. If that length is insuf-
     security. Refer to Figure 5-6.                ficient, Navy type DHOF-4 cable is suitable
                                                   for connections. Ensure that all wiring is well
Regardless of the securing method, an inde-        secured and protected from damage by the el-
pendent check (by an industrial facility,          ements.
structural engineer, or mechanical engineer)
of the rudder securing method should be ac-        Table 5-4 lists battery capacity requirements
complished to ensure they are strong enough        for one 60-watt, 12-volt DC sidelight or stern
to withstand the forces generated by the rud-      light for tows of various durations. Individual
der. Forces on the rudder, even at low speeds      batteries for each light may be used to elimi-
through the water may be very large due to         nate the power loss in long cables. Standard
wave impact and other sea action. These            Navy 12-volt lead-acid batteries protected by
loads will be transmitted through the steer-       steel containers provide the necessary am-
ing gear and absorbed by the ship’s structure.     pere-hour capacity.

It may not be possible to use any of the illus-    5-7.8   Selecting the Rig
trated arrangements, as in the cases of rescue
and towing at sea under unfavorable weather        Tow rig selection is best based on past perfor-
conditions. A temporary means may then be          mance and the unique needs of the upcoming
employed. Chain falls or come-alongs may           tow. Although most Navy tows are simple,
also be used in conjunction with tiller arms or    single-tug, single-unit operations, some tows
quadrants. Where practical, chain should be        are considerably more complex, consisting of
used instead of wire rope. Ram hydraulic sys-      a single tug with multiple towed units. Occa-
tems may be isolated in some installations to      sionally the displacement of the towed unit
assist rudder locking. These methods are only      requires using more than one tug. The follow-
temporary; a permanent locking arrangement         ing factors should always be considered when
should be installed.                               selecting a towing rig:


5-18
                 U.S. Navy Towing Manual




   Structure Angle                            Full Penetration Weld
or W ide Flange Beam




                                              Tiller Arm
                                                              Shell
                          Rudder Post



                         Fore & Aft Tiller Arm




                             Shear Plate(s)


                       Athw artship Tiller Arm




                               NO TE

                 To m axim ize lever a rm , it m ay
                b e n ece ssary to use sh e ar
                p la te (s) to secure tille r a rm to
                d eck. Se e F igu re 5 -6 for typ ical
                sh ea r pla te exam ple.




             Figure 5-5. Securing the Rudder.




                                                                      5-19
          U.S. Navy Towing Manual




       Figure 5-6. Securing the Rudder.




5-20
                                     U.S. Navy Towing Manual



                            Table 5-4. Battery Capacity Requirements.

                                                                12 Hr/Day Operation
              Length of Tow (Days)
                                                               60 Watt Light (Amp-Hr.)

                        5                                           300 Amp-Hr.

                       10                                           600 Amp-Hr.

                       16                                           960 Amp-Hr.

                       21                                           1260 Amp-Hr.

                       30                                           1800 Amp-Hr.


   • Identify the type of towing rig required      increase of atmospheric temperature. Barge
     for all conditions anticipated during the     sides and decks have been known to bulge se-
     transit and at either end of the tow.         verely when vents are plugged. Ensure hatch-
   • Ensure that all rigging is adequate. If       es, scuttles, doors, portholes and other water-
     in doubt, use a higher safety factor. Pay     tight closures are provided with pliable
     particular attention to protection from       gaskets and that material condition ZEBRA is
     chafing.                                      set throughout the tow.

   • Ensure that multiple tows are config-         If vents may be subject to heavy weather
     ured for optimum seakeeping ability.          flooding (such as vents near the waterline), it
   • Provide a secondary towing rig on the         may be necessary to weld a blank over the
     tow in case the primary system fails.         opening to minimize the risk of flooding.

   • Provide for anchoring the tow in case of
     emergency.                                    5-8 Emergency Systems

   • Provide for all contingencies as out-
                                                   Adequate fire fighting equipment and materi-
     lined in the checklist (see Appendix H).
                                                   als, as well as damage control equipment and
Before towing a new or unique configuration,       associated fuel, should be placed on board
ensure design of the rig conforms to appropri-     prior to starting the tow. For long voyages,
ate engineering and design criteria. A number      tows should have bilge pumping equipment.
of towing configurations and arrangements          If permanent bilge pumps on the tow are in-
are shown in Appendix I. Consult NAVSEA            operative portable lightweight pumps or edu-
00C to resolve any technical matters regard-       cator systems should be provided, that can be
ing towing.                                        handled by the riding crew or inspection party
                                                   from the tug. Tests should demonstrate that
5-7.9 Preparing Tank Vents                         the pumps have adequate suction lift and dis-
Vents to tanks and other closed spaces should      charge head. For larger ships, installation of
be covered with canvas socks to prevent wa-        portable fire-fighting systems should be con-
ter entry, but not plugged so as to prevent the    sidered. A portable system could make use of
escape of air or gas. Plugged vents allow          the existing firemain system aboard the towed
pressure to build up within the tank with an       vessel for distribution of fire fighting water.


                                                                                             5-21
                                      U.S. Navy Towing Manual


5-8.1    Electrical Power                           towing scenario will likely have serious po-
                                                    tential for fire.
Electrical power is required on the tow for the
following systems:                                  Risk of fire can be greatly reduced by elimi-
                                                    nating as much of the combustible material
   •    Fire alarms
                                                    on board as possible. It is virtually impossible
   •    Lights                                      to remove all combustible material from a
   •    Flooding alarms (audible and visual)        tow as items like insulation and cabling are
   •    Pumps                                       difficult and expensive to eliminate complete-
   •    Communications equipment                    ly. But paper products, furniture and combus-
   •    Crew accommodations                         tible liquids and paints are relatively easy to
   •    Winches and capstans                        remove and greatly reduce risk of a fire. A
   •    Radiological alarms                         full walk through of all compartments should
                                                    be done to identify any areas that may be a
All electrical and other systems should be in-      potential ignition source. Maintaining water-
spected and tested periodically to ensure reli-     tightness and sealing as many compartments
able operation. If electrical power on the tow      as possible will reduce the chance of a fire
is supplied by an installed or portable genera-     spreading.
tor, a sufficient amount of fuel for the tow
should be provided. A simple rule of thumb is       Fire fighting equipment should be compatible
to allow two gallons of fuel per day, per gen-      with determined risk. The capacity and porta-
erator horsepower, or to allow 2.7 gallons of       bility of installed equipment will determine
fuel per day, per kilowatt.                         the effectiveness of any fire fighting effort.
                                                    Active Navy ships should have three or more
Batteries in a battery powered system should        portable fire fighting pumps on board. The
be checked for capacity and condition. Bat-         Navy has replaced gasoline driven P-250s
teries exposed to the weather must be protect-      with newer self-priming, diesel-driven P-
ed in watertight containers that will not per-      100s. These pumps produce about 100 gal-
mit the batteries to leak to ground. It is          lons per minute at around 85 psi. The 3-inch
essential that all exposed wires and connec-        suction hose (same as the P-250) is used to
tions be adequately waterproofed. Wires             pull a maximum of about 20 feet of suction.
should be secured to prevent chafing and            The discharge connection is typically a 2 1/2-
grounding. Provisions must be made to vent          inch Y-gate that can be connected to two 1 1/
hydrogen gas from all batteries.                    2-inch standard fire hoses. Only one hose
                                                    should be used at a time due to pressure and
5-8.2    Fire-fighting                              flow limitations. It is prudent to connect two,
                                                    however, to allow quick response in the event
The need for fire fighting equipment must be
                                                    of a ruptured line. Pumps and associated gear
evaluated by the tow planner and will depend
                                                    are generally located on the weather deck
on several factors. The value of the tow, po-
                                                    near repair lockers.
tential sources of ignition, the consequences
of fire, and the effectiveness of fire fighting     It may also be possible to connect to the
equipment (including personnel), should all         ship's installed firemain. An assessment of
be considered when deciding how to ap-              the pressure and flow rate needed to meet the
proach this requirement. Potential for fire on      fire fighting capability should be made to en-
a planned unmanned tow should be relatively         sure that adequate pumps are installed. Navy
small, but this may not be the case. For in-        ships will typically use 1 1/2-inch fire hose
stance, ships involved in a rescue and salvage      (50-foot lengths) from a 6 to 8 inch header.


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                                       U.S. Navy Towing Manual


This header should be charged to approxi-            A vessel prepared for tow may have been pre-
mately 150 psi. Access to spaces deemed as           pared with extensive compartmentation.
potential fire hazards should contain extin-         Compartmentation will serve to limit the
guishers and fire hose. These items should be        flooding to certain areas of the ship and limit
staged in a place to allow the boarding crew         the amount of water taken on. When prepar-
to begin fire fighting without endangering           ing a tow, level of compartmentation should
their safety.                                        be a major consideration when determining
                                                     the need for dewatering equipment.
5-8.3 Dewatering
                                                     Operational ships should have a fairly large
It is common practice to outfit a tow with           capacity for dewatering. If one of these ships
flooding alarms. These tell the ship that there      has been picked up in a distress status, tow
is some problem on board and allow the tow           ship personnel should become familiar with
ship to assess the severity of flooding to some      it’s installed systems. Operational USN ships
degree (using high and low level alarms). De-        have flooding effect diagrams as part of their
watering equipment, such as pumps and hos-           damage control package. These diagrams will
es, is used to control flooding or remove wa-        tell the effect of flooding of a particular com-
ter. If the flooding can be stopped with             partment. In the absence of these drawings, a
patches or other repair, water can be removed        flooding matrix can be developed to show the
to restore the vessel to its stable condition. If    extent and effect of flooding certain compart-
the flooding cannot be stopped, this equip-          ments.
ment can be used to limit the effects of flood-
ing. If the rate of flooding is slow, dewatering     Effectiveness of the installed equipment must
pumps may be able to keep the vessel in a sta-       also be evaluated. Pumps operating on a ship
ble condition until port is reached or repairs       with a very high freeboard will have a diffi-
can be made.                                         cult time transferring the water from low
5-8.3.1 Deciding to Use Dewatering                   down in the hold, over the side and into the
        Equipment                                    sea. If large pumps are used, it will likely be
                                                     impossible for a boarding crew to move them
The decision to install or use dewatering            around. Sufficient lengths of hose must be in-
equipment should be made by both the tow             cluded to reach all areas of the vessel that
planner and the tow ship Commanding Offic-           may require dewatering.
er or Master. Not every tow will need to be
rigged with dewatering equipment. It may be          5-8.3.2 Choosing Equipment
desirable to use dewatering equipment on
high value tows or tows with little compart-         When preparing a compartment for dewater-
mentation. Critical compartments or compart-         ing, it is wise to identify potential flooding
ments with damage can be rigged for dewa-            sources. For example, a damaged ship may
tering while leaving other areas alone. Many         have some patching, or a ship may have had a
tows are decommissioned vessels and have             rudder casualty. The size and amount of
been prepared for long term storage. Often           equipment chosen should be able to over-
these hulls have a high degree of water-tight-       come any flooding from these sources. Leak-
ness with all hull openings having welded            age from a large patch may produce a greater
blanks. Flooding is a very unlikely event in         amount of flooding than leakage around a
these cases. But not every scenario can be           rudder post. Pumps should be sized accord-
foreseen and things break and accidents hap-         ingly. P-250s, P-100s or equivalent pumps
pen and flooding is still a possibility.             are often readily available on Navy ships and


                                                                                                5-23
                                      U.S. Navy Towing Manual


provide good pumping capability. Submers-           Sufficient means for personnel to board a tow
ible pumps may also be used effectively.            at sea should be provided. See Figure 5-7 for
Pumps may need to run continuously to over-         examples.
come flooding until repairs have been made.         Access markings should be as reflective as
Sufficient fuel should be carried on board to       possible to allow access in a low light or
operate the pumps for at least 24 hours, al-        smoky environment. They should be located
though more is desirable. Adequate fuel stor-       in enough areas to ensure that personnel will
age should also be included with the capacity       have no confusion when attempting to locate
to hold this amount of fuel. Provisions can be      an exit in an emergency. A line painted down
made to refuel if the operation will last long-     the center of the passage provides a continu-
er. The tow planner should be aware of the          ous route. Adding reflective arrows will assist
capabilities of the tow ship and boarding           in locating the access route.
crew. If refueling is not an option, additional
fuel storage will be required.                      5-8.5   Preparing for Emergency Anchor-
                                                            ing of the Tow
5-8.3.3 Pre-staging Hoses
                                                    Anchoring the tow in an emergency should be
Consideration must be given to the capabili-
                                                    considered. The following provisions should
ties of a boarding crew to rig hoses and oper-
                                                    be made when preparing the tow:
ate the pumps. Hoses can be pre-staged to the
maximum extent possible, but watertight in-            • Provide sufficient ground tackle or oth-
tegrity should not be sacrificed to rig hoses in         er anchor-handling equipment. The an-
advance. Locating suctions in the bilge and              choring system should allow anchoring
running hoses throughout a compartment will              in a minimum of 60 feet of water with a
eliminate a lot of effort by the boarding crew           scope to depth ratio of 3:1. If deeper
and save valuable time in an emergency. Hos-             capacity is required for the proposed
es should be rigged as high up and as near to            route, maintain a 3:1 ratio. The ship’s
the compartment access as possible. Final                anchor and chain may be used if in ser-
connections can be completed in the event                viceable condition. If other jewelry is
that a decision to open the compartment and              brought on board, it should be of simi-
run pumps is made.                                       lar size to the ship’s normal anchor.
                                                       • It may be necessary to seal the chain
5-8.4   Marking Access Areas on Tow
                                                         hawse pipe to prevent water from enter-
A riding crew or boarding party from the tug             ing compartments or tanks through this
may find itself in an unfamiliar setting on a            opening. The simplest method of seal-
large tow. The preparing activity should es-             ing a chain hawse pipe is to pack it with
tablish route markings to areas susceptible to           cloth filler and plug with cement.
either flooding or fire. Painted route markings
                                                       • Consideration should also be given to
from a central location and/or from the board-
                                                         power requirements for raising the an-
ing point would allow personnel to go by the
                                                         chor if the tow will be anchored at the
most direct route to the scene of possible
                                                         port of delivery.
emergency. Established route markings to aid
a security patrol in making his rounds also            • The anchoring system should be rigged
would eliminate missed areas, adding to the              for quick release, it can be rigged on a
efficiency of the patrol. Whenever possible a            specially made billboard (see Figure
potential boarding party should become fa-               5-8). Chain and wire should be able to
miliar with the tow prior to getting underway.           run over the side without risk of ob-


5-24
                     U.S. Navy Towing Manual




Cargo Netting




                                               W elded Rungs




    Jacob’s Ladder




Figure 5-7. Sample Provisions for Emergency Boarding of Tow at Sea.




                                                                      5-25
                                       U.S. Navy Towing Manual


        struction. The bitter end of the ground      cate shall be issued to the Commanding Of-
        tackle should be connected to a padeye       ficer or the Master of the vessel as well as the
        or other fixture capable of withstanding     sponsoring command.
        anchoring loads.
                                                     5-9.3   Tow Hawser Certification
5-9 Completing the Checklist for                     The towing vessel shall provide a certifica-
    Ocean Tows                                       tion of the tow hawser with respect to its in-
                                                     spection, construction, and type.
Appendix H provides a simple checklist for
preparing a vessel for ocean tow. The check-         5-9.4 Commercial Vessels (U.S. Coast
list is to be used by the preparing activity to      Guard Inspected) Master’s Towing Certifi-
aid in preparing a tow for sea and acceptance        cate
by the towing unit. It lists general require-        The towing vessel shall provide a copy of the
ments, most of which must be completed be-           Master’s Towing Certificate that became ef-
fore a towing unit will accept it for sea. If the    fective by the USCG TASC of 21 May 2001.
preparing activity has questions concerning
this checklist or preparations required to           5-9.5   Preparing for a Riding Crew
ready the tow, it should communicate via
                                                     After receiving approval to use a riding crew,
message or phone with the towing unit or its
                                                     the Commanding Officer or the Officer-in-
Immediate Superior in Command (ISIC). The
                                                     Charge of the riding crew must ensure that:
preparing activity must fully complete this
checklist. Items which are not applicable or            • Adequate training and drills are per-
cannot be accomplished must be cleared                    formed. These include fire fighting;
through the towing unit’s ISIC or the towing              flooding and other material condition
unit.                                                     drills; drills for abandoning ship, boat
                                                          launching, communications with the
5-9.1    Determining Seaworthiness                        tug and securing a secondary towline.
To be considered seaworthy, a towed vessel              • Security watches of machinery, water-
must have adequate watertight integrity,                  tight integrity, the towline, navigational
structural soundness, and intact stability.               lights, communications, and other
                                                          watches as necessary shall be stationed.
A representative of the preparing command
shall complete a Certificate of Seaworthi-              • There is an adequate method of board-
ness for ocean tows. The certificate includes             ing the tow at sea. When feasible, fixed
general characteristics, type of cargo, towing            ladder rungs are preferred. Figure 5-7
gear, lights, speed limitation, and similar               depicts several methods for boarding
items. A sample Certificate of Seaworthi-                 ladders.
ness and its endorsements can be found in               • Radios, pumps, hoses, tools, fire-fight-
Appendix H.                                               ing equipment, and handling gear are
                                                          positioned and ready for use by the
5-9.2 Towing Machine/ Towing Winch
                                                          riding crew or tug personnel who board
Certification
                                                          the tow. The towing plan also considers
The towing machine/towing winch shall be                  requirements for messing and berthing
inspected and tested prior to the tow by a                quarters for the riding crew, auxiliary
NAVSEA designated representative. After all               power, fuel, damage-control equip-
discrepancies are addressed, an annual certifi-           ment, and life-saving gear.


5-26
U.S. Navy Towing Manual




                          Line to
                          Crow n Buoy




  Figure 5-8. Billboard




                                        5-27
                                        U.S. Navy Towing Manual


   • Communication between ships is pro-                 • Inspect the towline, bridle, and associ-
     vided as stated in Section 6-2.9.                     ated towing gear for wear and to ensure
                                                           that improper substitutions have not
5-10 Accepting the Tow                                     been made in fittings and materials.
                                                           Typical items to look for include:
5-10.1 Inspecting the Tow
                                                             — Mild steel substituted for forged
Prior to accepting a tow, the Commanding                       steel in safety shackle pins.
Officer or Master of the towing ship must in-                — Stainless steel substituted for
spect the tow to confirm its seaworthiness and                 other high strength alloys.
readiness for tow. The inspection should in-
                                                             — Improperly sized components.
clude, but not be limited to items listed in
Section 5-7 and this section.                               Note whether a retrieving wire is
   • Review the towing inspection checklist,                rigged and if proper mooring lines are
     shown in Appendix H, to ensure it is                   available.
     thorough, adequate, and properly com-
                                                         • Ensure cargo on the tow is properly
     pleted.
                                                           secured to prevent shifting in heavy
   • Inspect tow rig, appendages, and at-                  weather.
     tachment point to ensure that the tow is
     properly rigged per, applicable instruc-            • Ensure liquid cargo tanks are pressed
     tions, or guidance from the tow spon-                 full or left empty.
     sor.
                                                                       WARNING
                   WARNING                                  Use the applicable safety pre-
                                                            cautions for entering voids and
       Substituting materials can be                        unventilated spaces. Failure to
       dangerous as well as detrimen-                       do so may result in injury or
       tal to the tow. Substitutions shall                  death to personnel.
       not be made unless there is a
       complete knowledge of the ma-
       terial being substituted. Material                • Check all accessible spaces to make sure
       substitutes frequently introduce                    they are completely dry and watertight.
       a new and unpredictable weak
       link. Substituting a stronger ma-                 • Check to ensure that vents to tanks and
       terial may change the location of
       the potential failure point in the
                                                           other closed spaces are properly cov-
       rig to a position that is hazard-                   ered or sealed.
       ous to personnel.
                                                         • Ensure hatches, scuttles, doors, port-
                                                           holes, and other watertight closures are
                   CAUTION                                 provided with pliable gaskets and that
       A screw-pin shackle shall not be                    material condition ZEBRA is set.
       used as a replacement for a safe-
       ty shackle in towing. A safety                    • Ensure that running lights and flooding
       shackle will deform under load                      alarms are operating properly, that
       and still hold, while a screw-pin                   batteries are fully charged and battery
       shackle's pin can work itself out of                life is computed to be sufficient for the
       the shackle.                                        transit.


5-28
                                      U.S. Navy Towing Manual


   • Ensure any required salvage pumps              5-10.4 Rejecting the Tow
     and associated equipment with fuel are
     safely stowed on board the tow.                If the tow is in such poor condition that
                                                    towing would potentially endanger the tow or
   • Ensure that any required fire fighting         the tow ship, the towing unit may reject the
     equipment with fuel, hoses, chemicals,         tow. Every effort should be made to correct
     and overhaul gear, is safely stowed on         any unsatisfactory conditions prior to reach-
     board. Require an operational                  ing the decision to reject a tow. But if the
     demonstration that fire pumps can take         Commanding Officer or Master of the towing
     a suction.                                     ship feels that the tow poses a serious risk, he
                                                    should notify his operational commander
   • Ensure that all high-value items on the
                                                    stating why the tow is unsatisfactory. The
     tow are locked up and inventoried on
     the tow report form.                           report should include recommendations for
                                                    correcting the deficiencies.
   • Ensure that provisions have been made
     for quickly releasing the towline in an        5-10.5 Preparing for Departure
     emergency.
                                                    With all other prerequisites completed, the
   • Ensure a provision has been made for           suggested items to complete prior to depar-
     streaming a pickup line for the second-        ture include:
     ary towline.
                                                       • Reconfirm the date and time of depar-
5-10.2 Unconditionally Accepting the Tow                 ture with tasking authorities

Upon satisfactory completion of the tow                • Recheck the weather forecast and sug-
preparations and inspection, the Command-                gested track immediately prior to de-
ing Officer or Master of the tug shall accept            parture.
the tow, notify his operational commander,
and proceed with the mission.                          • Discuss harbor maneuvers with local
                                                         tug operators. A final tow conference of
                                                         all parties involved with local charts
5-10.3 Accepting the Tow as a Calculated
       Risk                                              will provide a forum for clearing any
                                                         uncertainty about maneuvers. This is
If unsatisfactory conditions of seaworthiness            particularly useful when accepting a
or readiness are found and the differences               tow in an unfamiliar port.
cannot be resolved at a local level, the Com-
manding Officer or Master of the towing ship        5-11 Completing the Delivery Letter
should notify his operational commander stat-
                                                         or Message
ing why the tow is unsatisfactory. The report
should include recommendations for correct-
ing each deficiency. If conditions or circum-       Once the tow has been completed, the Com-
stances are such that a calculated risk is in-      manding Officer of the receiving activity will
volved, the Commanding Officer or Master            complete a delivery letter confirming receipt
of the towing ship should state that he will ac-    of the tow. A sample delivery letter is includ-
cept the tow only on a calculated risk basis.       ed in Appendix H.


                                                                                               5-29
              U.S. Navy Towing Manual




       This Page is Intentionally Left Blank




5-30
                                       U.S. Navy Towing Manual


                                                     When getting a tow underway, always build up
                 Chapter 6                           speed slowly. Judicious acceleration and de-
                                                     celeration prevent damage to the towing gear.
      TOWING PROCEDURES                              Sudden speed increases will cause dramatic in-
                                                     creases in towline tension and potentially
                                                     place the tow and crew in danger. An increase
6-1 Introduction                                     in towline scope should accompany speed in-
                                                     creases. This will help maintain catenary depth
This chapter will provide some guidelines for        and reduce towline tensions. Good communi-
operating while underway with a tow, pick-           cation between the tow ship and any assist tugs
ing-up a tow, and releasing a tow. This infor-       is also necessary for a safe underway.
mation represents the cumulative knowledge
of many operators gained during years of             Frequently the tow begins in restricted waters
towing. Although this will provide guidance          or a narrow channel. Beam winds or waves
for a number of situations, each tow is a            may force the tow out of its channel or into
unique event with its own unique hazards.            the path of other ship traffic. Even if the tow
Caution and adherence to safety guidelines           has operable steering machinery, the initial
will help minimize risk to personnel during          towing speed is often insufficient for control.
this dangerous evolution.                            For these reasons, it is prudent to retain har-
                                                     bor tugs alongside the tow, or at least close
6-2 Initiating the Tow                               by, until the towing ship’s Commanding Of-
                                                     ficer has control of the tow within navigation-
A tow can be picked up at a pier, in the             al constraints.
stream, or at anchorage. When rescuing a dis-
abled vessel or recovering a lost tow, it may        Tow resistance increases with speed, yet
be necessary to pick up a tow at sea. Ocean-         water depth may not permit sufficient hawser
going tugs should not be asked to maneuver           payout to establish a catenary. A towing
unassisted in restricted waters. If possible, the    machine’s automatic features are especially
tow should be delivered to the ocean-going           useful in this situation. Also, synthetic
tug by harbor tugs. At the very least, harbor        springs can provide an excellent means of
tugs should be available to assist the tug and       tension reduction while getting underway (see
tow to navigable waters.                             Section 4-6.5).
Positive communication between the tow
ship, pilot, and assist tugs is essential and        6-2.2   Getting Underway from a Pier
should be established as early as practical.
                                                     Getting underway from a pier with a tow re-
6-2.1 Accelerating with a Tow                        quires that the Conning Officer be particular-
                                                     ly aware of tides, currents, and wind. In addi-
                   CAUTION                           tion, the Conning Officer should discuss
                                                     intended procedures with the harbor tug mas-
       When picking up a tow, increase
                                                     ter and pilot before getting underway.
       speed slowly and gradually and
       maintain an even strain on the tow-
       ing gear. If a tow hawser tension             When determining tugs to be used for assis-
       readout is not available on the               tance, consideration must be given to expect-
       bridge, have this information pro-            ed sea conditions. The size and number of
       vided by the Towing Watch.                    tugs must be sufficient to control the tow until


                                                                                                 6-1
6-2
                                                                  M essenger                   Stop              W ire Strap




                                                                                                            Pendant
                                                                Shackle

                                                                                             8 - 10 Feet


                                                                               21 - Thread Stops
                                                                                                                                         U.S. Navy Towing Manual




                                                                                                                               Pendant




      Figure 6-1. Methods for Securing Messenger to Towline.
                                                                                    Half - Hitch           Rolling Hitch

                                                               M essenger
                                          U.S. Navy Towing Manual


the tow ship can establish sufficient speed to          If the harbor tugs are limited in their control
take control.                                           of the tow or are not available (rescue towing)
                                                        or if the tug desires to control the tow with its
                    CAUTION                             own power, riding lines can be used (see Fig-
      Care should be exercised when
                                                        ure 6-2). When the tow is brought close to a
      alongside in a seaway. The mo-                    tug’s stern, a riding slip line is rigged with its
      tions of the tug and tow may be                   eye on the bitts and then passed to the tow,
      sufficient to part mooring lines, re-             reeved through a suitable deck chock on the
      sulting in damage and causing the                 tow, and led back to bitts on the tug. A second
      tug to lose control of the tow.                   riding line may be rigged for increased con-
                                                        trol. A messenger line is then sent to the tow
Good communication between the tow ship                 and attached to the primary pendant. The pri-
and harbor tugs is critical at this phase. If the       mary tow pendant is heaved in and the tow
tow and tug are not kept in line, at a near con-        connection is made.
stant distance, large strains and damaged tow
gear could result. If the tow gear breaks, the          Using two riding lines is also a good method
harbor tug should be large enough to keep the           for lateral control, especially when towing a
tow under control to avoid a catastrophe.               small vessel at very short scope in shallow re-
                                                        stricted waters prior to final streaming of the
Once the towing ship and the tow are in the             tow.
channel, the towline should be set at short
stay in keeping with the depth of confined              Regardless of attachment method, it is best to
waters to be crossed. Keep the catenary shal-           have all items to be used in passing the pen-
low to avoid snags.                                     dant rigged on the tow before leaving the pier.
                                                        An evaluation of the capabilities of the assets
6-2.3 Getting Underway in the Stream                    available should be made when deciding the
                                                        correct method for hook-up.
At times it is necessary to accept a tow in the
stream. In this case, use the following procedure.      6-2.4   Getting Underway while at Anchor
The approximate channel course should be taken
by the tow ship with bare steerage and assisting        At times it is necessary for the tug to make up
tugs should bring the tow to the tug’s stern.           to a tow with either or both vessels at anchor.
                                                        This may be due to limited pier space, shal-
Heaving lines are used to send a messenger              low harbors, or simply the master’s prefer-
line to the tow which is then attached to the           ence. Suggested procedures for getting under-
primary pendant (see Figure 6-1). Depending             way in several situations are listed below.
on the height of the tow’s bow or other con-
                                                            • Tug underway/tow anchored. In mod-
figuration considerations, it may be desirable
                                                              erate seas, the tug should come along-
to send a heaving line from the tow to the tug.
                                                              side the anchored tow and tie up with
Either way, the tug should always have spare
                                                              her stern as close as possible to the bow
heaving lines on deck in case they are needed.
                                                              of the tow. The tow then passes a line to
Once a messenger is passed, the pendant is
                                                              the tug, which is used to pull a messen-
heaved in and the tow connection is made.
                                                              ger and then a portion of the tow’s
                                                              chain pendant to the tug. As the chain
                   CAUTION
                                                              comes down on the tug’s faintail, a
      The tow should be steadied on the                       stopper is passed on it to restrain it
      riding lines p rior to attempting                       while the tug’s crew rigs the remaining
      hookup. Surging can produce high                        towline connection. When the connec-
      loads on the riding lines very quickly.                 tion is made, the chain stopper is re-
                                                                                                      6-3
                                        U.S. Navy Towing Manual


        leased and the tug maneuvers clear. As-               dant or lead chain is passed to the stern
        sistance of a harbor tug is usually                   of the tug. Using the tug’s stern cap-
        required. When headed fair, the tow                   stan, a messenger is heaved on board
        weighs anchor, once the anchor is                     until a sufficient amount of chain is
        housed, the tug can start ahead slowly                brought on board to pass a chain stop-
        accelerating. Significant time is re-                 per. The connection is made, the chain
        quired to establish sufficient catenary in            stopper released, and wire paid out as
        the tow hawser and come up to towing                  appropriate. The tug weighs anchor and
        speed. If the tow has no power to its an-             begins accelerating at a very slow rate
        chor windlass, the crew should rig an                 of speed. This method is safe, simple,
        appropriate retrieval line and buoy so                and expeditious.
        that the anchor can be slipped and re-
        covered later.                                6-2.5    Recovering a Lost Tow
        If unfavorable conditions for going
        alongside prevail, passing the hawser         There are occasions when a tug must recover
        can be difficult. Expert seamanship is        a lost tow at sea. Towline chafing, a mechani-
        required to prevent the tug from drifting     cal break, or other circumstances may cause
        out of range on a downwind approach.          the tow to separate from the tug, making it
        It may be preferable to anchor, as dis-       necessary to recover the tow. In other cases,
        cussed below.                                 the original tug may become disabled or even
                                                      abandon a tow. Procedures used to recover
      • Tug anchored/tow anchored. Rather             the lost tow will be affected by the presence
        than passing the towline while under-         of personnel on the tow, sea and weather con-
        way, it is often advantageous for the tug     ditions, existing contingency plans, and assets
        to anchor upwind or upcurrent from a          available. See Section 6-2.7 for a discussion
        large ship. While at anchor, the tug can      of approaching a drifting tow.
        prepare the towline for passing. The tug
        veers its anchor chain until within a            • If the tow is unmanned and the weather
        short distance of the tow’s bow. When              and seas favorable, a boarding party
        the tug’s stern is close aboard the tow’s          may be put on board the tow, a messen-
        bow, the towline can be passed and the             ger passed, and the tow reconnected by
        connection made. With the towline                  routine procedures. The risks involved
        connected, the tug can use its engines to          in sending a boarding party and the dif-
        come ahead and weigh its anchor, veer-             ficulty of passing a new towline justify
        ing towline as necessary. With the tug             rigging a secondary, emergency tow-
        free to navigate, the tow weighs anchor            line. If the emergency towline has been
        and the tow commences. If the tow                  used, consider rigging another emer-
        does not have power, it may be neces-              gency towline.
        sary to slip the chain and anchor and
        mark the anchor’s position with a buoy
                                                         • If the tow is unmanned and the weather
        for later retrieval.
                                                           does not permit sending a boarding par-
      • Tug anchored/tow underway with                     ty, the tow ship should attempt to re-
        steering tugs. The tug anchors and set-            trieve the secondary pendant by means
        tles out into the wind and current. A              of the floating pendant or marker buoy.
        steering tug brings the tow up to the              The tow ship can either recover this us-
        stern into the current or wind. A pen-             ing one of its small boats or by grap-


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                          U.S. Navy Towing Manual




                                              Yard tug hold station so
                                              tow does not overrun tug.


                                        Lead Chain
       Tug has slight w ay on.            or W ire
                                         Pendant        Bitts
                     Riding Line
                                                                Pad




                                           Leave end
Tow Haw ser                                free up to tow
                          M essenger       haw ser or pendant
                             hauls         supplied by tug.
              Hogging       tow ing
                Strap      pendant
               Laying       aboard.
                Lazy




                                   NOTE

                  Riding lines may not be necessary if
                  there is sufficient tug power available
                  to control the tow.




                  Figure 6-2. Accepting a Tow in the Stream.




                                                                          6-5
                                        U.S. Navy Towing Manual


        pling the floating pendant directly from      For situations where a padeye must be welded
        the tow ship. The secondary tow pen-          to the deck, refer to Section 4-5.4 and Figure
        dant is rigged to deploy as the tow ship      4-7 for acceptable padeye design specifica-
        takes a strain. (See Section 4-4.)            tions. These figures provide means for con-
                                                      structing a well-designed padeye. In an emer-
      • If the tow is manned, it may still be         gency situation, however, when detailed
        necessary to send a boarding party on         calculations cannot be performed, it is recom-
        board. If the riding crew is not suffi-       mended that the largest available material be
        ciently large or able to safely and ade-      used. These calculations can be performed af-
        quately handle re-rigging of the tow,         ter installation, when the tow is out of danger,
        the tug should provide knowledgeable          as a check against proposed towing speeds. If
        assistance.                                   the installed padeye is too small, speed
                                                      should be limited until a more appropriate
      • The tug may use one or more of its            padeye can be constructed.
        small boats to act as a warping tug on a
                                                      All towing bridles, when rigged correctly,
        drifting tow, if the tow is not too large.
                                                      must have a backup securing system. This is
        The small boat can keep way on the tow
                                                      normally accomplished by using wire rope of
        near shoal water, or maintain a tows
                                                      appropriate size (able to lace through chain
        head into the seas, thereby facilitating
                                                      links) and taking sufficient bights of wire
        recovery. The small boat may also
                                                      from a second securing point (bitts, heavy
        change the heading of the tow as neces-
                                                      cleats, etc.) and lacing the wire rope through
        sary.
                                                      the after end of links in the chain bridle (no
                                                      less than four bights). Size and number of
6-2.6    Emergency Connection to a Dis-               bights of wire should equal the strength of the
         abled Vessel or Derelict
                                                      chain used in the bridle. If a towing pad is
Devising a means of attachment is a critical          used to connect the bridle to the tow, the
concern when rescuing a disabled vessel or            backup wires must be laced forward of the
derelict. This is particularly important in the       towing pads. The securing point should be aft
case of rescue towing, when time and shore-           of the towing pad to prevent snap-loading. If
side support may not be available for install-        a set of mooring bitts is used as a securing
ing padeyes and fairleads. Suggested attach-          point for the bridle on the tow, the wire
ment points of sufficient strength to tow in an       should be laced thorough the chain links that
emergency include:                                    remain astern of the bitts after the three or
                                                      more “figure eights” are secured on the bitts.
      • Using the ship’s anchor chain                 There must be a sufficient number of wire
      • Using installed bitts or padeyes              clips (see Table 4-1) on each bitter end of the
      • Wrapping a chain around a foundation          backup wire, aligned in the same direction
        structure such as a gun mount or winch        (See Appendix I and Appendix J for tow rig
      • Welding a padeye to the deck                  design plans.)

The preferred methods are to use the ship’s           It may not always be possible or practical to
anchor chain or installed padeyes. The other          rig a backup system (i.e., submarine towing).
methods are to be used in emergency situa-            In these cases, additional analysis of the main
tions and may be necessary due to damage to           to wi ng a t ta c hm en t ma y pr ov id e som e
the tow or other unusual operating con-               reduction in uncertainty. Where possible, the
straints.                                             attachment should be designed to a breaking


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                                                  U.S. Navy Towing Manual




                                                              To w in g
                                       W ire
                                     P end an t
                  S hack le                                               B acku p B itts
                                                  F irst B itts           (S ee D etail B )
                                                  O n e Tu rn O n ly
                                                  (S ee D etail A )
             C h ain
            P end an t




                        W ire                                                            To
                                                                                      B acku p
                     P end an ts                                                        B itts
                                                                       N o te O ne
                                                                       Tu rn O n ly
    To C hain
    P end an ts                              D eta il A                                  O n e R o u n d Tu rn        D eta il B
                                           L ead B itts                               T h en ‘F igu re Eigh ts ’   B acku p B itts




                                   Figure 6-3. Sharing Towing Load Between Bitts.


strength well in excess of the other com-                                  The usual method is to stop off the anchor
ponents.                                                                   and break the chain. Make sure that the in-
However the attachment point is affected, it                               board section will not be pulled down into the
may also be necessary to cut through the bul-                              chain locker due to its own weight after it is
wark or to remove other fittings from the                                  cut. This can be accomplished by rigging two
deck in order to provide a clean sweep for the                             stoppers and cutting between them. In an
towing pendant. When rigging a special at-                                 emergency situation, it may be easiest to cut
tachment for towing, twin problems of attach-                              the chain and lose the anchor. However, it is
ment point and fairlead must be resolved.                                  safest and economical to rig stoppers and
                                                                           save the anchor.
6-2.6.1 Using the Anchor Chain
                                                                           Next, connect the bitter end of the chain di-
Often the simplest, strongest, and most effi-
                                                                           rectly to the towing pendant brought through
cient connection method is to shackle the
                                                                           an appropriate deck edge chock. The anchor
pendant into the tow’s anchor chain with the
                                                                           chain can then be veered to provide chafing
correct connecting link.
                                                                           protection and any desired additional catena-
                                                                           ry to the towline system for improved dynam-
                        WARNING
                                                                           ic load mitigation. In this case, the ship’s
      In no case should the stud of the                                    chain stopper system may not align ideally
      common chain link be removed                                         with the fleet angle of the chain, but in most
      to provide a connecting point to                                     cases the alignment will be sufficient. If the
      a chain.
                                                                           alignment produces sharp bends or other po-


                                                                                                                                     6-7
                                             U.S. Navy Towing Manual



                                     TABLE 6-1. Information on U.S. Navy Bitts.




                                                                        Maximum        Maximum
                      A           B             C                                                    Maximum
                                                        Maximum           Pull at       Pull at
      Nominal      Barrel        Top          Barrel                                                   Size
                                                        Moment*           Upper          Mid-
      Bitt Size     Size        Plate        Height                                                  Synthetic
                                                        (inch-lbs)        Edge*         Barrel*
                  (inches)    (inches)      (inches)                                                   Line
                                                                        (pounds)       (pounds)

       4           4 1/2        6            10          134,000         13,400         26,800        3

       8           8 5/8        11 1/2       13          475,000         36,500         73,000        5

       10          10 3/4       13 3/4       17          1,046,000       61,500         123,000       6 1/2

       12          12 3/4       15 3/4       21          1,901,000       90,500         181,000       8

       14          14           17           26          3,601,000       138,500        277,000       10

       18          18           21           32          6,672,000       208,500        417,000       12

       *These numbers are safe working load with a factor of safety of 3 on Material Ultimate Strength.


tential failure spots, this area should be in-                lines will have different breaking strengths,
spected periodically and appropriate opera-                   Table 6-1 lists the capacities of Navy bitts.
tional steps taken to reduce risk of a failure.               The chart also contains some typical dimen-
                                                              sions that will help to identify existing bitts
Another method involving a tow’s anchor
                                                              and shows how each of these dimensions are
chain is to suspend the anchor from a wire
                                                              measured. The maximum pull can be applied
strap, or cut it loose completely, and tow
                                                              to either barrel (not both), in any direction.
through the hawsepipe. The rigging for this
procedure is complex and sometimes hazard-                    The strength criterion for bitts in commercial
ous. Furthermore, this method often results in                ships is similar, except older ships and Navy
the chain bearing against a sharp forward or                  support craft often have been designed for
upper outer lip of the hawsepipe, which may                   manila mooring lines. Consider this when
consist of a much smaller radius than would                   employing bitts for towing of commercial or
be ideal for chain.                                           older Navy ships. In all cases, the strength of
6-2.6.2 Using Installed Bitts                                 the bitts must be discounted if obvious corro-
                                                              sion or poor maintenance is evident.
Mooring bitts are a possible choice for secur-
ing a tow hawser. U.S. Navy bitts are de-                     Attaching a chain directly to the typical-sized
signed to withstand the breaking strength of                  bitts found aboard ships is feasible, but re-
the mooring line for which they are designed,                 moving slack is difficult. Such a connection
with a factor of safety of 3 on ultimate                      is susceptible to shock load from sudden ren-
strength. Since different types of synthetic                  dering and has a higher possibility of failure.


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                                      U.S. Navy Towing Manual


An improved connection where slack can be           6-2.6.4 Placing a Crew on Board
minimized can be made using wire that is the
same size as the towing pendant. (See Figure                          WARNING
6-3.)
                                                           Boarding a derelict vessel can
In Figure 6-3, note that the chain provides                present many unknown haz-
chafing protection at the deck edge, but wire              ards. Safety is paramount during
                                                           these operations.
is used to make the final connection. As stat-
ed earlier, when using mooring bitts as an at-
tachment point, a backup securing system            In an emergency, the presence of a function-
should be used. The reason for using backup         ing crew aboard a disabled ship is of consid-
bitts is to share the load. To accomplish this,     erable help when making the connection. If
the loaded part should make only one turn           the ship has auxiliary power and is able to op-
around one barrel of the first bitts. The first     erate its anchor windlass or other winches,
turn will absorb 50 to 75 percent of the total      passing the towline assembly is a relatively
load on the wire, depending on the coefficient      simple task, complicated only by adverse sea
of friction, and pass along 25 to 50 percent to     and weather conditions.
the backup bitts. The wire should be secured
                                                    Connecting to a derelict poses the immediate
to the second bitts by making one turn on the
                                                    problem of placing a boarding party on board.
first barrel and then making figure-eights
                                                    A derelict vessel can present many unknown
with the remaining line. Backing up to a third
                                                    hazards to personnel. The boarding crew
set of bitts is not necessary.
                                                    should consider personnel safety as para-
If two turns are taken around the first set of      mount. If any potentially dangerous condi-
bitts, only about 6 to 12 percent of the total      tions were found during the pre-tow inspec-
load is passed on to the second bitts. Thus, ef-    tion, these should be briefed to the boarding
fective load sharing is voided.                     party prior to attempting to board the tow.

If the wire required is too large to fit on the     If there are no means of boarding, grapnels
bitts, synthetic line may be used. This syn-        may be heaved on deck or fabricated pipe
thetic line is subject to the same restrictions     boarding ladders may be used to get a man
as synthetic towing hawsers. Minimum bend           aboard. This person can then lower more con-
radius for all components should be checked.        ventional means such as a Jacob’s ladder. The
The same principles are applicable to synthet-      boarding party may have to carry an assort-
ic line load sharing.                               ment of tools and rigging devices to help haul
                                                    the messenger on board and hook up a tow.
When using mooring bitts as bridle attach-          These tools may include:
ment points, heavy channel iron must be
welded across the bitts to prevent the bridle          •   Welding and cutting equipment
from jumping out.                                      •   Various size shackles
                                                       •   Wire straps
6-2.6.3 Using a Gun Mount or Foundation
                                                       •   Rigging lines
Another way to make an attachment is to pass           •   Battle lanterns
a chain around a gun mount or foundation of
                                                       •   Personal safety gear
a deck machinery installation or to rig a wire
rope strap with a large eye on one end around          •   Sheaves for rigging
the bitts (see Figure 4-11).                           •   Hand-held radios


                                                                                               6-9
                                      U.S. Navy Towing Manual


6-2.7   Approaching a Drifting Tow                  Figure 6-5. When approaching a ship lying
                                                    broadside to the wind, tug speed should be
There are as many variations of approaching
                                                    slow, but fast enough to offer good steerage-
a drifting tow as there are variables in wind
                                                    way. Because on-station time is short, a mes-
and sea. Good seamanship is required to ap-
                                                    senger must be passed quickly. The towline
proach and safely take in a drifting tow of any
                                                    can be passed in the lee of the ship’s bow.
size. Absolute coordination between the Con-
                                                    This situation requires a special effort to keep
ning Officer and the fantail crew is essential.
                                                    all lines clear of the propellers. Once connect-
Direct communication with personnel on the
                                                    ed, acceleration should be slow and maneu-
tow and all parties is crucial.
                                                    vering sequences gradual.
6-2.7.1 Establishing the Relative Drift

The first step in approaching a tow to be                                CAUTION
picked up at sea is to establish differential               Approaching at too small an angle
drift between the vessels involved. This is                 in the lee of a larger vessel can be
critical for positioning the tow properly and               dangerous. When working in the
avoiding a collision. Despite obvious differ-               lee of a larger ship, establish an at-
ences in size and configuration, vessels’ rates             titude that permits the tug to main-
                                                            tain a safe distance from the more
of drift are also affected by a host of other               rapidly drifting tow.
variables, including displacement, draft, sta-
bility, trim, damage, seas, wind, sail area, lo-
cation of the superstructure, and currents. The     6-2.8    Passing the Towline
above water hull configuration determines the       A towline is passed by messenger to the tow.
tow’s relative heading into the wind. Depend-       It is generally preferable to have the tug pass
ing on trim, ships having a greater portion of      the messenger and towline. The messenger
their superstructure aft tend to head into the      may be passed by a hand-thrown heaving
wind; ships having a greater portion of super-      line, rocket, line-throwing gun, small boat,
structure forward tend to lie with the wind         buoyant float, helicopter, or any other expedi-
from aft of the beam to astern. A midship su-       ent means. The hand-thrown heaving line,
perstructure will normally cause a ship to lie      backed up with a line-throwing gun, is a com-
with the wind abeam. With relative drift be-        mon and practical way of passing a messen-
tween tug and tow determined, and the state         ger. An experienced seaman, under favorable
of the seas and wind taken into consideration,      circumstances, can accurately throw a heav-
the tug can make its approach.                      ing line over 100 feet. Backup heaving lines
6-2.7.2 Similar Drift Rate                          should be coiled and ready on deck to mini-
                                                    mize time between attempts, should the first
Figure 6-4 describes a tug’s approach across        attempt fail. Time considerations and atten-
the wind and seas where similar drift rates ex-     dant dangers, however, make it prudent to
ist. The tug begins an approach leading to          give as much time as possible to pass the
pass close aboard on the weather bow; the           messenger. Use of a line-throwing gun, there-
messenger and towline can then be passed.           fore, is the preferable procedure.
The tug keeps station while passing messen-
gers and making the connection.                        • In some cases it may be imprudent to
                                                         navigate close to a distressed ship. In
6-2.7.3 Dissimilar Drift Rate
                                                         this event, a boat can be used to pass
Where dissimilar drift rates exist, a down-              the messenger. Line, free for running,
wind approach may be executed, as seen in                should be faked down in the boat and


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                U.S. Navy Towing Manual




Figure 6-4. Across Sea/Wind Approach - Similar Drift Rate.




                                                             6-11
                                          U.S. Navy Towing Manual


        on board the tug, with the maximum              6-3 Ship Handling and Maneuvering
        amount possible in the boat.                        with a Tow

   • Buoys, life jackets, salvage floats, foam
     fenders, or drums can be attached to the                                CAUTION
     messenger’s bitter end and floated to                      Small increments of rudder angle
     the distressed ship. This can be expedit-                  are recommended when changing
     ed by the tug crossing the disabled                        course under tow. This will ensure
     s h i p ’s b o w w i t h t h e m e s s e n g e r           that the tug maintains control of the
     streamed.                                                  tow and prevents the tow from
                                                                ranging up on the tug. Never permit
   • Line-throwing guns can carry the bitter                    the tow to pass forward of the tug's
                                                                beam, as the tug or tow hawser
     end of the messenger; an experienced                       may be severely damaged.
     seaman can safely and accurately fire
     the gun a distance of over 300 feet. A
     heaving line can also be used effective-           When the tow is underway, the tug begins to
     ly for shorter passing distances.                  accelerate slowly to towing speed. Rudder or-
                                                        ders should permit slow and orderly course
After a sufficient length of the initial messen-        changes. It is important not to subject a tow
ger is on board, it may be run through a block          or towline to excessive dynamic loading.
and the bitter end passed back to the tug               Slow course and speed changes will prevent
where the tug’s machinery can haul the heavy            excessive strain. If an automatic towing ma-
messenger and towing assembly on deck. The              chine is installed, a low tension setting can be
tow pendant is then made up to an available             employed and the tow streamed as speed is
strong point on the derelict.                           increased. Once desired scope is achieved,
6-2.9    Communications between Ships
                                                        the setting on the automatic towing machine
                                                        may be increased to the desired value.
In a towing situation, most communication               6-3.1    Tug Steering
between ships is by radio. Loss of radio, radio
                                                        Maneuvering characteristics of the tug can be
silence, weather, or foreign language barriers
                                                        dramatically affected when towing another
may require an alternate means of communi-
                                                        vessel. The ability of the tug to maneuver it-
cating. The most commonly accepted meth-
                                                        self under all conditions is essential.
ods for communicating between ships at sea
are identified in the International Code of             The position of the tow point (the point where
Signals, Communicating Ship-to-Ship NWP -               towline tension is applied to the tug) and the
14-1 (Ref. J). These are by no means the only           tension on the towline can create a moment
means of communicating. Prearranged sig-                that opposes the rudder moment and hence re-
nals and codes, as well as standard Navy pro-           stricts the turning motion of the tug. The tug’s
cedures such as those in NWP 14, are valu-              ability to steer is increasingly hampered as
able and highly useful tools available for              the tow point is located farther aft. The effect
communicating during towing operations.                 is aggravated at low or zero speed. The term




6-12
                             U.S. Navy Towing Manual




Figure 6-5. Downwind Approach Crossing the “T” to Ship Lying Broadside to Wind/Sea.




                                                                                      6-13
                                      U.S. Navy Towing Manual


“in irons” describes a condition where the op-      6-3.2   Keeping a Tug and Tow in Step
posing moment of the towline is the same as         When a tug is at sea with a tow, the two ves-
or greater than the turning moment created by       sels move distinctly and separately in surge,
rudder and other hydrodynamic forces. The           sway, heave, roll, pitch, and yaw in response
tug is then rendered incapable of steering (see     to the surface waves. The degree and timing
Figure 6-6). Being in irons can be catastroph-      of motion that either vessel experiences de-
ic for a tug, especially when maneuvering in        pend on the individual vessel’s characteris-
confined waters or in a poor orientation with       tics. No two vessels will respond to the sur-
respect to the sea. A tug also can be rendered      face waves in exactly the same pattern. In
in irons when it cannot make headway under          cases where the surface wave pattern is char-
its own power because of the towline making         acterized by a single predominant wave-
contact with the bottom. In this case, the tug      length, it may be possible to minimize the dif-
is effectively anchored by the stern. The tow,      ference in the timing of the tug and tow
however, is not anchored and may close rap-         motions. This involves adjusting the towline
idly. To avoid being run down, the tug should       scope to place the tug and the tow on crests of
shorten the wire and regain headway at once.        the predominant waves at the same time. By
                                                    placing both vessels on the crest at the same
Ideally, the position of the tow point should       moment, they will move in response to the
be located at the tug’s natural pivot point, to     waves in the same direction at approximately
allow the tug maximum freedom of rotation           the same time. Adjusting timing of a vessel
in steering. The tug’s natural pivot point is       motions in this way will reduce dynamic ten-
dependent on hull and rudder design; it is          sion in the towline. This practice has been re-
usually located on the center line at about         ferred to as keeping the tug and tow “in step.”
one-third of the tug’s length from the bow.         In tandem tows, this is rarely possible.
This is why the towing winch is mounted as          As the tug approaches shallow water, such as
far forward from the stern as possible, al-         a coastline or channel, the wave frequency
though it is doubtful that any towing winch is      will change (increase). The length of the tow
located exactly at the pivot point itself. From     should be adjusted, in conjunction with a pos-
a practical standpoint, the towing point is des-    sible change in speed.
ignated as the towing winch or towing bitts, if     Keeping “in step” applies equally to all tow-
installed.                                          ing situations, whether towing on the dog,
                                                    hook, brake, or on an automatic towing ma-
There are times, however, when the towing           chine. The benefit of being in step is lower
point is located farther aft—for example, on a      peak tensions.
Norman pin, hogging strap, or stern roller.
During long ocean tows, these configurations        6-3.3   Controlling the Tow
may be preferred since they will restrict line      6-3.3.1 Active Control of the Tow's Rudder
sweep and therefore chafing of the towline. If      The tow’s rudder can be used to stabilize an
little maneuvering is needed, moving the tow        unwieldy tow or to maneuver in close quar-
point aft may be acceptable. In any configura-      ters. Improper or excessive use of the rudder,
tion, it is imperative that the operator be         however, can cause the tow to become direc-
aware of the possible maneuvering restric-          tionally unstable. The decision to use active
tions imposed on the tug and take the neces-        steering of the tow will depend on the reli-
sary precautions to avoid being placed in           ability of the tow’s steering machinery and
irons.                                              qualifications of the riding crew. A decision


6-14
                      U.S. Navy Towing Manual




                                    Pivot point m oves tow ard
                                    the stern w hen haw ser is
                                         captured astern.
                Tow ing Tension Force                         Pivot Point
                                                             W ithout Tow



                                           Rudder Force


                                    A


                 For heavy tow line tension, the pivot
                  point m ay coincide w ith the stern
                   rollers, placing the tug in irons.




                                                       W inch

                                                                Pivot Point
                                                                W ithout Tow

                 Tow ing Tension Force


                                        Rudder Force

                                                Approxim ate
                                                 Pivot Point
                                               w ith Tow w hen
                                              Haw ser Captured
                                                   at H-Bitt
                                   B

Condition A has a decreased turning m om ent com pared to Condition B.
           Condition A can place a tug “in irons”, therefore,
   recom m ended tow point for m aneuvering is at the tow ing w inch
                  or H-bitts as show n in Condition B




              Figure 6-6. Effect of Towpoint on Steering.




                                                                               6-15
                                     U.S. Navy Towing Manual


whether to use active steering rests with the      stern makes the towed vessel less susceptible
tug.                                               to yawing.
6-3.3.2 Yawing and Sheering of the Tow             6-3.3.4 Speed

Most tows will yaw somewhat—that is, oscil-        Yaw of the tow may be increased or de-
late in heading about the base towing course,      creased with a change in speed; a range of
usually in response to wave action on the          tow speeds may be attempted in an effort to
tow’s bow or stern. This is not a serious prob-    obtain a desired reduction in yaw.
lem in itself. Many tows, however, will also       6-3.3.5 Use of Rudder or Skegs
sheer off to the side, where the tow’s track is
                                                   If the tow is tracking poorly but is steerable,
offset from the tug’s track. This may be espe-
                                                   the rudder can be used to reduce or eliminate
cially prevalent in beam winds for ships with
                                                   yawing and sheering. Active use of the rud-
large deck houses aft.
                                                   der, however, increases drag and adds the risk
The vessel may remain at a nearly constant         of steering machinery failure at a permanent
sheer angle or sheer from side to side, re-        rudder angle. Hull damage may cause the tow
maining at each side for as much as 10 min-        to take up a permanent sheer angle. In this
utes or more. Excessive sheering will cause        case, permanent adjustment of the rudder can
excessive chafing of the towing rig, addition-     significantly improve the tow’s behavior.
al strain on the towline, reduction in tow
                                                   If excessive yawing occurs on a movable
speed, and possible collision or stranding in
                                                   twin-skegged tow, each skeg can be splayed
restricted waters. In extreme cases, the tow
                                                   at an outboard angle. Although the drag will
can range up to a position abeam or even
                                                   increase, the directional stability should im-
ahead of the tug.
                                                   prove. Outboard splaying is commonly done
Sheering may be initiated by an external force     on barges and the technique has been success-
or disturbance such as wind or wave action.        fully applied to twin-ruddered ships and float-
Tows with bulbous bows tend to sheer more          ing dry docks. All such rudder or skeg move-
than those with “fine” bows. Improperly            ments should be made in moderation to
rigged bridles can also cause sheering. Legs       achieve optimum towing performance with
of unequal length can generate a sheer prob-       minimum increase in drag.
lem with the tow.                                  6-3.3.6 Location of the Attachment Point
Yaw can also lead to sheering. Depending on        A point of bridle entry into the tow may be
the tow’s inherent maneuvering characteris-        selected to offer an optimum angle, and thus
tics, the amount of yaw and sheer may range        eliminate or reduce excessive yaw or sheer.
from small to substantial. In general, a tow is    Steps must be taken to prevent towline chaf-
considered directionally unstable if the sheer     ing and to ensure that a fairlead is sufficiently
angle continues to increase from swing to          robust. As an example, the LST 1179 Class
swing, despite an absence in the force that        requires either a bridle or an off-centerline
initially caused the motion. The following         pendant because of the bow doors. Towing is
paragraphs discuss ways to control the factors     performed through a mooring chock on the
that influence yawing and sheering.                side. These ships tow quite steadily with a
6-3.3.3 Trim                                       very slight sheer.
                                                   6-3.3.7 Propellers
Before undertaking the tow, the towed vessel
should be trimmed by the stern slightly as de-     A locked propeller will create a larger drag
scribed in Section 5-7.2. Trimming by the          than a free-wheeling propeller, thereby result-


6-16
                                      U.S. Navy Towing Manual


ing in reduced towing speed. The additional         position and speed of advance must be con-
drag in the stern due to a locked propeller,        sidered to avoid collision.
however, may decrease the tendency of the
                                                    For information on anchoring with a tow, re-
vessel to sheer off from the intended track.
                                                    fer to Section 6-7.5.
Refer to Section 5-7.1.7 through 5-7.1.11 for
information on preparing the propellers for
tow.                                                6-4 Routine Procedures While at Sea

6-3.3.8 Steering Tug
                                                                       WARNING
The addition of an operational ship astern of
                                                            Motions of the tug and tow can
the tug can offer effective steering control of
                                                            cause the towline to change po-
a tow. The trailing ship can use its engines                sitions rapidly and without warn-
and rudder to maintain a light tension on a                 ing. Personnel must be aware of
line to the tow. Following steering orders                  the potential danger of a sweep-
from the tow ship, it can assist in keeping a               ing towline and remain clear of
tow from sheering off.                                      all areas that may be within this
                                                            sweep.
6-3.3.9 Sea Anchor or Drogue

An object towed from the stern of a tow will        This section deals with procedures that are
create a drag that acts to resist yawing mo-        performed at sea on a routine basis but deal
tions. Nets, anchor chain, line, wire, kite an-     specifically with towing. Each tow is unique,
chors, mine-sweeping gear, and a wide vari-         of course, and will present unique problems
ety of other drogues have been used as              and challenges, but some general guidelines
stabilizing devices on small tonnage or shal-       apply.
low draft ships, especially those with fine hull
forms. Care should be taken to prevent snag-        6-4.1   Setting Course
ging of the drogue in shallow water.                When adequate sea room is achieved, maneu-
6-3.3.10   Bridle vs. Single Lead Pendant           ver to set course and begin streaming the tow.
                                                    Do not stream to full scope until sufficient
Certain hull forms are more conducive to be-
                                                    water depth is available to keep the towline
ing towed by a single lead pendant. Subma-
                                                    from dragging.
rines and ships with bulbous bows or forward
sonar domes tow better on a single pendant          6-4.2   Towing Speed
than on a bridle. In general, fine lined ships      Safe towing speed is determined by many
should be towed with a single leg and broad         factors, including material condition of the
beamed ships towed with a bridle.                   tow, currents, sea states, towing direction rel-
6-3.4 Backing Down with a Tow                       ative to the surface waves, wind velocity and
                                                    direction, hull type of the tow, tug horsepow-
                   CAUTION                          er, capacity of the towline system, and avail-
                                                    able powering assistance from other tugs or
      Except in an emergency, backing               the tow’s power plant.
      down with a tow is not recommend-
      ed. It may be attempted if a colli-           The towing speed should be chosen to mini-
      sion with another ship is imminent.           mize the probability of damage to the tow.
                                                    When towing damaged vessels and flat-bot-
If backing down is necessary, take great care       tomed craft, try to avoid excessive seakeep-
not to foul the towline in the propeller. Tow       ing motions and pounding. When necessary,


                                                                                                6-17
                                     U.S. Navy Towing Manual


towing course and speed should be chosen           tion. Also, once the hawser is in contact with
relative to the sea state and wind direction to    the bottom, the tug no longer has control of
keep a towed vessel motions within safe lim-       the tow and is in danger of being overtaken.
its.
                                                   If the surface wave pattern has a predominant
Barges generally should not be towed faster        wavelength, attempt to adjust the towline
than about 8 knots under mild sea conditions.      scope so that tug and tow ride on crests of the
Small service craft and some dry docks             predominant wave components at the same
should be limited to about 6 knots. Deteriora-     time. Adjusting the towline this way may
tion of weather conditions requires appropri-      keep tug and tow “in step,” thus reducing
ate speed reduction to ensure continued safe       changes in towline tension caused by
towline loading. When towing larger surface        seakeeping motions (see Section 3-4.1.4).
ships, the speed limitation usually is a func-
tion of the tug’s capabilities. Sometimes,         6-4.4   Towing Watch
however, the dynamic loads induced by the          With the tow streamed, the towing watch
ship motions of a tug and tow in a seaway          must be set to observe the tow, towing loads,
will be the controlling factor in determining a    towing machine, towline, and the tow’s
safe towing speed (as opposed to the safe          seakeeping performance. The tow watch must
towing speed of the towed vessel or the capa-      routinely advise the Officer of the Deck of
bilities of the tug). Appendix M contains data     conditions observed. On board newer tugs,
about the way ship motion affects dynamic          much of the information is automatically dis-
towline loads.                                     played in the pilot house and control stations.
6-4.3   Towline Scope
                                                   6-4.5   Periodic Inspection of Tow
To estimate the towline scope required, fol-
low these steps:                                   Elements of the tow’s material condition
                                                   should be visually inspected and continuously
1. Choose a candidate scope                        monitored, even at night. They include:
2. Estimate steady towline tension (see Sec-
                                                      • Flooding and fire alarms, navigation
   tion 3-4.1.3)
                                                        lights, draft marks, and trim
3. Compute catenary (see Section 3-4.2)               • Sheer angle and seakeeping
4. Estimate maximum and minimum towline               • Timing of roll period for stability.
   tensions
                                                   To supplement the flooding alarms, tug watch
5. Ensure that catenary will not exceed wa-        personnel should be alert to signs of flooding
   ter depth at minimum tension (A maxi-           such as list, excessive drag (increase in tow-
   mum scope for the water depths expected         line tension without a change in conditions),
   should also be calculated.)                     change in roll period, or unexpected trim in
6. Adjust the scope as necessary and repeat        the tow. The towline should also be inspected
   steps 1 through 5.                              frequently for chafing and damage during a
                                                   tow.
The scope should be adjusted to provide an
adequate catenary for absorbing changes in         It is common practice to “freshen the nip” or
towline tension without exceeding water            “nip the tow” to avoid chafing. This is the
depth. Dragging a towline on the sea floor         practice of changing the scope of the towline
will damage the hawser through abrasion and        so no single point is continuously in contact
could lead to fouling on a sea floor obstruc-      with the caprail.


6-18
                                     U.S. Navy Towing Manual


6-4.5.1 Boarding the Tow for Inspection               • Check flooding and fire alarm system.
When carrying a riding crew, the crew per-            • Visually check open habitable compart-
forms most of the inspection functions. Long
                                                        ments and topside areas.
distance and valuable tows without a riding
crew should be periodically boarded and in-           • Sound any suspicious or questionable
spected, preferably by the same personnel on
                                                        voids, double bottoms, and liquid tanks.
each inspection. Because this operation is of-
ten difficult and hampered by weather and sea         • If indicated, visually check structural
conditions, inspection preparations should be
                                                        framing and hull plating in the bow.
well planned and promptly and efficiently ex-
ecuted. The following suggestions may aid             • Operationally check fire fighting and
this process:
                                                        dewatering equipment weekly, or more
   • When possible, consider seeking the lee            often if conditions warrant.
     of a land mass to make the operation
     safer, easier, and more controlled.              • Upon completing the inspection, close
                                                        and make watertight all hull access
   • Shorten up the tow, this provides an op-
     portunity to inspect the towline and any           openings. Any additional checks appro-
     part of the tow rig that can be brought            priate to the peculiarities of the tow
     aboard safely.                                     should be incorporated as needed into
                                                        the inspection checklist.
6-4.5.2 Inspection Guidelines
                                                   6-4.6   Towing in Heavy Weather
                  WARNING
                                                   Long ocean passages rarely offer the opportu-
      Carefully adhere to safety re-
      quirements when entering                     nity to plan for favorable weather during the
      closed spaces. See Naval Ship’s              entire tow. Seasonal storms and sudden, un-
      Technical Manual (NSTM) S9086-               expected weather can cause difficulty for
      CH-STM-030, Chapter 074, Gas
      Free Engineering (Ref. K).                   both the tug and the tow. Hurricanes and ty-
                                                   phoons are the most dangerous and destruc-
A written account should be kept of each in-       tive of all storms. Advice on actions to take in
spection, to be used as a reference for follow-    the event of such storms is contained in Chap-
ing inspections. The tow inspection party          ter 18 of Knight's Modern Seamanship (Ref.
should perform the following:                      L).
   • Check the tow connections and bridle
                                                   Upon receiving a hurricane warning, take
     for integrity and unusual wear.
                                                   these steps:
   • Check propeller shaft locking system.
                                                      • Determine the location and track of the
   • Check rudder locking system.
                                                        hurricane to plan a course that avoids
   • Check navigational lights and batteries.           the dangerous semicircle.




                                                                                              6-19
                                        U.S. Navy Towing Manual




                   CAUTION                                                NOTE

       Running before the sea and wind                      If water depth permits, increase the
       can cause difficulty in steering and                 towline scope and use the auto-
       in keeping the tow in the desired                    matic feature of the towing ma-
       position. The tow may become                         chine in heavy weather. This en-
       awash or start to overtake the tug.                  hances shock load reduction for
       If the tow begins to close on the                    the towline system. Every vessel
       tug, the tension in the towline will                 rides differently in severe storms.
       be reduced and cause an increase                     Tug captains should use good sea-
       in the catenary, which may also                      manship to determine how their
       cause the towline to snag on the                     tugs and tows ride best. They
       bottom or bring the tug and tow to                   should use the best combination of
       collision. The recommended                           towline scope, speed, and heading.
       course of action is to head into the                 Generally, heading into the weather
       weather and maintain steerage-                       allows better control of the tow.
       way, increase hawser scope and,
       as long as there is enough sea
       room, tolerate a negative speed                Estimate size and direction of the waves. Re-
       over the ground. There is no rea-              view applicable data in Appendix M to estab-
       son to slip the tow unless the tow-            lish average hawser tension limits for differ-
       ing ship is in danger of grounding.            ent wave heights and directions. Determine
                                                      whether extreme tension predictions can be
   • If necessary, change course to avoid or          eased by slight changes in course away from
     ride out the storm. It is far better to de-      towing directly into the wind.
     part from the projected track, ride out             • Recognize that the tug and tow likely
     the storm, and accept delays than to en-              will make negative speed over the
     danger the ship and tow by remaining                  ground. Sail for a position that will
     on a dangerous course and speed.                      minimize navigational hazards on a
                                                           downwind track.
                   CAUTION
                                                         • Rig the fantail for heavy weather. Stern
       Under more strenuous sea condi-                     rollers and Norman pins should be
       tions, dynamic hawser tensions can                  down and other obstructions to the tow-
       be significantly higher when towing                 line cleared.
       downwind than when heading into
       wind and seas at the same speed
                                                         • Increase hawser scope, if possible.
       and power. Turning into the wind                  • Set the towing machine on automatic if
       and seas and slowing to maintain                    it has an automatic feature. Otherwise,
       steerageway are appropriate ac-                     tow on the brake, rather than on the
       tions under such conditions.
                                                           dog, to ensure rapid reaction to chang-
                                                           ing circumstances.




6-20
                                       U.S. Navy Towing Manual


    • Arrange for quick disconnection of the         alongside the tow to effect necessary replen-
      towline. Methods for slipping the tow-         ishment. This requires disconnecting the tow,
      line are discussed in Section 6-7.3.2.         but in calm seas reconnecting should pose no
6-4.7 Replenishment at Sea
                                                     problem.

Long ocean tows or emergency circumstanc-            6-4.7.3 Rigging and Use of Fueling Rigs
es may require the tug to replenish at sea. Re-
plenishment at sea is a well-established rou-        Surging, often experienced in towing, may re-
tine, with procedures documented in Naval            quire that the replenishment ship keep station
Warfare Publication (NWP) MSC Handbook               on the tug. The greater maneuverability of the
for Refueling at Sea, NWP 14-2 (Ref. M).             oiler and the lack of complete control by the
The methods outlined there are suitable for          tug recommend this procedure. The tug des-
passing fuel, water, and other logistic necessi-     ignates the fueling station, receives the hose,
ties to a tug with a tow. The method selected        and proceeds to take on fuel while employing
is influenced mostly by sea and weather con-         standard precautions of proper stability, safe-
ditions, bearing in mind other factors that af-      ty on deck, adequate communication, and
fect safe and efficient ship handling. Due to        proper navigation. Astern refueling is also
reduced maneuverability of a tug with a tow,         recommended.
consideration should be given to having the
supply ship maintain station on the tug, vice        6-4.7.4 Astern Refueling from Another Tug
the receiving ship maintaining station. It may
                                                     Being refueled astern from another tug while
be advantageous to replenish from astern of
                                                     towing has become a common procedure due
the replenishment ship due to speed and ma-
                                                     to the limited number of replenishment ships.
neuvering limitations. It is also possible to re-
                                                     This process is somewhat different than that
plenish from the tow.
                                                     described in NWP 14 and can be accom-
6-4.7.1 Transferring Personnel and Freight           plished with or without the sending ship tak-
Simple light line procedures are used for            ing the receiving tug in tow. Due to the slow
transferring small freight. During these trans-      pumping rates available, however, taking the
fers it may be advantageous, as in fueling, for      receiving tug in tow does simplify station
a transferring ship to keep station on the tug.      keeping in what is sometimes a 24 to 36 hour
                                                     operation.
Personnel and mail should be transferred by
boat or helicopter. In unusual circumstances,        6-4.7.5 Replenishment Near a Port
personnel can be transferred by rigging a high
line, or, if necessary, a Stokes stretcher. Con-     The towing ship can arrange a temporary
ditions permitting, a rubber raft or boat            transfer of the tow to a local tug or tugs (see
should be used to avoid the maneuvering re-          Section 6-5.5). Then the ocean tug enters port
strictions of underway replenishment.                to replenish, while the tow is maintained off-
                                                     shore by a temporary replacement tug, or off-
6-4.7.2 Emergency Replenishment
                                                     shore mooring. When replenishment is fin-
Emergency conditions, wartime operations,            ished, the towing ship returns, the tow is re-
or heavy weather may require great ingenuity         transferred, and the journey resumes.
to replenish the tug or tow. Water and fuel
can be received from the tow, if available, by       If a long replenishment is anticipated, it may
shortening the towline and streaming hoses           be more economical to seek temporary dock-
from the tug. In calm seas, the tug may go           ing for the tow.


                                                                                               6-21
                                       U.S. Navy Towing Manual


6-5 Terminating the Tow                              6-5.2   Shortening the Towline

Terminating the tow at its destination requires      When approaching restricted waters, a shorter
as much planning as any other phase in tow-          scope and slower speed will make the tow
ing. If the schedule and the condition of the        easier to handle. It may be necessary to bring
tow permit, it is generally best to adjust speed     a tow to short stay to prevent the towline from
to arrive at destination during daylight hours.      fouling on the bottom. Bringing a tow to short
Darkness can easily magnify a routine evolu-         stay avoids being overtaken and fouling the
tion into a more difficult and dangerous situa-      towline. A delicate balance must be main-
tion. Based on the nature of the tow, pilot as-
                                                     tained between scope and speed. In this situa-
sistance and/or harbor tug assistance might be
                                                     tion, an automatic towing machine is invalu-
required.
                                                     able. Because there will be little or no
6-5.1   Requesting Assistance                        catenary, automatic control of the towline or
The Commanding Officer decides when to               the use of a synthetic spring (see Section
use a pilot, unless an order from senior au-         4-6.5) are the only means of surge control
thority supersedes. Some pilots, however,            available. An automatic towing machine can
may be unfamiliar with towing and with the           shorten the scope in either automatic or man-
characteristics of the tug. If a pilot is not fa-    ual modes. Often where there is a long dis-
miliar with towing, it may be preferable to          tance from sea buoy to berth, the ocean tug
employ him as an advisor to the Conning Of-
                                                     may continue to tow, at short stay, to a conve-
ficer rather than giving him the conn. The
Commanding Officer should be alert to diffi-         nient and safe location well inside the harbor.
culty and relieve the pilot if he deems it nec-
                                                     6-5.3   Disconnecting the Tow
essary.
Harbor tug assistance may also be necessary.         Before actually disconnecting the tow, lay out
Sea conditions may not permit harbor tugs to         necessary equipment, energize potentially in-
make up alongside. In this case, the only sig-       volved machinery, and brief all personnel on
nificant assistance that can be rendered is for      procedures. A well-drilled, disciplined team
the harbor tug to put a head line to the tow’s       will perform the routine smartly and will also
stern to assist in steering the tow. Once within     be responsive to any unexpected occurrences.
sheltered waters, harbor tug assistance can be
used as required. If an additional tug is avail-     Disconnecting procedures start by reducing
able, it and the original tug can be made up,        speed to bare steerageway and bringing the
each on a quarter, to effectively keep the tow       tow up short with the towing winch. With as-
heading fair to the channel. If the tow is large
                                                     sistance from whatever harbor tugs are in at-
and unwieldy, additional tugs may provide
both steering assistance and propulsion pow-         tendance, the towline is shortened until the
er. When using multiple tugs, it is advisable        connection fittings are on deck. A stopper is
to have pilots on board both the tug and the         passed onto the pendant, the connection is
tow to coordinate control of the assisting           broken and with all personnel clear, the stop-
tugs.                                                per is released.




6-22
                                       U.S. Navy Towing Manual


                                                     dure will ensure success and minimize diffi-
                                                     culty. If possible, personnel from both vessels
                  CAUTION                            should review and agree on a plan prior to
                                                     any action. Emergency conditions may not al-
      Do not permit the disconnected                 low this.
      pendant or bridle to drag on the
      bottom — it can cause consider-                The following procedure may be used for dis-
      able additional resistance and seri-           connecting the towline and passing the tow
      ously disrupt maneuvering.                     (see Figure 6-7).

When a tow bridle is long enough, the pen-           a. Set a course into the seas and reduce spe-
dant can be brought fully aboard the tug and            ed.
disconnected at the bridle apex. This may            b. Heave in until the pendant is on deck.
keep the pendant from dragging the bottom.
The bridle and pendant may also be retrieved         c. Signal the receiving tug to come close
on the tow by using a previously rigged re-             aboard on the designated side on a paral-
trieving line at the bridles apex. (see Figure          lel course.
4-18).                                               d. Secure tow bridle or pendant on deck with
When slowing, the towline scope may need to             a chain stopper; allow sufficient length to
be reduced to prevent dragging the bottom. A            lay on deck to facilitate disconnection
decrease in speed will cause a decrease in              from the hawser.
towline tension when the tow closes on the           e. Break the tow hawser from the pendant.
tug. As the tug and tow separate again, an in-
crease in tension will occur. Deceleration,          f. Receiving tug passes a messenger con-
like acceleration, must also be done in a con-          nected to the bitter end of its hawser or to
trolled and judicious manner. (See Section              a messenger strong enough to control the
6-7.5 for information on anchoring with a               tow.
tow.)                                                g. Bring the receiving tug’s hawser or heavy
6-5.4 Towing Delivery Receipt and                       messenger on deck and bend it onto the
      Reports                                           tow pendant.
If a harbor tug master is authorized to receive      h. With all lines and personnel clear, trip the
the tow formally, he should be asked to do so.          stopper and transfer the tow to the receiv-
This allows physical and legal transfer in              ing tug.
stream without having to dock or anchor. If          i. If a messenger was used, the receiving tug
the harbor tug master is not authorized, it may         makes the final connection to the tow
be necessary to send personnel ashore to ob-            pendant on its own stern.
tain necessary signatures on the letter of ac-
ceptance. Sample forms for receipt of tow            j. All special equipment and personnel asso-
and towing reports can be found in Appendix             ciated with the tow are then transferred
H.                                                      and appropriate documentation completed.
6-5.5 Transferring the Tow at Sea                    6-6 Tow and Be Towed by Naval
Casualty, operational orders, weather, or oth-           Vessels
er unusual circumstances may require trans-
ferring the tow to another tug. Preparing for        All U.S. Navy ships (except submarines and
transfer and understanding the transfer proce-       aircraft carriers) are capable of towing, using


                                                                                                 6-23
                                    U.S. Navy Towing Manual


their own emergency towing hawsers. When          Some submarines carry a towing bridle on
two Navy ships are involved in a “tow and be      board, but in some cases (SSBN 726 Class),
towed” operation, each provides its own           the towing gear is stored ashore. In this case,
emergency towing hawser to form half of the       this equipment shall be provided by the tow-
total towing system (see Figure 6-8).             ing ship. Submarines are built with the neces-
                                                  sary towing pads, cleats and chocks for being
Some Navy ships may be equipped with old,
                                                  towed. When not in use, the cleats and chocks
little-used hawsers. These ships may not be       are arranged to retract and are housed inside
aware of the recently understood problems
                                                  the faired lines of the hull. See Appendix J for
with deterioration of nylon rope over time.       more detail concerning submarine towing.
All should be alerted to current directives
concerning replacement of emergency towing        6-6.2   Towing Procedures
hawsers. Double-braided polyester hawsers         The information presented here is taken from
(MIL-R-24677) are preferred.                      NSTM CH-582 (Ref. A) which is the govern-
6-6.1   Towing Systems                            ing document for emergency ship-to-ship
                                                  towing. Where this manual and NSTM CH-
Navy combatant surface ships have a towing        582 differ on this topic, NSTM CH-582 shall
pad and stern chock aft and a chain stopper       take precedence. Consult this reference for
pad (towing pad) and bow chock forward.           greater detail.
Sometimes, because of equipment interfer-
ence, the stern chock and towing pad are lo-      6-6.2.1 Procedure for the Towing Ship
cated on the quarter.                             1. Connect the pelican hook to the after-tow-
                                                     ing pad with a shackle.
In addition to these deck fittings, Navy sur-
face ships carry a towing hawser, chafing         2. Connect the chafing chain with an end
chain, pelican hook, shackles and other ap-          link to the pelican hook. Lead the chafing
pendages needed for emergency towing oper-           chain through the stern chock.
ations.
                                                  3. Connect the towing hawser end fitting to
Each ship in the Navy is provided with a tow-        the chafing chain with a detachable link.
ing drawing that shows how to rig the ship for
                                                  4. Fake down the towing hawser clear for
being towed or for towing another ship. This
                                                     running fore and aft. Stop off each bight
drawing also shows such details as towing
                                                     of the towing hawser to a jack stay with
hawser size, chafing chain, and other append-
                                                     21-thread. Place shoring under the stops
ages. For surface ships and some submarines,
                                                     for ease in cutting.
the Ship’s Information Book (SIB) has details
on their towing gear and also contains dia-       5. Connect the NATO towing link to the free
grams that illustrate how to rig for being           end of the towing hawser. If a NATO
towed or for towing another ship.                    towing link is not available use an appro-
                                                     priate size long link or shackle. This fit-
Aircraft carriers are only equipped to be
                                                     ting should be capable of being connected
towed. They do not have a padeye or other
                                                     to the hawser of the towed ship.
towing equipment located aft for towing an-
other ship. Carriers are equipped with a 2-1/2    6. Connect a messenger, composed of ap-
inch diameter 6 x 37 galvanized wire rope,           proximately 100 fathoms (600 feet) of
900-foot towing hawser. The towing hawser            three-inch circumference line and 50 fath-
is stored in the anchor handling compartment         oms (300 feet) of 1-1/2 inch circumfer-
on a horizontal storage reel.                        ence line (For a 10-inch circumference or


6-24
    U.S. Navy Towing Manual




Figure 6-7. Passing a Tow at Sea.




                                    6-25
         U.S. Navy Towing Manual




       Figure 6-8. Tow-and-Be-Towed.



6-26
                                     U.S. Navy Towing Manual


   larger hawser, use four-inch line instead       4. Pay out sufficient anchor chain (5 to 45
   of three-inch), to the outboard end of the         fathoms [30 to 270 feet]) to provide a sub-
   towing hawser. Lead the free end of the            stantial towing catenary when the towing
   messenger through the stern chock.                 hawser has been payed out. Synthetic
                                                      line, by itself, will provide very little cate-
7. Pass the messenger to the towed ship us-
                                                      nary.
   ing a heaving line. Preparing extra heav-
   ing lines prior to hook up will allow sev-      5. Set the brake on the wildcat and pass and
   eral attempts to complete this pass during         equalize the chain stoppers one outboard
   maneuvering. Control the pay out of the            and one inboard of a detachable link, to
   tow line messenger and hawser by cutting           take the strain on the towed ship’s anchor
   the stops. The tow line messenger and              chain. Disengage the wildcat.
   hawser should be payed out gradually to         6-6.2.3 Quick Release of Towed Ship
   ease handling of the tow line by the towed
   ship and to avoid fouling the towing ships      1. Pay out the anchor chain connected to the
   propellers.                                        tow line on board the towed ship so that a
                                                      detachable link is just forward of the an-
6-6.2.2 Procedure for the Towed Ship                  chor windlass.
1. Stop off the anchor (port or starboard) of      2. To prevent the chain from returning to the
   the anchor chain to be used. Set up on the         chain locker when detached, pass chain
   anchor windlass brake. Pass a pinch bar            stoppers on the anchor chain and lash the
   through the chain, letting the bar rest on         anchor chain just abaft of the detachable
   the lip of the chain pipe, or pass a preven-       link or apply the chain compressor where
   ter to prevent the chain from backing              fitted.
   down into the chain locker and a preven-
   ter on the anchor to back up the stopper.       3. Disconnect the anchor chain between the
   Break the anchor chain at the detachable           anchor windlass and the chain stoppers so
   link inboard of the swivel. If power is            that only the chain stoppers are holding
   available, haul out the desired length of          both the anchor chain and tow line. This
   chain using the anchor windlass. If power          arrangement allows quick release of the
   is not available, the chain will have to be        towing hawser and chain.
   hauled out manually.
                                                                       CAUTION
2. Shackle the towing chain stopper to the
   designated (towing) padeye on the fore-                 In case of emergency, for quick re-
   castle, for stopping off the anchor chain               lease, tripping the pelican hook on
   after the tow is properly adjusted.                     the towing ship is faster than the
                                                           above procedure.
3. Fake out the towed ship’s hawser on
   deck, fore and aft, on the forecastle for       6-6.3   Getting Underway with Tow
   clear running, prior to connecting it to the
                                                   Implement the following steps when the tow-
   anchor chain. Use the towing ship’s mes-
                                                   ing hawsers are connected and both ships are
   senger to haul the towing hawser from the
                                                   ready to start the tow:
   towing ship on board through the
   bullnose. Connect it to the towed ship’s        1. The towing ship should come ahead as
   hawser secured to the end of the anchor            slowly as possible as the hawser begins to
   chain. If the towed ship’s hawser is not to        take strain. Increase turns slowly until the
   be used, connect it to the anchor chain.           inertia of the tow is overcome and both


                                                                                                 6-27
                                       U.S. Navy Towing Manual


   ships are moving with a steady tension in         When a riding crew is on board, the fire po-
   the hawser. Increase speed slowly until           tential should be evaluated. If equipment is
   the desired speed is reached. At no time          being operated for propulsion, auxiliary pow-
   should the tow speed be such that the tow         er, pumps, or allied systems, the danger of
   hawser lifts completely out of the water.         fire can be significant. Prudent and adequate
   The course of the tow may be changed              placement of pumps, hoses, fire extinguish-
   gradually, as necessary. Getting under-           ers, axes, foam, and fire fighting equipment is
   way with a tow will likely result in the          required to help the riding crew fight fires. If
   largest tensions and requires the most            necessary, personnel may be transferred from
   care.                                             tug to tow to perform fire fighting and dam-
                                                     age control. The tug, if it can be brought
2. Pay out or haul in (assuming power is             alongside, can deliver large quantities of wa-
   available to the anchor windlass) anchor          ter for use on board the tow; associated pow-
   chain as desired to keep both ships in step       er, foam, hoses, and personnel from the tug
   (that is, taking wave crests at the same          can be of valuable assistance. A charged 2½-
   time). When a comfortable distance is             inch fire hose can be streamed aft on salvage
   found, the chain stoppers are passed on           balloons if alongside fire fighting is not prac-
   the anchor chain and the strain is equal-         tical.
   ized between stopper and wildcat. Lock-
   ing plates are installed and set on both the      6-7.2    Tug and Tow Collision
   chain stoppers.
                                                                          CAUTION
6-7 Emergency Towing Procedures                              When towing under unfavorable
                                                             conditions, inclement weather, or
                    CAUTION                                  at short stay, danger exists of be-
                                                             ing overridden. In such a situation,
        Riding crews normally consist of a                   particular care is advised in setting
        minimum crew and can be expect-                      an underway material condition so
        ed to perform only limited emer-                     that watertight doors, hatches, and
        gency functions on board.                            other openings are secured.


This section presents general guidelines for         The tug and tow may collide when maneuver-
handling emergency situations unique to tow-         ing in restricted waters with the tow at short
ing. As in all emergencies, prudent seaman-          stay, or under other operationally complex
ship and adherence to safety guidelines are          circumstances. A collision may also occur
primary assets in bringing a situation safely        when:
under control.                                          • There is a loss of propulsion power or
6-7.1   Fire                                              sudden reduction of the tug’s speed.
                                                          With sufficient way on, the tow may
Fire on board is a well-known hazard; fire                override the tug, and in extreme cir-
prevention and methods of fighting fires                  cumstances sink it. The possibility is
should be drilled with the riding crew. There             greater if a tow is at short stay. If pro-
should be little danger of fire on board an un-           pulsion power is lost on the tug, put the
manned tow. One exception is the possibility              rudder hard over to the weather and
that a shaft locking device might fail and                slack the towline. With sufficient way
cause an engine room fire.                                on, the tug may fall clear of the advanc-


6-28
                                      U.S. Navy Towing Manual


      ing tow. If power loss is imminent but        Flooding, structural damage, shifting of ballast
      the tug can still make turns, consider        or cargo, or other events may degrade the tow’s
      going alongside or otherwise clearing         stability. When stability decreases, the tow may
      the tow.                                      be in danger of sinking. Excessive force placed
                                                    on the tug as the tow sinks can damage and se-
   • Tug and tow will experience different
                                                    riously endanger the tug before the towline
     set and drift from seas, currents, winds,
                                                    parts. Prompt action is necessary to save the
     or towline drag. To avoid collision, re-
                                                    tow and to ensure the safety of the tug.
     duce speed and increase the towline
     scope. If possible, the tug should turn        It is vital to monitor the condition of the tow
     into the predominant set, if the tow has       during transit. Trim, list, roll period, seakeep-
     a larger sail area. This will cause the        ing, and draft are monitored from the tug or
     tow to drift away from the tug. Follow         by a riding crew. Upon noting an irregularity,
     the opposite course if the tow’s sail area     a boarding party should be dispatched, if pos-
     is smaller than that of the tug, as in the     sible, to investigate and correct any deficien-
     case of a submarine.                           cy on board the tow. If the material condition
   • A tug and tow are dead in the water, al-       of the tow is so deteriorated that sinking is
     lowing towline cantenary to draw them          likely, the tug should consider the following
     together. The same situation can occur         courses of action.
     between two tandem tows. In an emer-
                                                    6-7.3.1 Beaching a Sinking Tow
     gency in shallow waters, it may be pos-
     sible to anchor both tug and tow by let-       When towing a casualty or a vessel that is
     ting the towline come into contact with        likely to sink, beaching may be the best way
     the bottom. (Routine use of this prac-         to save the tow. The decision to beach the tow
     tice is discouraged because of possible        is operational and should be based on an as-
     towline damage.)                               sessment of conditions. Weather conditions,
If a collision appears unavoidable, deploying       rate of deterioration of the tow, damage con-
fenders may serve to reduce or eliminate            trol equipment available, and distance to safe
damage to both vessels.                             port should all be considered when deciding
                                                    whether to beach a tow. Permission to beach
6-7.3 Sinking Tow                                   should be obtained from the cognizant au-
Planning to sink a tow also requires special        thority when feasible. Authorization to beach
consideration and preparation. Often, special       a tow should be made by immediate message
permission must be obtained and adherence           or voice communications when feasible. Sig-
to environmental regulation can be difficult.       nificant time may be required to steam to a
Caution should be used when accepting a tow         suitable site. It may be impossible to locate a
with the intention of sinking. This may be the      smooth beach in time. If pumps are on board
case following a salvage operation.                 the tow and damage control procedures are
                                                    employed, the tow may be kept afloat for
                                                    days before beaching, but indecision has re-
                  CAUTION                           sulted in tows sinking.
      When combatting a sinking tow,
      conditions can deteriorate rapidly.           When beaching a tow, follow these guide-
      The boarding party should have                lines:
      sufficient survival gear and should
      be prepared to abandon at any giv-                • When possible, select a beach with a
      en moment.                                          smooth, gradually sloping bottom.
                                                          Avoid rocky shores with breaking surf.
                                                                                                6-29
                                     U.S. Navy Towing Manual


       Potential loss of the tow and danger to     6-7.3.2 Slipping the Tow Hawser
       the tug exist in shallow, rocky waters.
                                                                     CAUTION
   • Ground the tow with the bow toward
     the beach. The tug’s assistance may be              Releasing the hawser under ten-
                                                         sion, or even its own weight, can
     required to put the tow on the beach
                                                         be hazardous, due to retained en-
     bow first. If water depth is sufficient,            ergy in the hawser.
     the tug can tie up alongside in the lee of
     the tow and take the tow in. The alter-       In emergencies, wartime conditions, or heavy
     native practice of allowing a tow to          weather, it may be necessary to slip the tow
     drift onto the beach should be avoided.       hawser to remove the tug from a hazardous
     This can increase the likelihood of           condition. This condition could be a sinking
     broaching and cause increased damage          tow, danger to the tug from weather, or a
     to the tow as well as make recovery           grounded tow.
     more difficult. Assistance from a small,
     shallow draft harbor tug is very valu-        Options for slipping the hawser include:
     able when beaching a tow.
                                                      • Paying out the hawser and allowing it
   • Disconnect the pendant and bridle be-              to run off the towing machine (free-
     fore beaching, when possible, to pre-              wheeling).
     vent the tow from stopping short of the
     beach.                                           • Cutting the hawser with a torch or ex-
                                                        plosive cable cutter. Synthetic hawsers
   • Prevent the tow from broaching and                 under no tension can be cut with an axe.
     sustaining additional structural damage
     due to excessive hull loading. Flooding          • Rigging carpenter stoppers and cutting
     the tow can prevent it from broaching              the cable inboard of the stopper.
     or going further aground. The ship               • If a ship with power is being towed, it
     should be set down hard enough so that             can sometimes cast off the towing pen-
     it will not be too light and, consequent-          dant on the tow’s bow.
     ly, broach at high tide. It should be as-
     sumed that in time the tow will be            If time allows, attach a buoy to the bitter end
     pulled off; however, this does not elim-      of the towline before slipping the hawser,
     inate the need for securing it properly       otherwise, it will be difficult (if not impossi-
     and preserving it until it is extracted       ble) to recover. Use a messenger that is at
     from the beach. If the tow has a stern        least 200 feet longer than the water depth and
     anchor, it should be deployed to help         strong enough to lift the hawser. One end of
     prevent broaching.                            the messenger is connected to the hawser and
                                                   the other to a recovery buoy line. The buoy
   • Ballast the tow as soon as possible after     line must be long enough to reach the bottom
     grounding to hold it securely in posi-        and strong enough to lift the messenger, but it
     tion. Even in completely sheltered wa-        need not be strong enough to lift the hawser
     ters, the range of tides and consequent       itself. The buoy should be adequately marked
     currents can be powerful enough to al-        with a bright color, radar reflector, staff, or
     ter the position of a beached ship.           flag so it can be easily located.


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                                      U.S. Navy Towing Manual


6-7.4 Disabled Towing Machine                       ing. The use of divers may be possible but,
                                                    because of the inherent danger, should be
The main resource for recovering and storing
                                                    used when there is no other solution. Ship-
a towline is the tow machine. If this machine
fails, the tow ship should reduce speed and at-     handling and working with heavy gear in a
tempt to make repairs. The machine's me-            seaway are complicated evolutions that are
chanical brake should be set to prevent an ac-      made more so when there are people in the
cidental spooling off of the tow wire. If the       water.
machine cannot be fixed, it will likely be nec-
essary to disconnect the tow, transfer the tow      If a retrieving pendant has not been rigged,
and recover the towline by a more difficult         the procedure is far more complicated and
method. Since Navy tugs employ a 2 1/4-inch         divers may be the only solution. Sliding a
wire rope, these evolutions can be very diffi-      working line down a chain pendant has been
cult.                                               done with varied success. The loop may snag
                                                    on the way down or slide off during retrieval.
6-7.4.1 Disconnecting the Tow
                                                    It may be better to attempt to run a shackle
If the tow machine fails while the tow is still     along the tow wire, but this may also meet
connected, it will be necessary to break this       with varying success. Certainly, divers can
connection at some point along the towline. If      make these evolutions more successful by
a retrieving wire has been rigged, this may         providing assistance to keep the line un-
not be so difficult. If there is power on the       fouled. A better solution may be to use divers
tow, it should haul on the retrieving wire until    to rig a retrieval line. Depending on the
the connection point is on deck. A chain stop-      length of the pendant divers may be able to
per (or other appropriate stopper) should be
                                                    attach a line at the bitter end of the chain; at
passed on the towline with sufficient slack to
                                                    the connection point. This is unlikely, though,
break the connection without tension on the
line. It may be necessary to rig a stopper          since there will probably be a pendant longer
around the tow ship's towline to allow a con-       than 90 feet. If divers are working in SCUBA,
nection to be broken. It may be sufficient to       bottom times will limit the amount of work
haul in slightly on the main chain pendant,         that can be done. However, divers may be
and break the connection at the main towing         able to rig a retrieving line of sufficient length
padeye. Only stoppers with quick release ca-        to haul the main pendant on deck. By lacing
pability should be used.                            small wire through links of chain 40 or 50
                                                    feet below the surface, the chain can be
                  WARNING                           brought on the deck of tow ship (or an assist-
                                                    ing vessel), once it has maneuvered along-
      When stopping the tow line for
      breaking, only stoppers with                  side. The tow ship's deck machinery can be
      quick release capability should               used to haul the heavy gear on board once
      be used.                                      dive operations are completed.

The exact disconnect method and stopper to          Dive Supervisors must be provided with an
be used is an operational decision and de-          accurate and detailed sketch of the entire tow
pends on many factors. If the tow is at its         rig. This will enable them to develop a safe
point of destination, and the main tow rig can      and efficient plan and ensure that they are
be destroyed, explosive cutters or torches          prepared with the proper tools to accomplish
may be an appropriate method of disconnect-         the mission.


                                                                                                 6-31
                                      U.S. Navy Towing Manual


6-7.4.2 Recovering the Towline                      process will also be laborious and time con-
Once a tow has been disconnected, it is still       suming.
necessary to recover the towline without the        A third method that may be used is to turn the
assistance of the main towing engine. The           towing drum manually. A wire can be se-
tow ship is faced with two problems. The            cured to a point on the side of the towing
first, how to recover the wire and second, is       drum and looped around the drum in the di-
where to put it once its on deck. The weight        rection of reeling. A crane can pick up the bit-
of a 2 1/4-inch IWRC wire is almost 10              ter end and be used to lift this line and conse-
pounds per foot. A typical towline scope is         quently turn the drum. Careful coordination
1500 feet or more. This is a total weight of al-    between the crane operator and a crewman
most 15,000 pounds. It cannot be handled            manning the drum brake is required to pre-
easily without machinery.                           vent accidental un-spooling of the wire. If a
Recovering a towline can be accomplished in         crane is unavailable, deck machinery can be
several ways. One way is to slip the hawser         used if sufficient blocks can be rigged to
off the drum and recover it when repairs have       reeve the hauling wire in the right direction.
been made. A marker buoy and suitable mes-          These are by no means the only methods of
senger should be rigged to the bitter end so it     retrieving a tow wire, but are a few examples.
can be found later. A messenger should be           Any of these methods require substantial
strong enough to be able to lift the hawser on      manpower and large amounts of time. This
deck for the depth of water. It need not be         process, like all towing procedures should be
strong enough to lift the entire hawser, but if     performed with close attention to safety of
it breaks, divers will be required to rig anoth-    personnel.
er messenger, and it is very likely that the
hawser will not be found without the marker         6-7.5   Anchoring with a Tow
buoy. Moderately deep water will make this          In general, anchoring should always be con-
alternative impractical. Additionally, it is        sidered less desirable than remaining under-
probably unwise for the tow ship to try to          way. Steaming with a tow may prevent many
bring the hawser to shallow water. Assuming         difficulties encountered at anchor. Provided
the hawser is at a scope of 1500 feet or more,      that there are no limiting operational factors
the hawser will drag the bottom for some time       and there is sufficient sea room, steaming is
before sufficiently shallow water is reached.       usually the better choice. When anchoring
This may damage the hawser or cause maneu-          with a tow is necessary, the following alterna-
vering problems for the tow ship.                   tives should be considered.
Another way to recover towline is to use deck          • Reduce speed to bare steerageway,
machinery and carpenter stoppers. This meth-             head into the predominant set, allow the
od requires a great deal of time and a large             tow to remain well astern, and then re-
amount of manpower. Assistance from shore                duce speed and allow the tug and tow to
crews may be advisable. This method does                 come dead in the water at the anchor
not solve the problem of stowage. A large                drop point. Let the tug’s anchor go and
deck area will be needed to fake out (figure-            pay out the necessary scope of chain.
eight) this amount and size of wire. It may be           The tow will follow as affected by set.
possible to hang the wire from the side of the
tow ship and stop it off in bights, similar to         • Reduce speed and approach several
the method used when preparing a main tow                hundred yards to port or starboard of
pendant on a tow, or a leg of beach gear. This           the desired anchorage. With the anchor-


6-32
                                         U.S. Navy Towing Manual


      age position broad on the bow and ap-            will be significant, and must be considered in
      proaching abeam, put the tug’s rudder            selecting the means of disconnecting.
      hard over and reduce speed; maneuver
      to hold at the anchoring point, letting                              WARNING
      the tow pass by. When the tow clears
                                                               The tow wire or bridle will likely
      the tug, drop anchor.                                    be under tension when released,
                                                               creating an extremely hazardous
   • The tow can be taken alongside in fa-                     situation. All nonessential per-
     vorable sea and wind conditions. With                     sonnel must evacuate the area
     the tow alongside, the tug can maneu-                     to prevent serious injury.
     ver in restricted waters, back down as
     necessary, and drop anchor.                                           CAUTION

                                                               The towing ship should reduce the
                   CAUTION                                     tension on the towing assembly by
                                                               either slowing down or stopping pri-
      The mooring loads of the tug and
                                                               or to cutting or otherwise releasing
      tow may be greater than the hold-
                                                               the tow rig.
      ing power or strength of the tug's
      ground tackle. A dragging anchor
      or failure of the ground tackle is               In the case of a damaged ship, the tow pen-
      possible, resulting in loss of control           dant or bridle legs, if chain, should be secure-
      of the tug and tow.
                                                       ly held by multiple chain stoppers, each bear-
                                                       ing equal tension. If the pendant or bridle legs
In some circumstances, such as shallow wa-             are wire, then provision should be made for
ter, the towline itself may be used for light          cutting with an oxyacetylene torch, a cable
holding of the tow and tug when the towline            cutter, or any similar device. As cutting is ex-
comes in contact with the bottom. Routine              tremely hazardous, precautions should be tak-
use of this practice is discouraged because of         en to prevent whipping, and the wire should
                                                       be seized on both sides of the intended cut.
possible damage to the towline.
                                                       When an emergency quick disconnect is pro-
If there is little wind or current, the tug must       vided, make sure that all jewelry will fit
be alert to the probability of the hawser’s            through all fairleads.
weight pulling the tow toward the tug, until           6-7.7   Man Overboard
the hawser rests on the bottom.
                                                       Standard man overboard maneuvers may not
6-7.6 Quick Disconnect System                          be feasible in towing situations, primarily be-
                                                       cause of the time involved and the tug’s limit-
Most routine point-to-point tows are securely          ed maneuverability.
rigged with no provision for quick release,
                                                          • If maneuvering is limited, the tug sho-
other than slipping the tow wire from the tow-              uld stop, or at least reduce speed to bare
ing ship. When towing damaged ships, how-                   steerageway, and recover the man
ever, it may be desirable to provide for a                  overboard using a boat. If the tug is
quick release of the tow pendant or bridle to               stopped, take precautions to keep the
facilitate breakup of the tow. Even if the tow              tow from overriding the tug and to keep
hawser has already been disconnected, the                   the towline clear of the propellers.
weight of the chafing pendant or bridle legs                Communications should be available


                                                                                                      6-33
                                     U.S. Navy Towing Manual


       between the boat and the tug so that the    6-7.8.2 Use of an Orville Hook
       tug can direct the boat to the man.
                                                   Orville hooks are only recommended for re-
   • If the recovery requires maneuvering          covering tows which have a chain bridle.
     the ship back to the man, seamen              They are not recommended for recovery of
     should be stationed with heaving lines.       wire or synthetic tow pendants.
     Swimmers should be outfitted with im-
     mersion or wet suits and safety lines         Figure 6-9 and 6-10 depict the general config-
     ready to swim out to the man.                 uration of the Orville Hook and its various
                                                   components. Figure 6-11 depicts deployment
6-7.8 Using an Orville Hook to Recover a           of the Orville Hook.
Lost Tow
                                                   The Orville Hook is suspended in the hori-
Using an Orville Hook to recover a lost tow        zontal plane by a trailing buoy and is towed
with a chain bridle may be a viable option if a    parallel to the tow by the recovery tug. Once
secondary towline is not available or it is not    the recovery tug has overtaken the tow by a
possible to recover the secondary towline.         sufficient distance dictated by the length of
6-7.8.1 Origin of the Orville Hook
                                                   the towline, the recovery tug swings across
                                                   the bow of the tow thereby snagging the
The Orville Hook was initially designed and        mouth of the Orville hook on the chain bridle.
patented by SAUSE BROS TOWING, Inc to              The Orville hook is sized to fit between the
recover lost tows which had a chain bridle.        individual links of the chain. In most cases
While the patent for this device has expired it    the Orville hook will remain in place as long
still remains a useful tool for emergency re-      as tension is kept on the synthetic pendant.
covery of broken or lost tows. This device         The synthetic pendant can then be retrieved
was successfully used to recover the dry dock      along with the chain bridle so a more perma-
SUSTAIN when recovery of the secondary             nent connection can be made between the tow
towline was not possible due to fouling.           wire and the chain bridle.




6-34
        U.S. Navy Towing Manual




Figure 6-9. Orville Hook Retrieval Assembly




                                              6-35
             U.S. Navy Towing Manual




       Figure 6-10. Orville Hook Configuration




6-36
         U.S. Navy Towing Manual




Figure 6-11. Deployment of the Orville Hook




                                              6-37
              U.S. Navy Towing Manual




       This Page is Intentionally Left Blank




6-38
                                         U.S. Navy Towing Manual


                                                                in the past. When faced with a similar situa-
                   Chapter 7                                    tions, they should refer to reports of actual
                                                                operations.
                SPECIAL TOWS
                                                                7-2 Target Towing
7-1 Introduction
                                                                The primary functions of ships such as the
This chapter addresses tows of unusual con-                     T-ATF, and ARS classes are salvage and
figuration that occur infrequently or are of a                  ocean towing; target towing is a secondary
highly specialized nature. Topics include                       function routinely assigned to them. Most
towing in ice and towing targets, submarines,                   combatant ships can tow target sleds with
merchant ships, and NATO ships in peril.                        their standard shipboard equipment.
Emphasis has been placed on rigging and pro-
                                                                7-2.1      Williams Target Sled
cedural differences between these types of
tows and towing operations previously dis-                      Currently the catamaran-hulled Williams Tar-
cussed.                                                         get Sled is the target used most for gunnery
As their recurrence is unpredictable, these                     exercises (see Figure 7-1). The Navy also us-
types of tows are not treated in depth in this                  es sonar buoys, arrays, drones, and remotely
manual. Instead, these topics are presented to                  operated boats as targets. Targets are towed,
make planners and operators aware that such                     escorted, or carried as deck cargo to the oper-
operations have been successfully completed                     ations area.




                                                                S hackle
                                            R ig hting S trap




                                                                                C oil E xcess Line in H arb or
                                                                                and D ep loy P rior to S tream ing Target

                                                                                       60-F oot Length of O ld
                                                                                       1-Inch C ircum feren ce To w line




      To To w                                                                                            Float




                Figure 7-1. Williams Target Sled Rigged for Tow with Righting Line Streamed.




                                                                                                                            7-1
                                              U.S. Navy Towing Manual


7-2.2    Towing Equipment                                   If the tow is made up in the water, slip the tar-
The Williams Target Sled is towed from a                    get mooring lines when clear of the pier. Tow
synthetic line bridle shackled to the inboard               target at short stay until clearing congested
sides of the catamaran hulls. The two bridle                waters. Steaming at short stay does not affect
legs are joined by a triangular flounder plate;             maneuverability or speed. When clear of the
a 30-foot pendant of synthetic line is also                 harbor and congested waters, about 600 feet
shackled to the flounder plate. The pendant is              of towline is usually streamed. If the towline
shackled to the main synthetic towline.                     is not on the drum of a winch, it may be paid
                                                            out using a gypsy head or capstan to maintain
The towline is        generated might cause the             control. Ships with towing bitts can control
target to list or a damaged sled to capsize.                the payout of towline by taking turns around
7-2.3    Routine Procedures                                 the bitts. When enough line has been paid out,
                                                            the towline is stopped off to the towing bitts
7-2.3.1 Transporting the Target to the
        Exercise                                            with the towline passing over the stern roller.
                                                            Speed is then built up slowly until the target
                                                            is towing steadily. If towing at night, make
                      CAUTION
                                                            sure that the target’s stern and side lights are
        If the target is made up bow-to-                    lit.
        stern, it should reverse direction
        a n d s w in g in t o p o s it io n wh e n          7-2.3.2 Streaming the Target
        slipped. Too much way on, howev-                    The towing ship times its arrival at the firing
        er, will cause the target to be towed
        stern first. In a stern-first position,
                                                            range long enough before the exercise begins
        the target has a tendency to stream                 to allow time to stream the target. Slowing to
        aft without reversing itself and can                about 4 knots and paying line out at 150 feet
        end up straddling the towline.                      per minute is a safe way to stream.
                                                            7-2.3.3 Making Turns with the Target
The tug can either pick up the target at its
berth or have the target brought out of the                 Depending on the weather, turns can be made
harbor by a delivery ship, usually a work                   in one increment by using a small amount of
boat. If a delivery ship is used to bring the tar-          rudder so as to have about a 1,000-yard diam-
get out of the harbor to the towing ship, slow              eter turning circle. When making turns, keep
down and maintain steerageway so that the                   the target aft of the towing ship’s beam, pref-
delivery ship can easily approach the stern.                erably broad on the quarter.
For tows that begin at the target’s berth, the              When proceeding on a circular course, the
target can be made up to the towing ship                    target’s tendency to capsize is determined by
bow-to-stern alongside (with the towline                    the speed of the tow, length and depth of tow-
shackled to the target’s bridle pendant),                   line, and the sea state and heading relative to
bow-to-bow alongside, or bow-to-stern aft of                the wind. Turns to windward are different
the towing ship. The target can also be made                from turns to leeward. When turning into the
up on the fantail of the towing ship. (It may               wind, the target screen area acts as a mainsail
be similarly made up on the fantail during                  and holds the target away from the turn, re-
protracted delays between exercises or in the               quiring an increased rudder angle and giving
event of impending heavy weather.) If the tar-              a smaller transfer with a slightly greater ad-
get is made up on the fantail, use the ship’s               vance. When turning leeward, the screen acts
crane or boom to set the target overboard up-               as a sail effect and propels the target toward
on arrival at the operations area.                          the inner part of the turn, requiring less rud-


7-2
                                      U.S. Navy Towing Manual


der and performing a greater transfer with          7-2.4    Special Procedures
less advance.
                                                    7-2.4.1 Passing the Target to a Combatant
                                                            Ship
In all turns the target acts as a sea anchor,
making a small tactical diameter while the
                                                                       WARNING
ship turns around the target with a larger tac-
tical diameter. Turns with the current increase             When tows are passed, most ca-
transfer; turns against the current reduce                  sualties occur because the ships
transfer. The advance in all turns is small.                do not maintain a steady course
                                                            or speed or because the towing
                                                            ship releases the tow before the
To keep the towline tension low and to avoid
                                                            other ship is ready to accept the
capsizing the tow, keep the rudder angle as                 strain.
low as is practical. A mean rudder angle of 12
or 13 degrees is satisfactory. A good practice
                                                    It is sometimes necessary to pass a target
is to make the turn in small increments,
                                                    from one ship to another on the open sea. The
steadying up until the target is directly astern
                                                    towing ship selects the side and speed for
before going to each new increment.                 passing and signals this information to the re-
7-2.3.4 Recovering the Target
                                                    ceiving ship well in advance of passing the
                                                    tow. Stop off the hawser along one side of the
When the exercise is over, the towline is           towing ship. The receiving ship steams into
heaved in to a shorter stay for the tow home        the wind alongside the towing ship.
or brought up short so the target can be lifted
                                                    The receiving ship signals and sends a mes-
aboard. Combatant ships use their capstans to       senger when it is ready to receive the tow.
heave the towline, MSOs use one drum of the         The towing ship receives the messenger and
sweep-wire winch, and salvage ships use             secures it to the hawser. The receiving ship
their capstans or traction winches. Significant     hauls the messenger through its towing
time must be allowed to bring the hawser in at      chock. The towing ship frees the hawser and
even maximum capstan speed. Hawser recov-           the receiving ship hauls it away (see Figure
ery typically proceeds at 40 to 60 feet per         6-7).
minute. As soon as the towline is on board, it
should be faked on deck or spooled on a reel.       7-2.4.2 Recovering a Capsized Target

Upon entering port, the tow can either be                              WARNING
brought alongside, brought to short stay, or
lifted aboard. The use of riding lines that have            Always remain with a target sled
                                                            until it is recovered or righted
been stopped off on the tow hawser during                   and towed to port; it will become
streaming contributes to the ease of bringing               a navigational hazard if left to
the sled alongside (see Section 6-2.3). For                 drift.
leaving and entering port, some ships prefer
two-blocking the bow of the sled against their      A capsized target must be righted immediate-
stern. When the sled is firmly snugged into         ly because it cannot be towed at any speed.
position and riding lines are added, this meth-     Safety precautions must be strictly observed
od allows good maneuvering.                         because of the hazards of recovery work.




                                                                                                7-3
                                       U.S. Navy Towing Manual


If the target capsizes, the towing ship should          • Alter course gradually with a target
heave in slowly. The ship may be required to              under tow in order not to capsize the
back slowly while heaving in, being careful               sled
not to foul the towline in the propellers or            • Approach the target with caution. The
rudder. Another method is to reverse course               shallow draft of the target sled causes
and place the ship alongside the target.                  considerable pitching and rolling at
Weather conditions will determine the best                slow speeds or when drifting.
method to use for approaching the target.
                                                     7-2.6 Other Targets
Before getting underway, the target should           SEPTARs (Seaborne-Propelled Targets) are
have been prepared for righting. A recovery          remote controlled, high speed surface targets
pendant can be made from 60 feet of line and a       that are transported to the operating area by
float. Attach one end of the line to the middle      a tug and then operated from the tug. Simi-
of the pipe framework at the apex of the tar-        larly, tugs can carry drone-type targets for
get. Coil the remainder of the line and secure it    antiaircraft and antimissile training exercis-
with small stuff to one of the pipe frames near      es. They can also carry transducers and ar-
the trailing edge of one of the catamaran floats     rays for submarine and antisubmarine train-
(see Figure 7-1). Tie the bitter end of the line     ing exercises. Each of these services is
with a bowline onto the float. Before stream-        unique and presents special problems not
ing the target, release the line and float to        found with standard target sled towing.
stream aft of the tow. If the sled capsizes, ma-     Some of the information necessary to sup-
neuver the ship alongside the sled and bring         port specialized target services is classified.
the float and recovery line aboard the ship. By      Generally, range personnel will provide spe-
leading the recovery line over the caprail to a      cific information regarding these special
capstan and heaving in, the sled can be made         systems.
to rotate to an upright position in a motion that
carries the target away from the hull of the         7-3 Towing Through the Panama
ship. Once upright, inspect the target to make           Canal
sure that it is not damaged and is fit for tow.
                                                     Tows of unmanned vessels through the Pana-
7-2.5 Target Towing Precautions
                                                     ma Canal present some unique concerns and
Take the following precautions when towing           often require additional preparations. A canal
a target:                                            tow may be the result of an East Coast de-
                                                     commissioning of a nuclear vessel that needs
      • Avoid surges                                 to go to the West Coast for final disposal. It
                                                     may be the result of an asset being transferred
      • Maintain a steady course, avoiding tig-      from a West Coast activity to an East Coast
        ht turns                                     activity. Either way, as more and more ves-
      • Ensure that the target’s stern and side      sels are decommissioned, and assets become
        lights are lit at night                      fewer in number, tows through the canal have
                                                     become more frequent.
      • Do not tow the Williams Target Sled
        at speeds in excess of those authorized      The Panama Canal is unique and has restric-
        by Fleet directives                          tions on size and requirements for special
                                                     bitts and chocks to accommodate tow wires.
      • Do not tow a capsized Williams Target        While many vessels that are designed for
        Sled                                         service through the canal have the necessary


7-4
                                       U.S. Navy Towing Manual


installed fittings, other ships, particularly                vent the hawser from coming into con-
warships, may not meet all the specific re-                  tact with the ice, adjust the catenary so
quirements.                                                  the chain bridle, or chain pendant enters
It is essential for a tow planner to fully under-            the water at the towed vessel. It may
stand the requirements when towing through                   also be desirable to rig additional chain
the Panama Canal. Code of Federal Regula-                    to help make this easier. This is ad-
tions (CFR) 35, Panama Canal (Ref. N) con-                   dressed in Allied Tactical Publication
tains this information and is an invaluable re-              (ATP) 15, Arctic Towing Operations
source when planning a canal tow. It has also                (Ref. O).
proven to be well worth the investment to fly
a representative from the Panama Canal                  • Selecting the appropriate towing me-
Commission to the preparing yard. A walk                  thod. When towing in ice, a tow should
through of the vessel by knowledgeable per-               be close to the tug’s stern to keep an ice
sonnel can identify any changes that need to              passage open ahead of the tow. The tow
be made while the ship is still in the preparing          may not have an ice-strengthened bow
yard. If the tow arrives at the breakwater in             and could sustain impact damage from
Panama, and does not meet the requirements
                                                          floating ice. Two approved towing
to go through, arrangements must be made to
                                                          methods for keeping the tow close are
effect repairs and modifications. This can re-
sult in both substantial costs and delays. Ad-            the short-scope method and the saddle
vance preparation is essential.                           method. The method used depends
                                                          upon type of towing ship and the de-
7-4 Towing in Ice                                         sign of the ship being towed. The sad-
                                                          dle method will ensure that the tow will
Arctic operations may require towing through              not encounter ice, but, if not rigged
ice. Towing ships may also be required to                 properly, could cause damage to both
recover ships with no steering or propulsion              the tug and the tow.
capabilities that have been stranded in ice
conditions.                                          7-4.1    Short-Scope Method

An icebreaker may be required for breaking           Navy ocean tugs should use the short-scope
through heavy ice, but Navy ocean tugs can           method because they have no saddles. A haw-
tow through thin ice or broken ice. The Navy         ser scope of 150 to 300 feet should be main-
ARS 50 and T-ATF Classes were built to               tained. The tow’s rudder can be used, if nec-
modified ice strengthening rules, but those
                                                     essary, to keep the tow in the tug’s wake.
with Kort nozzles are less suitable for heavy
                                                     Occasional kicks from the tow’s propeller
ice operations.
                                                     may also be necessary to augment the rud-
The major considerations when towing in ice          der’s force. The tug’s propeller wash should
are:                                                 keep the tow from riding up on the stern; if it
    • Protecting the hawser from ice dam-            does not, the propeller of the tow should be
      age. Long periods of exposure to ice           backed, if possible. Riding lines may be used
      will chafe and wear the hawser. To pre-        for increased lateral stability (See Section




                                                                                                  7-5
                                        U.S. Navy Towing Manual


6-2.3). These lines will be very susceptible to       to the tow’s forecastle bitts to help keep the
chafing.                                              tow following fair. The tow’s engines can be
                                                      used. If the tow begins to jackknife or sheer
                    CAUTION                           or yaw badly, however, it should slow at once
                                                      until it is again under control. A fire hose
        The tug should follow these recom-            should be kept ready at the saddle or stern be-
        mendations and guidelines when
                                                      cause friction may cause fires in the chafing
        towing at short scope:
                                                      material.
          • The pull on the towline will be           7-4.3   Rigging for Tow
            severe if the towed ship sud-
            denly contacts heavy ice.                 The recommended gear for towing in ice con-
         • Take special precautions to                sists of:
            prevent the chain bridle, chain
            pendants, and hawser from                    • Wire rope towing hawser
            chafing. An automatic towing
            machine makes this easier.                   • A 2 1/4-inch chain pendant and connec-
         • Avoid towing on the bitts they                  tion jewelry, or
            may be torn out by the sudden
            increases in tension if ice is               • A 2 1/4-inch chain bridle with flounder
            encountered when towing at                     plate and connection jewelry.
            short scope.
                                                      This heavy gear will provide protection
                                                      against the increased potential for chafing and
7-4.2   Saddle Method
                                                      impact damage. Synthetic lines are not rec-
The saddle method can be used by icebreak-            ommended as main towing gear.
ers and tugs with reinforced sterns and towing
                                                      In a convoy with no icebreaker, any ship may
machines. The U.S. Coast Guard has operated
                                                      be expected to tow and should be prepared to
some icebreakers equipped with towing ma-             both tow and be towed. Rigging the tow bri-
chines or strengthened saddles. Even when a           dle in advance quickly lowers the chance of
towing ship has a saddle, the saddle method           being caught in the ice. Gear should be pre-
may not be practical for tows with sharp              pared in advance; the crew should know how
prows, bulbous bows, or any other protuber-           to complete the rigging quickly and safely.
ances that can interfere with the tug’s propel-
lers and rudders. Normally, the tow can be            Before entering the ice, the bridle or anchor
brought up and held firmly in the saddle by           chains should be rigged to receive a towline.
the towing machine.                                   Even when using a bridle, it is necessary to
                                                      secure bow anchors to keep them from strik-
If the tug does not have a saddle and the short       ing hummocks in the ice. This is especially
scope method of towing in ice is not feasible,        important on low bowed ships.
a variation of the saddle method formerly
used by icebreakers may be possible for tow           7-5 Submarine Towing
ships having strong, broad sterns. The tow is
brought up snug against the tug’s stern, using        This section provides an overview of emer-
extensive chafing gear, and heavy fenders.            gency (unplanned) towing of submarines.
The towline is attached in the normal fashion;        Appendix J provides specific data that will be
the towing machine should be in automatic             useful in rigging submarines for emergency
mode to prevent the towline from parting if           tow. For planned tows and for tows of deacti-
the ships pitch or surge. Two mooring lines           vated submarines, consult NAVSEAINST
can also be passed from the tug’s quarter-bitts       4740.9E Towing of Unmanned Defueled Nu-


7-6
                                      U.S. Navy Towing Manual


clear Submarines (Ref. P). This instruction,        7-5.1.2 Tow Attachment Points
which takes precedence over this manual,
may be useful also in planning and executing                            CAUTION
an emergency submarine tow.
                                                          The submarine's designed towing
                                                          rig was intended for intra-harbor
Submarines are challenging tows. Even                     towing and is not generally accept-
though they may be equipped for towing, the               able for open-ocean towing.
towing arrangements are not as strong as on
typical surface ships, their configurations
                                                    The design of submarines is such that consid-
present serious topside personnel hazards,
                                                    erable ingenuity may be required to find suit-
and they can be very poor at tracking behind        able towing attachment points. See Appendix
the tow ship.                                       J of this manual for details on towing arrange-
                                                    ments for specific submarines.
7-5.1 Towing Arrangements

7-5.1.1 Retractable Deck Fittings                                       CAUTION

                                                          Few deck fittings on submarines
Modern submarines are built with essentially              are designed for loads that are
no flat surfaces on the main deck. All subma-             commonly considered in the design
rine deck fittings are either retractable or re-          of surface ships. Care must be ex-
cessed. They are normally constructed so that             ercised to ensure that the safe load
                                                          capacity of fittings, such as the
they can be retracted to form a flush deck,
                                                          bu lln o se fa irle ad , c le a ts , a nd
and rotated into position where they can be               padeyes, is not exceeded. Particu-
used. In most cases, deck fittings can be ex-             lar attention must be paid to the
pected to be safe for working up to the break-            loads that may develop when the
ing strength of the line with which they are              submarine yaws.
normally used.
                                                    On several classes of submarines, a tow pad
Most submarines have small hydraulic cap-           is installed on the forward portion of the sail,
stans, fore and aft, that can be useful in han-     where it is faired into the main deck. This is a
dling lines. They typically have a limited ca-      hard point with an SWL of about 47,000
pacity of 3,000 pounds line pull at a               pounds, depending on submarine class. On
maximum 40 fpm and a maximum pull of                some other classes, the tow pad is installed
4,500 pounds at creep speed. These capstans         forward of the forward escape trunk and is al-
are severely limited in assisting with the con-     so rated at 47,000 pounds. The latest subma-
nection of a towing hawser. The tug, accord-        rines are intended to be towed using a bri-
ingly, should plan its connecting procedure to      dle-flounder plate arrangement secured to a
minimize reliance on the submarine’s cap-           pair of 70,000-pound (SWL) mooring cleats.
stans.                                              On some submarines, the intended tow point
                                                    may have been removed. In such cases, an
The controls for the retractable capstan are
                                                    emergency tow may well involve use of some
usually designed so that the capstan can be
                                                    of the installed cleats or other deck fittings
operated from topside. The machinery, how-
                                                    such as capstans.
ever, is activated from inside the submarine
and is dependent on the submarine’s having          As a last resort, towing by the stern planes,
hydraulic power.                                    the propeller, or the sail may be the only al-


                                                                                                      7-7
                                      U.S. Navy Towing Manual


ternatives. In such an event, all parties must      Guidance from the submarine crew is particu-
be aware of the damage that will likely result.     larly valuable in the area of safety. They are
                                                    far more experienced in the problems of
7-5.2   Personnel Safety Issues
                                                    working topside on the submarine than
The main deck of a submarine is frequently          non-submarine personnel. The submarine can
inaccessible and dangerous to board in a sea-       also provide additional assistance if required.
way. There is very little freeboard, and if         The submarine, however, should follow the
there is any sea running, the decks will most       guidance provided by the tug. The tug is re-
likely be awash. Great care is required when        sponsible after the tow connection is made.
moving about on the deck; a tether or safety        7-5.2.4 Submarine Atmosphere Problems Re-
line should be used. A safety track is provid-              sulting from Fire
ed for attachment of personnel-restraining
                                                    If the submarine has had a fire or has dis-
safety lines. The necessary fittings and har-
                                                    ch a rg e d i ts e x tin gu ish in g syste m , t he
nesses are carried on the submarine for use
                                                    atmosphere inside may not be of breathing
with this track.
                                                    quality. If entering the submarine is neces-
7-5.2.1 Protection for Work on the Deck             sary, proper breathing equipment should be
                                                    used. The atmosphere in the submarine is
When connecting to a submarine in the open          difficult to clear unless it is possible to run
sea, all personnel working on the deck should       some of the equipment on the submarine.
wear full wet suits, survival suits, or other       Running the low-pressure blower or the
such dress that will provide both thermal and       emergency diesel engine will quickly pro-
physical protection if they are washed over-
                                                    vide a change of atmosphere.
board. No one should be permitted to work
without proper life preservers or other appro-      7-5.3   Tendency to Yaw and Sheer
priate safety equipment.                            Some model tests of the towing characteristics
7-5.2.2 Boarding the Submarine                      of the various classes of submarines have been
                                                    conducted. These tests confirm the observed
An inflatable boat may be the only successful       tendency for submarines to yaw and sheer far
means for boarding a submarine. It is helpful       off the towing track. This can be improved if
if the submarine is able to rig a Jacob’s ladder    the submarine is trimmed by the stern. This
alongside for boarding purposes.                    can be done by sealing ballast tanks and de-
7-5.2.3 Personnel Experience                        ballasting the sonar dome. These actions will
                                                    also provide more freeboard forward for rig-
Submarine deck hazards are frequently com-          ging the tow wire, thus facilitating the tow op-
pounded by limited personnel experience.            eration. In deep water, deploying the stern an-
Because submarines normally conduct inde-           chor of the submarine may also help
pendent operations, their personnel have few        (assuming that hydraulic power is available).
opportunities to become familiar with many          If rudders and planes are not being used to
of the deck seamanship procedures that are          control the submarine, they should be secured.
common to personnel on surface ships. At the
                                                    7-5.4   Rigging for the Tow
same time, personnel on the tug may have lit-
tle or no experience with submarines and may        Innovation is often required when rigging a
lack familiarity with the particular fittings,      submarine for towing. Creative thinking is
equipment, and limitations of the submarine.        needed both when making up a connection
Good communications between the subma-              and when selecting the hardware to use. In an
rine and tug crews are especially important.        emergency, it is better to rig something as


7-8
                                      U.S. Navy Towing Manual


strong as possible the first time, accepting        7-5.4.2 Underwater Projections
some possible damage, than to risk loss of the
tow at a more inopportune time in the future.                          CAUTION

See Appendix J for information on rigging                 Every retractable item forward of
specific submarines.                                      the tow fairlead (or flounder plate, if
                                                          used) must be retracted by the
7-5.4.1 Hardware                                          submarine crew to preclude dam-
                                                          age to the submarine and the tow
There is no assurance that towing hardware
                                                          hawser.
will be carried by the submarine. Occasional-
ly the submarine will carry special shackles
or other hull fittings to connect the towline to    The submarine crew can provide information
the tow point. In most cases, however, a Navy       on the location of all underwater projections.
tug should have sufficient gear to make up a        These projections must be rigged in to avoid
towing connection superior to that included in      problems. Submarine personnel may not ap-
the submarine design.                               preciate the deep angle of the tow hawser re-
                                                    sulting from an adequate catenary. In addi-
The ship conducting a tow must determine            tion, most submarines can take wide swings
what special jewelry is available or required,      from the direction of the tow, meaning that
from either the submarine crew or the appro-        any projections forward of its tow fairlead,
priate Squadron or Type Commander. If re-           including items on the keel, can damage or be
quired jewelry is stored ashore, it may be pos-     damaged by a tow hawser or pendant. If there
sible for the tug to pick it up before getting      is any doubt, a diving survey should be made
underway or to have it delivered to the scene       to assure the hull exterior is clear.
of the casualty. Modifications to submarine’s
designed towing jewelry may be necessary as         All tugs must also be aware that U.S. subma-
circumstances warrant. When jewelry is not          rines have keel anchors, often located aft. If
available, it may be necessary to manufacture       such a submarine is anchored, it will head
it. The necessity of providing for both ade-        downstream. See Appendix J for identifica-
quate strength and chafing capability for           tion of anchor location by submarine class.
whatever jewelry is employed must be kept in        7-5.4.3 Towing by the Stern
mind.
                                                                          NOTE
It is advisable to use a length of chain as a
chafing pendant where the tow connection                  Use of the submarine’s anchor
passes through the fairlead chock. It may be              chain for towing may be feasible if
necessary to include a wire between the con-              its windlass is operable.
nection point and a short length of chain to re-
duce the length (and weight) of chain used.         A submarine that has been damaged by a
The chain needs to be just long enough to take      grounding or collision may require a stern
the chafing at the fairlead. Assistance may be      tow. For submarines whose anchor is located
available from the submarine’s hydraulic            aft, the anchor chain is the first choice for a
deck capstan, if it can be rigged and operated.     stern tow. Careful coordination is critical. By
Keep in mind the limited capacity and speed         using divers, the tow ship may be able to con-
of the capstan. For submarines using a bridle       nect to the submarine’s anchor chain, with or
attached to a set of mooring cleats (chiefly        without the anchor removed. It also may be
SSN 688 and SSBN Classes), no fairlead is           possible to dip a wire around the anchor
used and a wire chafing pendant is sufficient.      chain. If the stern planes or the propeller must


                                                                                                    7-9
                                        U.S. Navy Towing Manual


be used for a tow point, take great care to en-       7-5.5    Towing Operations
sure that the attachment chain, strap, and so
forth, are wrapped close to the hull. When us-                             CAUTION
ing stern planes or rudder, the strong operat-
ing shaft extends only a short distance into the              Due to their severe sheering ten-
                                                              dencies, submarines should em-
control surface. It is important that the attach-             ploy active steering (if available) as
ment point be held against the hull and not at                directed by the towing vessel.
the outboard side of the rudder or plane.

7-5.4.4 Use of the Sail as a Tow Connection           Once a suitable tow connection is achieved,
                                                      come up to speed very carefully. A constant
For connection to a sail, consider chain, wire,
                                                      watch should be kept on the position and atti-
or a wide heavy strap that is fabricated from
                                                      tude of the submarine. At night, it may be
plate or from a wide synthetic lifting strap.
Chafing gear may be required to distribute the        necessary to require the submarine to contin-
load because the sails after edge may be              ually report its relative position until a stable
brought to a relatively sharp edge. Suitable          condition is achieved. If they cannot be
chafing gear can be fabricated from a short           steered, many submarines will tend to sheer
section of split pipe and plate. Rigging such a       off and hold a position as much as 70 degrees
device, however, is not a simple task at sea.         relative to the tow ship’s stern. Sometimes
                                                      the submarine will hold this extreme position
7-5.4.5 Welding to the Hull
                                                      for hours, only to veer suddenly to the other
                                                      side without warning. The tug’s Conning Of-
                   CAUTION
                                                      ficer must be advised immediately. In such a
       Contact NAVSEA to obtain techni-               case, the Conning Officer may have to reduce
       cal advice before any welding is               power to prevent the tug from surging ahead
       done to a submarine's pressure                 and compounding transient stresses devel-
       hull.
                                                      oped when a submarine fetches up on the oth-
                                                      er side. This sheering characteristic, coupled
If welding is required, make sure that towing         with a lack of strong fittings, is a major rea-
pads are fastened to the pressure hull (as op-        son for insisting upon relatively modest ten-
posed to the non-pressure hull) and that the          sions in towing submarines.
welding is done in accordance with the speci-
fications for the material of the hull.               7-5.5.1 Towing on the Automatic Towing
                                                              Machine
7-5.4.6 Passing a Messenger
                                                      Every effort should be made to tow with an
In establishing the initial connection, it is eas-    automatic towing machine. Controls should
ier for the submarine to pass an initial line to
                                                      be adjusted for a maximum tension setting
the tug than vice versa. Limited deck space
                                                      not to exceed the safe working loads of the
on the submarine makes it difficult to catch a
heaving line or the line from a line throwing         components used for the tow rigging and fit-
gun. It may be easier to rig a double messen-         tings on board the submarine. Deploying a
ger around the tow connection and use the             synthetic spring will also help to reduce peak
tug’s power to heave around on the hawser. A          tensions. More information about the use of
sufficiently long messenger should be pre-            springs is contained in Section 4-6.5 and
pared in this case.                                   NAVSEAINST 4740.9E (Ref. P).


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                                     U.S. Navy Towing Manual


7-5.5.2 Towline Tension and Towing Speeds          the distressed vessel is capable of surviving
Attainable towing speeds will be dependent         any increased seas.
upon weather, class of submarine, type of          Information on the events and circumstances
connection, equipment used, and the ability        surrounding the towing should be collected
of the submarine to use its rudder. In general,    and documented as soon as practical, if not
towline tension should be limited to 25,000        immediately. The following information may
pounds for all submarine classes built prior to    help the Navy in subsequent claims for reim-
the SSN 688/SSN21/SSBN 726 Class subma-            bursement:
rines. This will provide about five knots tow-
ing speed under favorable sea conditions. The      a. Note reasonable availability of adequate
688/21/726 Class submarines should be limit-          privately-owned or commercial towing
ed to a maximum of 35,000 pounds tension,             assets at the time that the Navy towed the
resulting in about four knots speed under fa-         distressed merchant ship. Examples of
vorable conditions. Normally, increasing ten-         such information are:
sion/speed should not be attempted without            • Location of the nearest privately-owned
first observing the tow’s behavior and con-             or commercial towing vessels.
sulting with appropriate operational and tech-        • How the existence of nearby towing
nical authority.                                        companies or vessels was known. Are
As with all towing operations, it may be nec-           they locals whose presence was known
essary to slow down and simply maintain                 from past incidents that resulted in the
steerage when the weather is severe.                    need for towing or salvage? Were they
                                                        discovered as a result of communica-
7-5.5.3 Drogue
                                                        tions at the time of the casualty that re-
If the submarine rudder is out of commission,           quired the tow?
a drogue rigged behind the submarine may
                                                   b. Nature and extent of services rendered.
assist it to stay on course. In deep water, the
stern anchor may be deployed. In narrow wa-        c. Location of the nearest safe haven. If a
terways or where interference from other traf-        merchant ship was towed elsewhere, the
fic is anticipated, docking (harbor) tugs             reason for towing to that farther point
should be used alongside to properly control          should be documented.
the submarine’s movements.
                                                   d. The citation of funds, cash deposit,
                                                      "promise to pay", or other agreements ar-
7-6 Towing Distressed Merchant
                                                      ranged by the merchant ship prior to the
    Ships
                                                      commencement of any operation.
Occasionally, during routine operations and        Supervisory control of the effort will ordi-
national emergencies, the Navy is called upon      narily remain in the Navy. A situation may
to engage in towing merchant-type ships in         arise, however, in which it may be advisable
distress. These may be MSC ships, chartered        to relinquish supervisory control to an owner
ships, ships engaged in support of operations,     or underwriter’s designated representative,
or any other merchant ships requiring assis-       even though Navy facilities are required. Re-
tance. In emergencies and in remote areas,         linquishment of supervisory control may be
these services also may be required to save        effected upon authorization by the cognizant
lives and valuable ships and cargo. If pollu-      naval commander or higher authority. Prompt
tion is a concern, towing the ship to sea will     notification should be made of such action to
likely reduce the impact of any spill. Be sure     CNO; the cognizant Fleet Commander in


                                                                                             7-11
                                     U.S. Navy Towing Manual


Chief; numbered Fleet Commanders; Naval            Bracket (see Figure 4-8). Alternative points
Surface Force Commander; COMNAVSEA-                for attachment include the bitts, the anchor
SYSCOM; and other interested authorities,          chain, and the foundations of deck machinery
because this may well affect the status of the     (see Section 4-5).
Navy’s claim. Relinquishment of superviso-
ry control shall in no case be construed to af-    Many commercial vessels have an emergency
fect the responsibility of commanding offic-       tow hawser and connecting jewelry in a pack-
ers for the safety of their ships.                 aged arrangement on the bow or stern. These
                                                   boxes usually require assistance from the
NAVSEA 00C should be contacted (703 607            crew or a boarding party. Light weight mate-
2753; DSN 327 2753; 24 hours: 703 602              rial is usually used and a connection can be
7527; DSN 332 7527) to assist in assessing         made very quickly. This arrangement should
commercially available assets. Information         be sufficient until a more permanent arrange-
can also be provided regarding towing proce-       ment can be made.
dures.
7-6.1   Information Sources                        7-7 Ships with Bow Ramp/Door
Various companies and trade groups have as-
sembled information intended primarily to          LST type tows are required to have hydraulic
provide guidance to merchant tanker opera-         rams connected with bow ramp operating in-
tors in contingency planning. This same in-        structions posted in the hydraulic control
formation can be equally valuable to Navy          room. Ensure that mud flaps at the bottom of
personnel who may become involved in res-          the doors are secured and that all dogs, heavy
cue responses to merchant ships in distress.       weather shackles, ratchet-type turnbuckles,
Some particular publications are cited in In-      and strongbacks are tightly and securely in
ternational Maritime Organization (IMO)            place so that they cannot work free. YFU/
Resolution A.535(13), Recommendations on           LCUs are inherently unseaworthy due to their
Emergency Towing Requirements for Tankers          wide beams and flat bottoms. A lift of oppor-
(Ref. Q) and International Chamber of Ship-        tunity should be used whenever possible. If it
ping Oil Companies International Marine Fo-        is absolutely necessary to tow these crafts, the
rum, Peril at Sea and Salvage: A Guide for         following must be strictly adhered to:
Masters (Ref. R).                                     • The bow ramp must be secured with a
7-6.2   Attachment Points                               minimum of four angle straps on each
Ideally, a distressed ship would present an             side, welded on the outside of the ramp.
easily reached connection to the rescuer. This          Straps should be at least 4 inches by 3/8
would be a complete system including the                inches and overlap the bow ramp and
hawser, or at least everything necessary to             sides of the craft by a minimum of 10
connect the hawser to the ship. The Oil Com-            inches.
panies International Marine Forum (OCIMF)             • All normal securing devices (such as
recommendations have been superseded by                 ramp chains, dogs, and turnbuckles)
similar IMO standards, but these have not               must be in place and in good mechani-
been formally adopted. Nonetheless, many of             cal condition.
the larger tanker operators have complied
                                                      • All hatches, scuttles, and doors must
with the IMO recommendations.
                                                        have good gaskets and all securing
Many ships employ a prearranged attachment              devices must be in proper operating
point on the tow such as the Smit Towing                condition.


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                                     U.S. Navy Towing Manual


7-8 Towing Distressed NATO Ships                   The operational data contained in ATP-43,
                                                   while accurate, are quite elementary com-
The NATO navies are concerned with emer-           pared to the background of the experienced
gency towing as part of their military mis-        tug crew. Nonetheless, knowledge of the con-
sions as well as normal maritime concerns for      tents of ATP-43 will be useful to the naval
safety of life at sea and pollution prevention     tug or salvage ship since it describes what the
for all ships.                                     crew of the casualty should know concerning
                                                   being towed.
7-8.1 Standardized Procedures (ATP-43)
Standardized NATO emergency towing pro-            7-8.2   Making the Tow Connection
cedures are found in the unclassified ATP-43
                                                   It may be prudent to use the casualty’s own
(Ref. B). It was written for the situation
                                                   hawser and end fitting to expedite the remov-
where one combatant tows another. In this
                                                   al of the casualty from immediate danger. In
type of operation, each ship typically pro-
                                                   such a case, the casualty may have already
vides its own towing hawser as half of an en-
                                                   rigged its own hawser ready to pass to the
tire rig of reasonable length. As in the U.S.
                                                   tug. The tug need only heave the casualty’s
Navy, this activity is sometimes referred to as
                                                   hawser on board the tug to make the final
“tow and be towed.” ATP-43 includes sec-
                                                   connection to its own hawser, thus being
tions on:
                                                   ready to commence towing shortly after arriv-
   • Principles of Operations                      ing at the scene. The towing system can be re-
   • Organization and Command (including           rigged with the tug’s more robust gear after
     Communications)                               the casualty is removed from immediate dan-
                                                   ger.
   • General Consideration of Towing Op-
     erations                                      This is not to suggest that the damaged ship’s
   • Preparation, Approaching the Casual-          towing gear is preferred over a tug or salvage
     ty, Passing and Connecting the Towing         ship’s gear. On the contrary, the tug’s gear
     Rig                                           will be more robust than that of all but the
   • Conduct of the Tow                            largest warships, and will almost always be
                                                   longer than the casualty’s hawser. Further-
   • Emergency Release or Parting of the Rig       more, unless the emergency hawser is con-
   • Transferring the Tow                          nected to the ship’s anchor chain, there will
                                                   be insufficient long-term chafing protection
The Annex to ATP-43 contains data on the
                                                   for the casualty’s own hawser, and possibly
emergency towing hawser carried by each
                                                   insufficient catenary as well. Use of the tug or
class of NATO warship and auxiliary ship, as
                                                   salvage ship’s towing gear is preferred for
well as the end fittings on the hawser. It also
                                                   towing a warship or naval auxiliary. Connect-
provides hawser strengths and dimensions
                                                   ing to the casualty’s hawser as an expedient
and the static tests of the end fittings.
                                                   means should be based on a careful balancing
ATP-43 should be available on board every          of the tactical circumstances, rapidity of com-
NATO warship and auxiliary vessel. The             mencing the tow, distance to be towed, and
assigned tow ship might remind the dis-            existing and forecast wind and sea conditions.
abled ship’s Commanding Officer of the             If the tactical situation requires initial use of
publication’s existence, so that the disabled      the casualty’s hawser, re-rigging to the more
ship can better prepare for the arrival of the     conventional connection is recommended at
tow ship.                                          the earliest possible opportunity.


                                                                                               7-13
                                    U.S. Navy Towing Manual




                                                     14 0 M M
                                                                 2 1 /8 ”
                                                                55 M M



                                                  13 15/16”
                                                 3 55 M M

                             Figure 7-2. NATO Standard Towing Link.


When connecting to the casualty’s own emer-                     c. The interface will be at the presented end
gency towing hawser, the towing ship should                        of one or both ships’ towing hawsers.
consider inserting a shot of chain between the                     (One of the ships will have to provide a
two hawsers to assist in maintaining a healthy                     joining shackle.)
catenary, provided that the water is deep
enough. This may complicate recovery if the                     d. The NATO Standard Towing Link shall
rig is to be changed at sea.                                       conform to the dimensions shown.
7-8.3   NATO Standard Towing Link                               e. The strength of the link is the responsibil-
Change 1 to the publication (May 1987) also                        ity of the providing nation.
specifies a NATO Standard Towing Link,
which should soon be found on NATO ships                        The link is quite large, so the largest conceiv-
of over 1,000 tons displacement (see Figure                     able tow shackle (4 inches) can be dipped
7-2).                                                           through it. Note that the strength of the link is
                                                                left to the Providing Nation. In the absence of
The ATP-43 comments relevant to the NATO                        information to the contrary, assume that the
Standard Towing Link are:                                       link strength exceeds the breaking strength of
a. The NATO Standard Towing link is to be                       the casualty’s emergency tow hawser. As-
   used during ship-to-ship towing opera-                       sume that the casualty’s attachment points al-
   tions as an interface between the towing                     so exceed the strength of its hawser.
   equipment of the towing ship and that of
   the ship towed, whichever of the two                         7-9 Unusual Tows
   ships provides the equipment, in order to
   improve interoperability.
                                                                Conditions may require towing floating struc-
b. Ships of less than 1,000 metric tons dis-                    tures that are in unusual positions. Many
   placement, other than tugs, are not obliged                  such tows have been successfully completed
   to have the Standard Towing Link.                            in the past.


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                                     U.S. Navy Towing Manual


7-9.1 Dry Dock (Careened)                          proach inland towing in a professional man-
                                                   ner.
One example of an unusual tow is the towing
of an AFDM through the Panama Canal.               7-9.4    Other Tows
These dry docks are approximately 124 feet
wide. Because the canal is only 109 feet wide,     Contact NAVSEA 00C for information con-
these docks must be careened for transit. This     cerning advice on unusual and unique tows
has become an established practice. When the       including:
transit operation has been completed, the ca-         •    NR-1, submerged tow
reening procedure is reversed to restore the
                                                      •    Towing of gravity structures
dock to its even keel condition for towing to
its destination. An attempt should be made to         •    Non-self-propelled floating structures
adjust the trim to improve the behavior.              •    Minesweeping devices
7-9.2 Damaged Ship (Stern First)                      •    Submerged and surface towing of sub-
                                                           mersibles
If a ship cannot be prepared properly for tow         •    SINKEX
due to bow damage, the feasibility of towing
                                                      •    Test bodies
by the stern may be considered. Some ships
will tow fairly easily by the stern, but most         •    Platforms
can be expected to track very poorly.                 •    Pipe structures
7-9.3 Inland Barge Towing                             •    Cable-layers
                                                      •    Acoustic arrays
Barge towing supports Navy logistic require-
ments. The basic techniques for inland barge          •    Semi-submersibles
towing are almost identical for harbor tugs           •    Ships of unusual hull forms (SWATHs,
and towing ships. The principles of alongside              PHMs, and so forth).
towing and handling become part of the
open-ocean tow in making up, streaming, and        7-9.5    Towing on the Hip
entering the harbor. Naval Education and
                                                   While it is common practice for harbor tugs
Training Command (NAVEDTRA) 10122-E,
The Boatswain's Mate First Class and Chief         to tow on the hip, it is somewhat unusual for
Rate Training Manual (Ref. S) provides a           an ocean tug to do so. However, if an ocean
thorough discussion of inland barge towing in      tug is involved in a salvage it may be neces-
its most common configuration, alongside.          sary to engage in this type of towing. Caution
Understanding the basic principles set forth in    should be taken if there is any sea state. Beam
that manual will enable personnel on board         waves, may cause the vessels to roll out of
the ocean going tug or salvage ship to ap-         sync and alternately separate and collide.




                                                                                             7-15
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       This Page Intentionally Left Blank




7-16
                                      U.S. Navy Towing Manual
                                                                                       DRAFT
                                                    8-1.1   Repair Work
                Chapter 8
                                                    Conducting a commercially chartered lift is
      HEAVY LIFT TRANSPORT                          an expensive undertaking therefore, any re-
                                                    pair work to the asset should be arranged so it
                                                    does not interfere with the heavy lift contrac-
                                                    tor. For example, on the USS COLE heavy
8-1    Introduction
                                                    lift the Heavy Lift Project Team prepared a
                                                    design sketch for a hull patch. The patch was
This chapter describes the personnel, proce-
                                                    not installed prior to departure due to cost and
dures, preparations, and safety precautions re-
                                                    operational considerations. The cost of delay-
quired for float on/float off (FLO/FLO) heavy
                                                    ing the operation for repairs is usually far
lift transports of Naval ships and craft. Heavy
                                                    greater than the cost of dry dock lay days and
lift, as used in this chapter, is defined as the
                                                    such costs should only be incurred in emer-
transportation of a ship, craft, or other asset
                                                    gency situations after careful consideration
aboard a larger semi-submersible ship or
                                                    by the Operational Commander. Having the
barge. FLO/FLO refers to the method of load-
                                                    asset completely out of the water does present
ing and unloading. This alternative to towing
                                                    a unique opportunity for inspection of hull fit-
was developed for the movement of large
                                                    tings and rudders/propellers and certainly
drilling rigs and other offshore structures.
                                                    should not go unrealized. For example, on the
The United States military has used this
                                                    Desert Storm lift of minesweepers four pro-
method to transport smaller vessels some of
                                                    pellers were found to be damaged and were
which were not suited for ocean transit as
                                                    repaired because the assets were going to
well as damaged vessels that could not transit
                                                    war. Because this is a transport operation, the
safely under their own power.
                                                    asset has to be ready for sea transit; in partic-
Heavy Lift was used to return the bomb dam-         ular the water-tight integrity of the hull must
aged destroyer, USS COLE (DDG 67), mine-            be maintained. The operational commander
damaged frigate, SAMUEL B. ROBERTS                  should identify a ship repair officer for the
(FFG 58), from the Persian Gulf and to trans-       team to coordinate their work.
port smaller assets such as mine warfare
ships, landing craft (LCU) and service craft        8-2 Special Considerations
across the ocean. Two separate lifts brought
minesweepers from the United States to the          8-2.1   Dry Docking Comparison
Persian Gulf and three lifts brought others
back. Since these were operational ships            A float on/float off procedure may be consid-
whose mission required rapid safe transport,        ered similar to operations involving a dry-
heavy lift was used. Tugs, barges, and float-       dock. Both involve positioning a floating as-
ing cranes have also been moved using the           set over docking blocks and then reducing the
FLO/FLO process.                                    amount of water or distance between the ves-
                                                    sel and the blocks. In the case of a graving
The Navy does not currently own any heavy
                                                    dock, the water is pumped out of the dock and
lift FLO/FLO ships and therefore uses con-
                                                    the asset settles on the blocks. In the case of a
tracted vessels to perform these services. This
                                                    floating drydock, the draft of the drydock is
chapter assumes that the heavy lift ship is a
                                                    decreased by removing water from tanks until
contracted vessel.
                                                    the blocks “lift” the asset. Although these
This chapter does not apply to nuclear pow-         procedures are similar, the FLO/FLO portion
ered ships with the core installed.                 of a heavy lift transport is much more in-


                                                                                                 8-1
DRAFT                                 U.S. Navy Towing Manual


volved. Take the following considerations in-       dock. The large wing walls provide some
to account:                                         added protection to weather, but assets must
                                                    be loaded from astern. Figure 8-3 shows a
      • The transport may be of a single asset,
                                                    vessel with a deckhouse fore and aft. In this
        multiple assets from the same squad-
                                                    case, assets must be floated on from port or
        ron, or multiple assets from different
                                                    starboard.
        operational commanders. Assets of oth-
        er services may also be transported.        Smaller ships of this type also exist but are
                                                    not used to perform lifts of larger vessels.
      • The asset(s) may be lifted in open water
                                                    Commercial submersible and semi-submers-
        areas.
                                                    ible barges may also meet the requirements of
      • Seafastening must be installed to ensure    some heavy lifts. Barges have the added com-
        that the asset remains secured on the       plexity of a tow arrangement and are also
        cargo deck of the heavy lift ship.          considered less desirable due to stability con-
                                                    cerns.
      • The assets must be secured internally
        for the sea transit.                        8-2.3   Choosing a Vessel

      • The asset being lifted may be in final      When deciding on which type of vessel to
        days of preparing for extended deploy-      use, several factors must be considered.
        ment and therefore may be topped off        These factors are similar to those used to de-
        with provisions, fuel, and water.           cide on a towing asset and whose significance
                                                    will vary depending on the mission being
      • The contract under which a FLO/FLO          supported. For instance, for a coastal or in-
        transport is accomplished is a vessel       land lift, a barge may be suitable but for an
        charter and differs significantly from a    trans-ocean voyage, the added seaworthiness
        dry docking contract.                       of a specially designed vessel will likely be
      • Some of the asset’s systems may be op-      worth the extra cost. Some of the factors to be
        erational during transit. Therefore all     considered are shown in Table 8-2.
        power and support interface require-
        ments must be identified.                   8-3 Procedures
8-2.2    Commercial Fleet
                                                    This section will discuss planning a FLO/
Some basic characteristics of the larger, com-      FLO operation. Few FLO/FLO operations are
mercial heavy lift ships are presented in Table     ever duplicates of earlier operations as there
8-1. These semi-submersible vessels are self-       will always be differences in season (weath-
powered and have large open decks to sup-           er), route, personnel, and configuration of the
port cargo. They contain enough internal            assets. Each FLO/FLO transport is unique
tankage to allow them to ballast down far           and requires careful planning, preparation,
enough that their cargo decks are well below        and execution to minimize error and maxi-
the water’s surface. This allows assets to be       mize safety. This section presents FLO/FLO
floated over the deck and lifted upon dewater-      transport procedures in general terms.
ing. This process is almost identical to float-
                                                    8-3.1   Designating the Lift
ing drydocks except that it is often done in
open water. Figure 8-1 shows a typical heavy        The cost of a heavy lift may make this option
lift ship where the assets can be loaded from       seem disadvantageous, but several situations
port, starboard, or astern. The vessel shown in     may dictate that this method may be an ap-
Figure 8-2 is more similar to a typical dry-        propriate way to relocate an asset. Moving


8-2
8-3
                                         Table 8-1. Commercial Submersible and Semi-Submersible Vessels.
      Company Name             Vessel Name                 LOA       Beam     Deck Dimensions      Wall Height          Draft Full Load    DWT abt
                                                                                                                                                        DRAFT


       Netherlands Freight      DEVELOPING ROAD         134.2 m     34.2 m     115.0 m       x   9.2 m           5.15 m                   13,230 tons
       Agencies                                                                29.2 m

       Netherlands Freight      SHA HE KOU              134.2 m     34.2 m     115.0 m       x   9.2 m           5.15 m                   13,230 tons
       Agencies                                                                29.2 m

       Smit Maritime            SMIT PIONEER            160 m       29.0 m     2,880 sq. m       7.06 m          4.43 m                   6,500 tons

       Smit Maritime            SMIT ENTERPRISE         160 m       29.0 m     2,880 sq. m                                                6,500 tons

       Condock                  CONDOCK I               92.4 m      20.13 m    74.6 m x 15 m     6.35 m          4.83 m                   3,603 tons

       Condock                  CONDOCK III             106.4 m     20.4 m     87.5 m x 15 m     7.95 m          4.83 m                   4,074 tons

       Condock                  CONDOCK IV              106 m       20.4 m     87.5 m x 15 m     7.95 m          4.95 m                   4,500 tons

       Condock                  CONDOCK V               106 m       20.4 m     87.5 m x 15 m     7.95 m          4.95 m                   4,600 tons

       Condock                  OSTARA                  106 m       19.6 m     N/A               N/A             4.85 m                   4,400 tons

       Dockwise NV              DOCK EXPRESS 10         153.8 m     24.2 m     2,130 sq. m       8m              8.89 m (max. sailing)    13,209 tons

       Dockwise NV              DOCK EXPRESS 11         159.2 m     24.2 m     2,130 sq. m       8m              8.89 m (max. sailing)    13,209 tons

       Dockwise NV              DOCK EXPRESS 12         159.2 m     24.2 m     2,130 sq. m       8m              8.89 m (max. sailing)    13,209 tons

       Dockwise NV              MIGHTY SERVANT 1        160 m       40 m       4,800 sq. m       N/A             22 m (submerged)         21,500 tons

       Dockwise NV              MIGHTY SERVANT 2        170 m       40 m       5,200 sq. m       N/A             22 m (submerged)         23,300 tons
                                                                                                                                                          U.S. Navy Towing Manual




       Dockwise NV              MIGHTY SERVANT 3        180 m       40 m       5,600 sq. m       N/A             22 m (submerged)         24,800 tons

       Dockwise NV              SUPER SERVANT 3         139 m       32 m       3,500 sq. m       N/A             6.26 m (transit)         14,112 tons
                                                                                                                 14.5 m (submerged)

       Dockwise NV              SUPER SERVANT 4         169 m       32 m       4,380 sq. m       N/A             6.02 m (transit)         17,600 tons
                                                                                                                 14.55 m (submerged)

       Dockwise NV              SWAN                    180.5 m     32.26 m    4,007 sq. m       N/A             10 m                     32,650 tons

       Dockwise NV              SWIFT                   180.5 m     32.26 m    4,007 sq. m       N/A             10 m                     32,101 tons

       Dockwise NV              TEAL                    180.5 m     32.26 m    4,007 sq. m       N/A             10 m                     32,101 tons

       Dockwise NV              TERN                    180.5 m     32.26 m    4,007 sq. m       N/A             10 m                     32,650 tons

       Dockwise NV              TRANSSHELF              173.5 m     40 m       5,280 sq. m       N/A             8.8 m (transit)          34,242 tons

       Hinode Kisen Co. Ltd.    SEA BARON               150 m       32 m       3,600 sq. m       4.9 m           5.025 m                  10,377 tons

                                AMERICAN CORMORANT      223.06 m    42.25 m                                                               47,230 tons
DRAFT     U.S. Navy Towing Manual




        Figure 8-1. Heavy Lift Vessel.




8-4
  U.S. Navy Towing Manual
                                 DRAFT




Figure 8-2. Heavy Lift Vessel.




                                    8-5
DRAFT     U.S. Navy Towing Manual




        Figure 8-3. Heavy Lift Vessel.




8-6
                                        U.S. Navy Towing Manual
                                                                                        DRAFT

                           Table 8-2. Heavy Lift Ship vs. Submersible Barge.

                            Heavy Lift Ship                       Submersible Barge
            Stability       Stable in all operational modes,      Relies on bottom contact for
                            Sheltered in head seas                stability during lift, limited
                                                                  shelter in head seas
     Access to Asset        Asset on deck of vessel, access       Access limited by weather and
                            through brow or ladder                small boat capability
             Support        Designed to support lift ops,         Tug may have limited addition-
                            usually good hotel services           al hotel services
                 Cost       Specialized craft, more expen-        Tug/barge combo may have
                            sive, but generally shorter tran-     cheaper day rate, but longer
                            sit time                              rent time
    Insurance/Risk          Generally insurance is less due       Insurance rates can be a sub-
                            to larger more controllable plat-     stantial cost
                            form
               Speed        Open ocean design, good speed         Tow will be slower
                 Risk       One unit, minimal risk with           With two craft and towline, risk
                            good seafastening plan                is inherently greater

small coastal vessels across the ocean can be          but other areas are more rigid. The heavy lift
slow, costly and a significant risk to both per-       ship will require a certain amount of time to
sonnel and the vessel. A vessel designed to            perform the lift and construct the blocking
operate in sheltered coastal waters is ill-suited      and seafastening. This process can be helped
to survive the winter storms of the North At-          by providing the most up to date documenta-
lantic. The asset may not have been designed           tion.
to have the endurance to make the trip. Tow-
                                                       Once an organization decides that a heavy lift
ing the assets is an option but a long ocean
                                                       is the preferred method of transfer, they
tow of a small vessel is not without its own
                                                       should begin the planning phase by determin-
risks. In the case of a multiple asset transfer, a
                                                       ing some basic details of the operation. Spe-
heavy lift is far safer than a multiple tow. In
                                                       cific information about the what, where, and
the case of a damaged vessel, a heavy lift may
                                                       when of the operation will be needed when
be the only option as towing may not be feasi-
                                                       developing the request for proposal (see
ble.
                                                       8-3.2). If the lift is a planned transfer, the
Figure 8-4 depicts a notional schedule for             Military Sealift Command has been used suc-
preparing a heavy lift. This schedule allows           cessfully to administer contracts with com-
sufficient time to perform all necessary docu-         mercial firms who are experienced in this
ment reviews as well as completion of all              field. In the case of an emergency, NAVSEA
block and seafastening builds. Some portions           00C (Supervisor of Salvage) should be con-
of this schedule are extremely flexible, such          tacted to expedite planning and execution of
as the market search and contract solicitation,        the lift. The remainder of this chapter as-


                                                                                                     8-7
8-8
                                                                                                                                                                            DRAFT



                                                                                 Float-O n / Float-O ff (FLO /FLO ) Heavy Lift Transport
                                                                                                (Nom inal Tim es in Days)



                                                                        -6 6               -5 2            -5 0             -4 5              -3 1          -2 1
                                                                                                                                                                       //

                                                   C onderation       M arket           D ecision        R equest         Request           R eceive     C ontract
                                                   to H eavy Lift     S u rvey          to H eavy           GFI              fo r          P ro posals    Aw ard
                                                   Tran spo rt                             Lift                          P ropo sals
                                                                                        Transp ort




                                                        -1 0             -5                 -4              -3               L                +3             D
                                                                                                                                                                       //


                                                      R eceive      P re-Loading        A pp rove       C om m ence        F loat          Co m plete     Depart/
                                                     Tran sport     C onference         Transp ort       B locking          On                 S ea      Tran spo rt
                                                                                                                                                                              U.S. Navy Towing Manual




                                                      M an ual                           M anual            B uild                         F astening
                                                         fo r
                                                     Ap proval




      Figure 8-4. Plan of Action and Milestones.
                                                        A -2             A                 +1               U


                                                   P re-U nlo ad      A rrival            U nrig          Float
                                                   C on feren ce                           S ea            O ff
                                                                                        Fasteners
                                      U.S. Navy Towing Manual
                                                                                        DRAFT
sumes that MSC has issued the charter, but          successful and timely execution of the lift.
NAVSEA 00C or any office issuing the con-           The items listed below should be accom-
tract would have the same responsibilities.         plished in advance of the date that the vessel
                                                    will arrive at the loading site.
8-3.2 Request for Proposal (RFP)
                                                    8-3.3.1 Choosing a Heavy Lift Team
Previous FLO/FLO operations have been ac-
complished through the Military Sealift Com-        The personnel chosen to be the MSC/Navy
mand (MSC) Headquarters Contracting Of-             coordination team functions much like a Su-
ficer. MSC issues a Request for Proposal            pervisor of Shipbuilding monitoring a dry
(RFP) or modifies an existing time charter to       docking availability. They review the con-
include the details of the particular operation.    tractor’s proposals and ensure that he is per-
When the need for a FLO/FLO operation is            forming the work in accordance with the con-
determined by an operational command, they          tract and the Transport Manual. It is a good
must specify certain requirements to be in-         idea to choose people that can be present
cluded in the RFP or contract modification.         throughout the process (plan development,
Information required may include:                   contract award, loading, off-loading), al-
   • Assets to be lifted                            though it is not necessary. A list of personnel
                                                    is shown here as an example of what has been
   • Dates and locations of load and dis-           used successfully in the past. This list may
     charge points                                  vary slightly depending on the assets to be
   • Supporting activities                          lifted, the personnel available, and location of
                                                    the lift.
   • Asset specifics - class/name, condition
     of readiness, loading condition, value,        Operational Commander
     date of last dry docking                       Generally the owner of the asset, the Opera-
   • Additional cargo                               tional Commander has cognizance of the op-
                                                    eration. He designates the need for a FLO/
   • Asset’s plant service requirements -           FLO operation, identifies the services re-
     both during the FLO/FLO operations             quired to support the asset during all phases
     and preparations and during sea trans-         of the lift, and prepares the asset for transport.
     port                                           The Operational Commander is responsible
Upon receipt of the proposals, MSC (and oth-        for selecting all the members of the heavy lift
er technical authorities; the operational com-      team.
mand, a dry docking authority, NAVSEA               Designated Docking Activity (DDA)
tech codes, SUPSALV, etc.) review the pro-
posals for technical correctness and cost com-      The DDA is the technical point of contact
parison. MSC will then award a contract or          during the planning and approval phases. The
contract modification.                              DDA provides on-site technical personnel
                                                    and requests technical and coordination assis-
8-3.3   Preparations
                                                    tance from cognizant commands as required.
Once a contract has been awarded, several           The activity designated as the DDA should
things need to be done to prepare for the lift.     have experience in docking and undocking
In this preparation phase, communication be-        evolutions and is often located in the vicinity
tween the various organizations is critical to a    of the loading area.




                                                                                                  8-9
DRAFT                                  U.S. Navy Towing Manual


Heavy Lift Project Officer (HLPO)                    tank and draft indicator system) to ensure that
                                                     it operates properly and should monitor this
The HLPO is a senior technical officer, pref-
                                                     system during loading and off-loading opera-
erably an Engineering Duty Officer. Once
                                                     tions.
designated, he or she will be responsible for
coordinating both technical and logistics sup-       Services Coordinator
port for the asset to be lifted and development      This individual coordinates the installation of
of lift requirements as well as review of the        asset plant services such as electrical power,
Transport Manual. The HLPO is the leader of          fire main, and potable water. He/she also co-
the Navy heavy lift team.                            ordinates general vessel support during the
Dry Docking Safety Officer (Docking Ob-              FLO/FLO operation such as line handling, se-
server)                                              curity watches, communications, access,
                                                     scupper overboard discharges, etc. The ser-
The Docking Observer is responsible for the          vices coordinator should be familiar with the
float on and float off portions of the transport     asset in order to verify all support require-
and for seafastening during transport. The           ments.
Docking Observer reviews and approves all
calculations required to conduct the FLO/            Ship Repair Officer
FLO operation. The Docking Observer must             A Ship Repair Officer is assigned at both the
be familiar with the local ship repair and ser-      loading and off-loading sites to coordinate
vice industry and the environmental condi-           any emergent/emergency repair work that
tions in the loading and off-loading sites. He/      may be necessary, using the lift as a docking
she is responsible to ensure that the asset is       of opportunity. Because this is a transport op-
safely positioned, lifted and secured for trans-     eration, the asset must be ready for sea tran-
port, and discharged.                                sit; in particular, the watertight integrity of
Blocking Expert                                      the hulls must be maintained. The Ship Re-
                                                     pair Officer should be familiar with local ship
The blocking expert oversees the construction        repair and other services that may be neces-
and installation of all necessary blocking and       sary to complete repair work.
seafastening. He should be familiar with Na-
vy docking drawings and the construction of          Riding Crew
various types of docking blocks. The block-          The riding crew should be familiar with or
ing expert verifies that all blocks, blocking,       come from the asset being lifted and include
seafasteners, and roll bars (spur shores) are        personnel of each of several rates. For multi-
installed properly and in accordance with the        asset lifts, the riding crew should include at
approved Transport Manual.                           least one representative from each asset. The
Stability Expert                                     size of the riding crew is determined by the
                                                     asset or assets being lifted and the berthing
The stability expert should monitor the opera-       and messing capabilities of the heavy lift
tion to ensure adequate stability of both the        ship. The riding crew is responsible for secu-
ship and the asset at all stages of the lift pro-    rity, damage control, maintenance, and other
cess. He/she should be familiar with the oper-       duties required for assets in a secured or part-
ation of a heavy lift ship or floating dry docks     ly secured status.
and can verify the ballast/deballast sequence
                                                     Independent Marine Surveyor (IMS)
is sound and performed in accordance with
the Transport Manual. He/she should inspect          An Independent Marine Surveyor (IMS),
the ballast/deballast system (including the          qualified by experience and credentials in the


8-10
                                     U.S. Navy Towing Manual
                                                                                     DRAFT
operation of FLO/FLO heavy lift ship opera-        contractual questions should be coordinated
tions and transports, should be appointed and      through this individual.
be present at all FLO/FLO operations for Na-
                                                   8-3.3.2 Contractor Preparations
vy assets. The IMS will be responsible for in-
dependently assessing the following items:         Once awarded the contract, the contractor
                                                   must provide information about the opera-
Transport Manual                                   tion. He must choose loading and unloading
Material condition of the heavy lift ship          sites and develop drawings and procedures
                                                   for the entire transfer including both loading
Ship systems                                       and unloading.
Blocking arrangement                               The primary document detailing the prepara-
Seafastening                                       tions and procedures is the Transport or Load
                                                   Manual. The contractor prepares and pro-
Loading/off-loading procedures                     vides this document in advance (exact dates
Voyage arrangements                                will be specified in the contract, but usually
                                                   no less than four days) of the transport ship’s
Preparation of the asset
                                                   arrival at the load site or the blocking build
The IMS is an independent third party to act       operations begin, whichever occurs first. It is
as a mediator between the Navy and the con-        recommended that the government loading
tractor to provide independent analysis of the     team be in contact with the contractor during
operation and to assist in settling disagree-      the development of the Transport Manual to
ments. The IMS selected should be agreed to        avoid delays if corrections or adjustments
by both the Navy representative and the            need to be made. The document should be re-
heavy lift contractor.                             viewed to:
Loadmaster                                            • Ensure adherence to technical require-
                                                        ments
The Loadmaster is the heavy lift contractor's
designated coordinator. The Loadmaster di-            • Ensure that proper information, includ-
rects the heavy lift ship's crew and subcon-            ing drawings, is provided
tractors during the blocking build, the posi-
                                                      • Verify references and their use
tioning of the asset over the submerged heavy
lift ship, ballasting/deballasting operations,        • Verify all engineering calculation and
and the installation of the seafastening. The           assure appropriate technical topics are
Loadmaster coordinates the off-loading pro-             addressed
cedure as well. The Loadmaster also ap-
                                                   Upon approval, this document will serve as
proves the loading and securing of the deck
                                                   the technical guide for all further events.
cargo before departure.
                                                   8-3.3.3 Transport (Load) Manual
Contract Coordinator
                                                   The contractor shall provide a Transport
The Contract Coordinator works with the            (Load) Manual that details the technical re-
members of the different parties represented       quirements of the lift. The manual includes,
during a FLO/FLO operation to resolve any          but is not limited to, the following:
contract disputes. He will often be a represen-
tative from the Military Sealift Command, the         • Description of the heavy lift ship.
organization that has contracted most Navy            • Particulars of cargo from the heavy lift
lifts in the past. Any modifications or other           contract used by the contractor in the


                                                                                             8-11
DRAFT                                 U.S. Navy Towing Manual


       development of the Transport (Load)               acceptable cargo deck load capacity or
       Manual.                                           drawing(s).
   •   Proposed route and probable sea states          • Structural data for the heavy lift ship:
       to be encountered.                                  - Maximum allowable bending mo-
   •   Motion analysis of the heavy lift ship as             ment calculation.
       loaded with the cargo, for determina-               - Transverse strength calculation sus-
       tion of roll and pitch angles and periods             taining the maximum allowable pon-
       and accelerations for use in developing               toon deck loading in long tons per
       blocking and seafastening arrange-                    linear foot.
       ments.                                              - Longitudinal deflection calculation.
   •   Critical motion curve or table depicting            - Maximum keel block, side block,
       the motions (angle and periods of roll,               and hauling block loading calcula-
       pitch, heave, and surge) to which the                 tions.
       blocking and seafastenings are de-                  - Maximum pontoon deck loading at
       signed and which must not be exceeded                 other than keel block and side block
       in transport.                                         locations, if different than that of the
   •   If an asset overhangs the edge of the                 blocking area.
       heavy lift ship’s deck (either over the             - Structural arrangement and scant-
       side or over the stern), a slamming                   lings.
       study must be prepared to determine the             - Longitudinal and transverse water-
       number of occurrences and accelera-                   tight bulkhead design calculations.
       tions to which the asset may be subject-            - Maximum allowable differential
       ed. The study should ensure the asset                 head between tanks.
       can be safely transported without sus-
                                                           - Maximum allowable differential
       taining damage.                                       head between tanks and exterior
   •   Stability analysis of the heavy lift ship             tank draft.
       as loaded, including calculation of
                                                       • Cribbing/blocking plan/drawings, in-
       loading conditions and intact stability
                                                         cluding table of keel and side block off-
       assessment including righting moment
                                                         sets as specified in the docking draw-
       curves and wind heeling moment, as
                                                         ings and calculation of loads including
       defined in Paragraph 5.3.3.1(b) of MIL-
                                                         analysis of worst-case block loading
       STD-1625 as specified in 8-5.2.3.
                                                         when the heavy lift ship is at extreme
   •   Stability analysis (GM curve) during
                                                         trim angle during ballasting/deballast-
       the ballast/deballast sequence as de-
                                                         ing.
       fined in Paragraph 5.3.3.1(b)(1) of
       MIL-STD-1625, except that the mini-
                                                                        NOTE
       mum GM must not be less than speci-
       fied in 8-5.2.2 unless specifically ap-            Experience with past heavy lifts
       proved by NAVSEA at the request of                 demonstrated that locating side
       the DDA.                                           blocks in the locations as specified
   •   Cargo deck arrangement plan/drawing.               on the Navy docking drawing offers
                                                          the best chance to have proper off-
   •   Structural analysis of longitudinal ben-
                                                          set heights. The Navy standard
       ding stress imposed on the heavy lift              docking drawing is a Selected
       ship by the proposed loading. Include a            Record Drawing (SRD) and takes
       cargo deck load diagram, plating thick-            precedence over all other drawings
       ness, and arrangement and size of trans-           in determining offsets for height of
       verse and longitudinal stiffeners with             side blocks.


8-12
                                       U.S. Navy Towing Manual
                                                                                        DRAFT
    • Descriptions of the docking blocks             The contractor may choose to do his prepara-
      showing the physical characteristics of        tions at a location other than the site of load-
      the blocks, including material and di-         ing. The preparations can be made at any full-
      mensions, and calculations to verify           service, easily accessible location and then
      that the blocks will be stable and struc-      moved to a staging area when ready for sea.
      turally adequate to withstand the load-
                                                     The location should be mutually agreed upon
      ing used in lifting capacity calculations
                                                     by all parties involved in the loading process
      and that side blocks (and shores) are ad-
                                                     including the IMS.
      equate in number to provide sufficient
      bearing area to resist overturning mo-         8-3.3.5 Preparing The Deck
      ments specified herein.
                                                     It is the contractor’s responsibility to prepare
    • Seafastening plan/drawings, including          the deck of the lift ship in accordance with the
      design forces.                                 approved Transport Manual and he will need
    • Loading/off-loading     sequence     plan/     to arrange for any necessary subcontractors.
      drawings.                                      To assist in deck preparations, the contractor
                                                     should be provided with the most up to date
    • The amount of damage the heavy lift            docking drawings available for the asset to be
      ship can withstand and survive without         lifted. The Planning Yard, or NAVSEA,
      dropping the asset off the blocking and        should ensure that information and drawings
      seafastening.                                  provided to MSC for use by the contractor are
8-3.3.4   Choosing A Load Site
                                                     accurate and current. Any activity reviewing
                                                     the Transport Manual should also ensure that
While the points of departure and destination        the docking drawing is the latest markup from
for the assets will be specified in the contract,    the last dry docking, and that blocking loca-
the contractor will select the actual load site,     tions and heights are correct. The minimum
subject to approval. Weather will be the ma-         number of keel and sideblocks is discussed in
jor factor in determining if a choice of load        8-6. These blocks should be installed and in-
sites is good or bad. The location should be as      spected a minimum of 24 hours prior to com-
protected as possible, although open water lo-       mencement of the lifting operation to accom-
cations offshore have been used successfully.        modate any last minute corrections. The
A poor choice of location for conducting             building drawings presented in the Transport
FLO/FLO preparations can lead to major               Manual should be reviewed and approved pri-
problems. If the operation is to be conducted        or to the start of the build.
offshore, take into consideration that the pre-      8-3.4   Pre-Load Conference
ferred anchoring/mooring method may be to
swing on a single anchor.                            A conference should be held prior to loading
                                                     where all parties involved are represented.
The site must have enough water depth to ac-         The pre-load conference covers all aspects of
commodate the heavy lift ship’s required             the procedure so that all parties are familiar
draft for loading or off-loading, plus at least      with their respective roles. Important topics
one meter clearance below the keel. Adequate         to cover at this meeting are personnel, sched-
water depth depends upon the draft of the as-        ules, procedures, and responsibilities. This is
set to be loaded and the height of the blocking      often the first opportunity for some parties to
installed. Semi-submersible barges may re-           have contact with each other. If possible, the
quire that one end of the barge rest on the bot-     conference should be held near the load site
tom (for stability reasons) during loading.          and/or the assets. This will allow site/asset in-


                                                                                                 8-13
DRAFT                                  U.S. Navy Towing Manual


spection and may identify potential problems         drafts, trim, and list. An internal survey to
early.                                               document the asset’s loading and any on
                                                     board weights should also be completed.
This conference should be held far enough in
advance to ensure that any changes or adjust-        A complete checklist for both the heavy lift
ments to the plan can be completed without           ship and the asset is included in Appendix R.
adversely impacting the schedule. It should
also be near enough to the date of the loading       8-3.5.2 Support Tugs/Divers
to allow for as many details as possible to be       To assist in the positioning of the assets over
finalized. Separate meetings should be held          the blocks, support tugs and divers may be
as part of that conference to discuss specific       used. It is important to keep in mind that the
operational details with line handlers, divers,      area of loading will likely not be as well shel-
block builders, tug masters, pilots, etc. It will    tered as a dry dock. Therefore, when selecting
likely be beneficial to have each team leader        the number and size of tugs, assume the worst
attend the conference. A similar conference          case for the weather. Tugs should be of suffi-
should be held prior to discharge.                   cient size to hold the assets in the greatest ex-
8-3.5   Load Site                                    pected wind and seas. If wind and seas are too
                                                     strong, the operation should be postponed or
Prior to the start of the float on operation, all
                                                     another suitable location found. If multiple
assets and support craft must be on scene and
                                                     tugs are used it is important to keep lines of
all preparations must be completed. The
                                                     communication clear. One person, a harbor
heavy lift ship must complete the blocking
                                                     pilot or the loadmaster, should direct the posi-
build (see 8-6) and any preparatory efforts.
                                                     tioning and operation of all the tugs.
These may be completed at the actual load
site or at another facility. All work must be
inspected to ensure compliance with the                                 WARNING
Transport Manual.                                           All sea suctions for the asset
8-3.5.1 Visual Survey                                       and the heavy lift vessel should
                                                            be secured during diver opera-
At the load site (or preparation site), the                 tions.
blocking placement, arrangement, and build
on the deck are inspected to ensure compli-
                                                                        WARNING
ance with the drawings referenced or includ-
ed in the Transport Manual. The materials                   All parties must be informed
used to build the blocks should be in service-              when divers are being used. Ex-
able condition. Any blocks with rotted wood                 treme caution must be used to
should be replaced. At a minimum, a visual                  ensure the safety of these indi-
                                                            viduals. No deballasting or other
survey of the heavy lift ship and its systems is            ship movements should occur
conducted, including the ballast/deballast                  while divers are working directly
system. This survey must be completed satis-                under the asset.
factorily before the heavy lift ship is accepted
(described in 8-9.2).
                                                     Divers should be used to check the final
A survey of the assets should also be conduct-       alignment of the assets on the blocks. There is
ed. The survey should include an inspection          increased risk for divers since the operation is
of the floating condition of the asset including     taking place in open water vice in a drydock.




8-14
                                      U.S. Navy Towing Manual
                                                                                         DRAFT
Blocking heights are generally minimized, al-               valves should be prepared and made
lowing little clearance between the asset and               available to watchstanders.
the cargo deck. Appropriate safety measures             •   All sounding tubes should be capped. A
must be taken and only divers with experi-                  list of sounding tubes and there condi-
ence in checking docking blocks should be                   tion should be prepared and made avail-
used.                                                       able to watchstanders.
Tug masters and divers should be briefed                •   All between tank sluice valves should
about the operation and should be present at                be closed.
the preload meeting. Divers should be thour-            •   All watertight boundaries should be
oughly briefed on the blocking arrangements,                sealed. Where gaskets show signs of
build and marking to include a walk around                  wear or deterioration, new gaskets
of the blocking build prior to submerging the               should be installed.
cargo deck.                                             •   Rudders should be secured against any
                                                            vessel motions. This may be accom-
8-3.6   Preparing the Asset                                 plished after the asset has landed firmly
An asset must be specially prepared to be lift-             on the blocks. It may be accomplished
ed and transported. Many preparations are                   prior to this if no steering is required
similar to preparations for a long deployment,              for docking.
docking, tow, or other special event. Appen-            •   All loose equipment should be secured.
dix P provides a thorough list of all items that    8-3.6.1 Arrival Conditions
should be checked prior to arrival at the load
                                                    When the asset is delivered to the load site, it
site.
                                                    should be in the condition (loading, drafts,
The preparing activity should ensure that the       trim, list, etc.) in which it is to be transported.
asset has complete watertight integrity. It is      If multiple assets are being transported, they
not necessary to go through the rigors of pre-      should be in a similar condition of draft, trim
paring a vessel for tow (locking propellers,        and list. The assets should arrive early
two-valve protection, etc.) but every effort        enough to allow for inspection by the heavy
should be made to make the hull as tight as         lift team, the IMS, and the Loadmaster.
possible. All of these items should be accom-
                                                    The trim should be less than one foot and list
plished as early as practicable, leaving only
                                                    should be less than 0.25 degrees. The final
those that are essential until the loading day.
                                                    configuration and details of loading should be
   • Condition Zebra should be set through-         completed and made available as early as
     out the ship.                                  possible, preferably at the preload confer-
   • All compartments and bilges should be          ence. This will ensure adequate time to pre-
     free from oil and water.                       pare the vessel and plan the lift. It may not be
                                                    possible to bring the asset into proper trim
   • All sea valves should be secured and
                                                    and list by simply adjusting tank levels. All
     tagged out in accordance with normal
                                                    tanks should be topped off or emptied to min-
     tag out procedures. This may need to be
                                                    imize free surface effect.
     done while the vessel is being lifted or
     shortly after float-on. If connections         Weights may be added to help achieve the
     from the heavy lift ship are to be used        right configuration. If weights are placed on
     for items like cooling water, these            the asset to adjust draft, trim, or list, the struc-
     valves should not be secured until the         tural adequacy of the asset to support the
     connection has been made. A list of sea        weights during the transport must be consid-


                                                                                                   8-15
DRAFT                                    U.S. Navy Towing Manual


ered. It must also be considered that the facil-       nications to ensure that all operations proceed
ities at the off-load site may preclude removal        smoothly and all needs are met. Often, align-
of the weights. The docking officer should be          ment columns will be constructed to assist in
notified as added weights may effect the               the athwartships alignment of the asset (See
blocking build. Assets of the same design that         Figure 8-5). Sufficient fendering or other sys-
are positioned alongside one another and in            tem must be employed on the alignment col-
the same longitudinal orientation should be at         umns to prevent damage to the asset. This
a similar draft and trim.                              will depend on the number and size of the as-
                                                       sets being lifted. Support tugs will position
8-3.6.2 Transport of Damaged Vessels
                                                       the asset over the blocks and against the
The transportation of a damaged asset re-              alignment columns or other guiding mecha-
quires careful assessment. Stability must be           nisms. The Loadmaster will verify fore and
assured but draft, trim, and list in excess of         aft position. Divers may also be used to verify
those indicated above may be accommodated              position. (See 8-3.5.2)
by the heavy lift ship's draft, trim, list, or free-
board. For example, USS COLE was lifted                Care should also be taken to ensure that there
with 4 feet of trim by the bow and 1 1/2o list to      is sufficient clearance for all underwater pro-
starboard. This condition was the maximum              jections such as sonars, propellers, bilge keels
that the heavy lift ship could accommodate by          and pit swords. During positioning, a mini-
the freeboard on the after starboard caisson.          mum of 1 foot of clearance should be main-
                                                       tained between the blocks and the asset (in-
                                                       cluding all underwater projections). This
8-4 Loading Operations
                                                       limit is to allow for ship motions, so, if the
                                                       ship is expected to pitch more than 1 foot,
This section will discuss the operations at the
                                                       more clearance should be allowed. No part of
load site. It should be understood that many
                                                       the asset should be closer than 1 foot to the
heavy lift vessels require deep water to oper-
                                                       blocks. A one foot clearance should also be
ate. This may preclude these vessels from
                                                       maintained between the asset and other parts
performing FLO/FLO functions in protected
                                                       of the heavy lift ship structure, such as wing
waters. It is essential that all preparations be
                                                       walls.
completed prior to the day of the lift. Favor-
able weather windows may be small and un-              Actual placement of the assets on the cargo
necessary delays may jeopardize the safety of          deck is dependent on adequacy of working
the operation or cause immense cost increas-           area between and around the cargo. This is al-
es. Furthermore, poor or incomplete prepara-           so affected by installation technique and con-
tion is a leading cause of accidents and haz-          figuration of blocking and sea fastening.
ards.                                                  Forklift trucks can be used to move material
                                                       around the assets on deck. Work space may
8-4.1    Positioning of the Asset(s)
                                                       be limited and spacing may dictate the work
When the heavy lift ship is in position and            flow. A minimum spacing between assets of
ballasted to the proper draft, the Loadmaster          2800 mm (9.2 ft) should be adequate for one
will assume control of the assets for final po-        directional work flow and walking space.
sitioning. The exact point of turnover should          However, twice the minimum spacing allows
be decided and agreed upon by all parties pri-         for two directional work flow and forklift
or to the event. Support tugs, riding crew, and        truck access between the thrust blocks of the
the heavy lift crew should have good commu-            spur shores. A minimum of 2500 mm (8.2 ft)



8-16
                                        U.S. Navy Towing Manual
                                                                        DRAFT




                                          K eel B locks
A lignm ent Colum ns




                                                     S ide B locks
                       W ater Line




                                     Figure 8-5. Assets Being Loaded.




                                                                           8-17
DRAFT                                    U.S. Navy Towing Manual


clearance between the ship and the edge of             dications of damage or stress. The riding
the cargo deck should be adequate for block-           crew should tend the fenders to ensure that no
ing, working and access requirements. If pos-          damage occurs to the asset. See 8-5.2.2 for
sible, additional spacing should be allowed so         more information concerning stability during
that forklifts can still pass between the ships        this critical phase.
after the sea fastening spur shores (roll bars)
                                                       8-4.5    Connection of Services
are installed.
8-4.2    Fendering                                                          CAUTION
Support tugs and alignment columns may be                      Connection of critical services,
used to assist in positioning the asset(s) over                such as fire-fighting, should be giv-
the pre-built blocking arrangement. The                        en priority over other events. Fire-
riding crew should be prepared to provide                      fighting services should be avail-
fendering from the asset in the event that in-                 able throughout the process.
sufficient fendering exists elsewhere. These
fenders should be tended during the deballast-                              CAUTION
ing operation until the vessel comes to its fi-
nal resting position. 4’ x 4’ sheets of plywood                Once the asset is lifted, overboard
                                                               discharge from the asset must be
may prove useful in preventing damage as the
                                                               avoided, restricted, or scuppered
asset is moved into its final position.                        over the side of the heavy lift ship.
8-4.3 Riding Crew Accommodations
During Loading
                                                                            CAUTION
The riding crew is required on board each as-
                                                               When the asset is on board the
set for handling lines and tending hand fend-                  heavy lift ship, a security watch
ers during loading and off-loading. This oper-                 should be established at the gang-
ation may extend for some time and crew size                   way of the heavy lift ship.
should be kept to a minimum. Since the asset
may be in a reduced operating status and have          During the transit, the assets may depend on
no power during the loading, sanitary facili-          the heavy lift vessel for all necessary servic-
ties and/or box meals for all personnel aboard         es. In planned operations, it is common for
the asset during the procedure must be ar-             the riding crew to live aboard the heavy lift
ranged.                                                vessel. However, even if no one lives aboard
8-4.4   Deballasting                                   the assets during the transfer, certain services
                                                       should be made available. Connection of
                    CAUTION                            these services should not begin until the asset
                                                       is in position and the heavy lift ship starts de-
        Personnel must be restricted from              ballasting.
        the heavy lift ship deck and on the
        lifted assets during both ballasting           Fire fighting and cooling water services
        operations and work periods asso-              should be connected as soon as possible after
        ciated with seafastening.                      the asset is secured in position. These connec-
                                                       tions should be completed prior to these sea
Once the assets are satisfactorily positioned,         suctions emerging from the water. Careful
the heavy lift ship should begin deballasting          preparations, including a ship check prior to
procedures. The assets should be observed              the event will ensure a quick and trouble free
carefully for any abnormal motion or any in-           process. Connection of critical services, such


8-18
                                          U.S. Navy Towing Manual
                                                                                          DRAFT
as fire fighting, should be given priority over         the number and size of assets and the com-
other events. Fire fighting services must be            plexity of blocking and seafastening required
available throughout the process. Power ca-             to accommodate the shape of each hull form,
bles and fire fighting hoses, can be pre-staged         the seafastening may require several days of
for ready use when required.                            round-the-clock operation. A qualified Navy
                                                        representative, normally the Docking Observ-
Additional services may be required if the
                                                        er or Blocking Expert, should be present to
riding crew is to remain aboard the asset dur-
                                                        inspect these operations in coordination with
ing the transit. These extra services should be
                                                        contractor personnel at all times. Personnel
considered a secondary priority compared to
                                                        should traverse the area with caution and
deballasting. If a problem occurs with one of
                                                        avoid the area as much as possible to prevent
these connections, deballasting should con-
                                                        accidents and/or delays. Because of the ex-
tinue without delay. A temporary means of
                                                        tensive amount of welding on the cargo deck,
access should be provided as soon as possible
                                                        personnel access and overboard discharge
after the deck is dry. Primary, all-weather ac-
                                                        from the lifted assets must be restricted. Des-
cess may be provided later, but must be in-
                                                        ignated access routes should be created to
stalled prior to departure.
                                                        minimize any interference from traffic. Any
After deballasting, the asset quarter deck              overboard discharges from the asset should
watch should be moved to the cargo deck of              be secured during the seafastening proce-
the heavy lift ship near the gangway. The as-           dures.
set may be expecting technical representa-
tives or other visitors who must be directed to         8-5 Seakeeping and Stability
the safest means of egress. Because these op-
erations are unique and interesting, sightseers
                                                        This section discusses some of the concerns
may be present; safety considerations should
                                                        associated with the stability of the asset, the
be made for them. Similar coordination is re-
                                                        heavy lift ship, and the combination of the
quired for loading and fastening of any "Lift-
                                                        two. Some calculations are presented here,
on/Lift-off" deck cargo.
                                                        but a qualified stability expert will be re-
8-4.6    Blocking and Seafastening                      quired to ensure the safety of all vessels in-
                                                        volved.
                     CAUTION
                                                        During a FLO/FLO operation, several distinct
        Welding and industrial facility safe-           stability considerations must be addressed,
        ty precautions must be followed                 namely:
        closely during blocking and seafas-
        tening.                                            • Stability of the asset
                                                           • Stability of the heavy lift ship
                     CAUTION
                                                           • Stability of the asset/heavy lift ship sys-
        Personnel must be restricted from                    tem during the ballast/deballast opera-
        the heavy lift ship deck and on the
                                                             tion
        lifted assets during both ballasting
        operations and work periods asso-
                                                           • Stability of the heavy lift ship with the
        ciated with seafastening.
                                                             asset secured aboard during transit
Once the ship is deballasted, the blocking and          The various phases of stability are depicted in
seafastening should begin. Depending upon               Figure 8-6.


                                                                                                   8-19
DRAFT     U.S. Navy Towing Manual




                                           Phase 1

                                           B a llaste d dow n,
                                           asset a floa t




                                           Phase 2

                                           A sset kee l
                                           contact




                                           Phase 3

                                           A sset la nded .
                                           H eavy lift vessel
                                           trim m ing to b ring cargo
                                           de ck out o f the w ate r.




                                           Phase 4

                                           F orw ard e nd of carg o
                                           de ck out o f the w ate r,
                                           rem o vin g trim to b ring
                                           aft en d out o f th e w a te r.
                                           A sset o ut of w a te r.




                                           Phase 5

                                           D eba llaste d w ith asset
                                           on bo ard, read y for
                                           trans it.




        Figure 8-6. Phases of Stability.




8-20
                                       U.S. Navy Towing Manual
                                                                                         DRAFT
8-5.1 Ship Motions                                   8-5.1.1 Wind Heel Criteria


                   CAUTION                                              WARNING

       All personnel must strictly adhere                    Loading and unloading shall not
       to the operational plan and safety                    be conducted in winds above 20
       guidelines.                                           knots or in a sea condition of
                                                             sea state 3 or higher.

A FLO/FLO transport is a very dynamic op-
eration. Each of several assets and the heavy        FLO/FLO operations are best conducted in
lift ship move independently when the assets         sheltered waters, however, currents or chan-
are being positioned on the deck of the heavy        nel depths may make this impossible. Wher-
lift ship. This difficult situation is further       ever the operation is conducted, the dominant
complicated if the operation takes place in          weather patterns should be studied. If the
unprotected waters or even in the open ocean.        loading operation is to be conducted in pro-
Once the assets are on the cargo deck of the         tected waters a minimum wind of 60 knots
heavy lift ship, they act as one. This at first      with a gust factor of 1.21 should be used to
would sound similar to the case when the as-         evaluate stability. If the operation is to be
set is in a floating dry dock. The dry dock,         conducted in an open ocean area, the histori-
however, is in protected waters and is not           cal data for that area should be consulted for
normally moved. In a normal dry docking, lit-        expected conditions. A gust factor of 1.21
tle else is done to secure the asset, (internally    should be applied to expected winds. In the
or within the dry dock) with the exception of        event that there is no data available, a com-
providing blocking for earthquake or hurri-          mercial standard of 100 mph (86.8 knots)
cane force winds. With a heavy lift transport,       shall be used as the expected wind and then
the assets must first be made fit for sea (se-       multiplied by the gust factor.
cured internally) and then secured aboard the        Weather routing during transit requires a sep-
cargo deck of the heavy lift ship for transit us-    arate analysis. Information concerning ex-
ing blocking and seafastening. The intent is to      pected sea states and winds should be ac-
hold the asset in position on the cargo deck         quired for planning purposes. Transits
and cradle the asset to keep it from sliding ei-
                                                     include both the transfer from the point of de-
ther transversely or longitudinally or rolling       parture to the unloading site as well and the
over.                                                transfer from the loading site to the building
As the heavy lift ship proceeds through the          site (if they are different). If no data is avail-
waves, it will flex (hog and sag). If the asset      able the commercial standard of 100 mph
is rigidly tied down on the heavy lift ship, this    (86.8 knots) shall be used. In no case shall a
flexing will be imparted to the asset and may        wind of less than 60 knots be used.
cause structural damage. It is therefore neces-      8-5.2   Stability of the Asset
sary to design a structure that will be both
strong enough to resist the motions of the           Stability of the asset, in the case of an unmod-
ship, yet flexible enough not to cause damage        ified or undamaged Navy commissioned ship,
to the asset. An understanding of the dynam-         can be determined by reviewing the data in
ics of the heavy lift ship and the asset is nec-     Chapter II(a) of the ship's Damage Control
essary to create such a structure. How to ana-       Book and a recent Inclining Experiment Re-
lyze the effects of ship motions is covered in       port. Similar information for commercial
Sections 8-6 and 8-7.                                ships should be available in the ship's Trim


                                                                                                  8-21
DRAFT                                 U.S. Navy Towing Manual


and Stability Booklet and the Deadweight            8-5.2.1 Stability Afloat
Survey. Ship's from other services (USCG,           A thorough assessment of the asset’s stability
US Army, etc.) should also have a consolidat-       should be performed prior to the start of the
ed source for this information. These docu-         FLO/FLO process. It is essential to evaluate
ments will provide a good source for informa-       the stability of the asset in its actual condi-
tion for planning purposes and contain              tion. A good weight survey (see 8-6.2.4)
specific measures to improve stability. These       should be conducted on the day of the asset’s
books also contain stability characteristics for    arrival to the loading site to ensure that the
various loading conditions that meet the Na-        actual condition is known. This includes draft
vy's stability criteria.                            readings, tank soundings and determination
For small craft and barges that do not have         of displacement (weight) and centers of grav-
Damage Control Books, follow these general          ity. However, such a detailed analysis may
guidelines when attempting to improve stabil-       not be possible if wartime or emergency con-
ity:                                                ditions mandate quick action. Still, some esti-
                                                    mate of the asset's stability should be ob-
   • Completely fill any slack tanks to re-         tained. If documentation of the ship's stability
     duce the free surface effect                   is not available, the stability may be approxi-
                                                    mated by timing the ship's roll period. This
   • Lower and secure or off-load high              method is reasonably accurate and is used by
     weights                                        the U.S. Navy, U.S. Coast Guard, and other
                                                    regulatory bodies to check the stability calcu-
   • Secure any large hanging weights and           lations to confirm the accuracy of the inclin-
     add ballast                                    ing experiments and other similar determina-
                                                    tions. This method is explained in 8-5.2.3
   • Ballast by completely filling low tanks        and Table 8-6.
Completely filling tanks or adding ballast will     This approximation method is not to be used
decrease freeboard but will generally improve       as a substitute for a thorough stability analy-
stability.                                          sis and weight determination. It only provides
                                                    a measure of a ship’s stability to be used to
Do not shift, add, or remove any weight from        validate stability estimates in emergent condi-
the asset once it is on the heavy lift ship un-     tions.
less specifically authorized by the Loadmas-
ter, including liquids such as fuel or water.       Equally important is frequent verification that
When permission is given to shift weights, an       the ship's roll period has not changed. Even if
accurate record of the amount and location of       overall criteria are satisfactory, any signifi-
the weight change must be kept. Always ac-          cant time increase in the period of roll should
count for weight changes to ensure that the         be promply investigated, since this suggests
asset lifts from the blocks without losing sta-     flooding or additional free surface.
bility or taking an undue list or trim.             8-5.2.2 Stability During Loading

The asset must meet stability requirements          A detailed report of the condition of the asset
for all potential environments of the FLO/          as it arrives at the loading site should be made
FLO evolution. Four different environments          available to all parties so it can be evaluated
should be examined; loading, unloading, tran-       and the heavy lift ship can make the final
sit, and any transitional periods (i.e., from       preparations. All assets of the same design
loading site to building site).                     that are to be loaded in the same fore and aft


8-22
                                                   U.S. Navy Towing Manual
                                                                                                                DRAFT



                                                                     ( ∆ - R KN )
                                                               R esid ual B uoya ncy




                                                                        ∆

                   WL                                                                                    WL
                                                                 GV
                   W L1                                                                                  W L1
                                                                G
                                                        R KN

                                                                    K
                                                                                                 D eck

                                   K e el B lock




                          W L = initia l w a te rlin e
                          W L 1 = d e ba llas ted w ate rline




                                               Figure 8-7. Draft at Instability.


orientation should arrive in a similar condi-                                      • Freeboard requirements and limiting
tion of list (no more than 0.25 degrees), trim                                       submerged draft may preclude exces-
(no more than 1 foot) and draft. It may not be                                       sive trim angles.
possible to meet these limits with damaged
assets.                                                                            • During the ballast/deballast operations,
                                                                                     the trim of the heavy lift ship must be
The heavy lift ship can trim and list to match                                       limited so that the asset does not float
that of the asset. Additional considerations                                         off or slide on the blocking.
may limit the trim and draft of the vessels.
                                                                              The deballasting operation will put the asset
   • Since this operation is performed in a                                   in an unusual stability condition. The reaction
     seaway, the trim may be limited by the                                   of the docking blocks on the asset is equiva-
     stability considerations addressed                                       lent to removing weight from the asset's keel.
     above.                                                                   This weight removal will serve to effectively
   • When deballasting the vessel, consider-                                  raise the asset's center of gravity and reduce
     ation must be given to knuckle loading                                   its metacentric height (GM) and thus reduce
     on forward or after most blocks.                                         its stability (see Figure 8-7). As more and
                                                                              more water is removed from the heavy lift
   • Channel drafts may preclude excessive                                    vessel, the asset will be raised more and more
     trim angles by the heavy lift vessel.                                    out of the water, the reaction on the docking


                                                                                                                        8-23
DRAFT                                  U.S. Navy Towing Manual

blocks will increase and this effect will be in-     8-5.2.3 Draft-at-Instability
creased. The amount of reaction from the             A good measure of a vessel's initial stability
docking blocks is equal to the difference be-        is the vessel's metacentric height (GM). It
tween the asset's floating displacement and          measures the ship's ability to recover from
the displacement at the waterline under con-         disturbances that cause small angles of heel.
sideration in the landed condition. Eventual-        If GM is positive, the ship will be stable and
ly, the reaction on the blocks will be large         return to its original heel angle when the dis-
enough to cause the center of gravity to rise to     turbing force (wind, waves, etc.) is removed.
a point where the GM, and thereby the stabil-        If GM is negative, the vessel will be unstable.
ity, will be zero. This is an extremely danger-      This means that if the vessel is disturbed, it
ous condition, and the asset will almost surely      will not be able to recover and will continue
capsize unless side blocks are in place. The         to roll in the direction that it moved at the on-
asset's draft at this condition is called the        set of the disturbing force. In other words it
                                                     will capsize.
"draft-at-instability."
                                                     GM can be determined from known or pre-
The asset must land firmly on the keel and           dictable quantities, KM and KG. The height
side blocks before this point is reached. If the     of the metacenter (KM) for a ship is the theo-
asset has trim, it must land fore and aft on the     retical point around which a ship rolls and
keel blocks before the draft-at-instability is       through which buoyancy acts for small angles
reached, or it will turn over. It is necessary to    of heel. This point is based on a ship's geome-
calculate both the draft-at-instability and the      try and is generally plotted on a ship's draft
draft-at-landing fore and aft to ensure that the     diagram or curves of form. A ship's vertical
vessel will not capsize during deballasting. A       center of gravity (KG) is the point that repre-
good analysis should be provided by the con-         sents the centroid of all the weights of a ship.
tractor in the Transport Manual. There shall         This value is derived from the asset’s current
be a minimum of one foot of difference be-           condition of loading. Both of these quantities
                                                     are measured from the keel and can be deter-
tween the draft-at-instability and the draft-at-
                                                     mined with some degree of certainty. GM is
landing fore and aft.
                                                     simply the distance between these two points
If the draft-at-instability is much lower than       or:
                                                                    GM = KM – KG
the draft-at-landing fore and aft, the asset will
have acceptable stability and dock safely. For       As stated previously, a positive value for GM
example, if the asset has a draft-at-instability     is required to be stable. By looking at this
of 13 feet and a draft-at-landing of 15 feet,        equation, it is easily seen that KM must be
the asset should remain stable until it lands on     greater than KG to have a stable vessel. As
the side blocks in calm water. Additional con-       the heavy lift ship deballasts, and the asset
sideration must be given to the local sea state      lands on the blocks, the draft of the asset will
conditions. As an example, if the weather cri-       begin to decrease. As this draft goes down,
teria to perform this operation allows for the       the buoyant force on the asset (or residual
asset to pitch such that it may lift off the         buoyancy) will decrease and the height of the
blocks after initial landing, a difference be-       metacenter will change. Additionally, the
tween draft-at-landing and draft-at-instability      amount of the vessel supported by the keel
of 1 foot would not be adequate for the asset        blocks (or reaction of the keel blocks) will in-
to safely dry dock. If these draft values can-       crease.
not be changed, this difference may dictate          The reaction of the keel blocks acts as weight
the operational weather criteria.                    removal at the asset's keel. This negative
8-24
                                       U.S. Navy Towing Manual
                                                                                                  DRAFT


                          (KG ) (∆)




                  D R A F T (ft.)



                                                                              K M (∆−R K N )




                                                                   D raft at Instability




                                           M O M E N T S (ft - tons)


                                      Figure 8-8. Limit of Stablility

weight at the keel causes the same effect as an             To determine the draft-at-instability, it is nec-
added weight high in the ship. Both will                    essary to determine the virtual reduction in
cause an increase in the height of the center of            metacentric height caused by the virtual rise
gravity (KG). Since the asset's weight did not              in KG. The draft at which this virtual meta-
actually change, this rise in KG is called a                centric height (GMv) equals zero, will be the
virtual rise. This virtual increase will effec-             draft where the asset is unstable. The virtual
tively cause a reduction in GM and reduce the               GM can be found by subtracting the virtual
stability of the asset. As draft continues to               center of gravity (KGv) from the height of the
decrease, this effect will become more pro-                 metacenter (KM) at the draft in question.
nounced and the asset will become unstable.
                                                            The virtual center of gravity can be found by
This point of instability occurs in every dock-             summing the weight moments of the asset:
ing. The asset must land on the keel and side
blocks before this point is reached or it may                 ( KG o ⋅ ∆ ) – ( R kn ⋅ 0 ) = KG v ⋅ ( ∆ – R kn )
capsize.




                                                                                                            8-25
DRAFT                                             U.S. Navy Towing Manual


or:                                                             Rkn      = Reaction at the keel blocks (tons)
                               KG o
              KG v = ∆ ⋅ ----------------------
                                              -                 Note: The term (∆ − Rkn) is the displacement
                         ( ∆ – R kn )                           at a reduced draft, i.e. the residual buoyancy
                                                                after keel contact. This equation can be
Where:
                                                                solved for a number of drafts, until a draft is
KGv      = Virtual center of gravity (ft)                       found where GMv equals zero. A shorter way
                                                                to determine this value is to set the equation
∆        = Ship’s displacement (tons)
                                                                equal to zero and solve graphically. By set-
KGo      = Afloat center of gravity (ft)                        ting GMv equal to zero we see that:
Rkn      = Reaction at the keel blocks (tons)                                          ( ∆ ⋅ KG o )
                                                                              0 = KM – -----------------------
                                                                                                             -
(∆ − Rkn) = Residual buoyancy at a reduced                                             ( ∆ – R kn )
draft (tons)
When the asset lands on the blocks and the                      or:
draft begins to decrease, the asset is support-
ed by two forces, the reaction of the keel                                KM ⋅ ( ∆ – R kn ) = ∆ ⋅ KG o
blocks (Rkn) and buoyancy. The total of these
two forces equals the displacement (weight)
of the asset. In other words, the difference be-                Both KM and the residual buoyancy (∆ - Rkn)
tween the displacement and the reaction of                      can be found on the asset's curves of form or
the keel blocks is equal to the buoyancy at the                 draft diagram. To solve this graphically:
reduced draft. This quantity (∆ - Rkn) is also                        • Determine a range of drafts, starting at
called the residual buoyancy. The residual                              the floating draft and decreasing in in-
buoyancy can be determined for a given draft                            crements of one foot.
from the asset's curves of form or draft dia-
                                                                      • For each draft, determine the asset's re-
gram. Knowing these values, the equation for
                                                                        sidual buoyancy and KM
GMv can be solved.
                                                                      • For each draft, calculate the residual
                                                                        buoyancy moment
               GM v = KM – KG v                                                  ( KM ⋅ ( ∆ – Rk n ) )

                               KG o                                   • Calculate the displacement moment
         GM v = KM – ∆ ⋅ -----------------------
                                               -                                    ( ∆ ⋅ KG o )
                         ( ∆ – R kn )
                                                                        (Note: This quantity is determined at
Where:                                                                  the assets floating draft and is not af-
                                                                        fected by the change in draft)
GMv      = Virtual metacentric height (ft)                            • Plot the residual buoyancy moment and
KM       = Height of the metacenter (ft) from                           the displacement moment for the range
           ship’s curves of afloat draft                                of drafts.
                                                                        (Note: The displacement moment
KGv      = Height of the virtual center of                              should be a vertical line)
           gravity (ft)
                                                                      • Where these two curves intersect will
∆        = Ship’s displacement (tons) from                              be the draft-at-instability.
           ship’s curves of afloat draft
                                                                A sample of this graph is presented in Figure
KGo      = Afloat center of gravity (ft)                        8-8. and in Appendix Q.

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                                      U.S. Navy Towing Manual
                                                                                       DRAFT
If information is not known, such as during                   Table 8-3. Sample Cc Values.
an emergency or rescue docking, an estimate
of the vessel’s condition can be made. By            SHIP TYPES                            Cc
measuring the asset’s roll period (see Table
                                                       Auxiliaries                        0.44
8-6), an estimate of the GM and hence KG
can be made. Using the formula:                        Aircraft Carriers                  0.58

                                                       Cruisers                           0.43
                 Cc ⋅ B
                           2     2
            GM = ------------------
                           2                           DD692 (short hull)                 0.42
                        T
                                                       Destroyers (other)                 0.44
where:                                                 Destroyer Escorts                  0.45
GM = metacentric height (ft)                           Landing Ships                      0.46
Cc   = a constant (sample values given in              Patrol Craft                       0.47
       Table 8-3)
                                                       Submarines
B    = beam of ship (ft)                                 Body of Revolution hull          0.41
                                                         Other (fleet type)               0.36
T    = period of roll for complete cycle,
       from a maximum on one side to a                 Tugs                               0.40
       maximum the other and back (sec)
                                                    8-5.2.4 Draft-at-Landing Fore and Aft
Thus, from the value of GM, KG may be ob-
tained from equation:                               A similar method can be used to determine
             KG = KM – GM                           the draft-at-landing fore and aft. Again, it will
                                                    be a balance of the residual buoyancy mo-
where:                                              ments and the moment created by the dis-
KG    = height of center of gravity of ship         placement and the keel blocks (see Figure
        above keel when waterborne (ft)             8-9). If the asset lands on the aftermost block
                                                    (method is similar for bow landings) it will
KM = height of metacenter above the                 begin to pivot about this point as the draft
     ship’s keel (ft)                               changes. The ship will land fore and aft when
GM = metacentric height (ft)                        the moment created by the buoyancy equals
                                                    the displacement moment (each acting about
The value of KM is obtainable from the              the aftermost keel block). To determine the
curves of form.                                     draft when this occurs, follow this procedure:

                      NOTE                              • Determine the displacement, LCG,
                                                          buoyancy, and LCB for the floating as-
      It is emphasized that the Cc value                  set.
      is only an approximation and en-
      ters the equation as the square of
      its value. The GM value thus ob-
                                                        • Determine buoyancy and LCB for se-
      tained is, therefore, an approxima-                 lected drafts below the floating water-
      tion. This approximation method                     line.
      should not be a substitute for a                    (If the asset or heavy lift ship has con-
      thorough weight analysis.                           siderable trim at the time of landing,
                                                          horizontal waterlines may not provide
                                                          an accurate estimate. In most cases,


                                                                                                 8-27
DRAFT                                 U.S. Navy Towing Manual




                              Figure 8-9. Draft at Landing Fore and Aft.


        however, differences will be negligi-         be classified by one of the commercial regu-
        ble.)                                         latory bodies (ABS, DNV, Lloyd's, etc.). The
                                                      contractor must provide documentation
   • Determine the distance between the
                                                      showing current certification. The regulatory
     knuckle block and the LCG.
                                                      body should be contacted to verify that the
   • Determine the distance between the               vessel has met the latest requirements. When
     knuckle block and the LCB                        transporting assets, heavy lift ships must be at
   • Calculate moments of residual buoyan-            or below specified load line drafts that are in-
     cy and moments of displacement about             tended to ensure adequate freeboard.
     the aftermost keel block.                        8-5.3.1 Intact Stability Requirements
     (Differences caused by the reaction              The calculated stability and buoyancy charac-
     point moving due to compression of the           teristics of the heavy lift vessel (including
     keel block can be ignored unless the             displacements and centers of gravity with and
     stability is marginal and a more precise         without the asset on board) must be provided.
     calculation is needed.)
                                                          • The intact stability must be determined
   • Plot these moments versus draft. The                   for all modes of operation, including
     displacement moment will be a vertical                 the five phases shown in Figure 8-6.
     line.                                                  Longitudinal stability must be included
   • Where the line of buoyancy moment                      for Phases 3 and 4 of Figure 8-6. Free
     crosses with the line of displacement                  surface effects must be determined and
     moment, will be the draft at landing                   included in the calculations.
     fore and aft.                                    8-5.3.2 Stability During Ballasting/
                                                              Deballasting
A sample of this graph and these calculations
is provided in Appendix N.                            The ballast/deballast operation presents some
                                                      unique stability concerns and must be evalu-
8-5.3    Stability of the Heavy Lift Ship
                                                      ated thoroughly. Stability is largely impacted
Any heavy lift ship considered for use as a           by the amount of waterplane area of the ship.
transport platform for US Navy assets shall           As the cargo deck of the heavy lift ship goes


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                                         U.S. Navy Towing Manual
                                                                                          DRAFT
into or out of the water, the stability of the lift    and the heavy lift ship may roll out of phase,
ship changes rapidly and substantially. If the         causing landing problems, or, even worse,
deck is completely submerged, only the wa-             causing the asset or the lift ship to become
terplane of the raised hull structure, which ex-       unstable, assume a large list or capsize.
tends above the cargo deck, will provide sta-
bility to the vessel. Additionally, during this        During operations involving lifting of U.S.
phase, the water level in the ballast tanks is         Navy assets, the heavy lift ship shall maintain
changing and may not be in either empty or             a GM (including free surface correction) of
pressed up condition. This may produce a               no less than 3.28 feet (1 meter). Trim of the
free surface effect which will also reduce sta-        heavy lift ship of up to 3° may be included to
bility. The result is that the heavy lift ship         meet the minimum GM. More trim than this
passes through a phase of minimum stability            may cause the asset to float off the blocks on
(minimum GM) while the cargo deck is under             one end or slide on the blocks. The effect of
water. To control the amount of this change,           this trim should be investigated to be sure it is
heavy lift ships generally go through this             satisfactory. Normally, if this trim on the
phase with some list and trim.                         heavy lift ship's cargo deck does not cause the
                                                       assets draft at one end to be zero while the
                                                       draft on the other end is less than 2 - 5 feet of
                                                       the afloat draft, the asset should not slide or
                    CAUTION                            float off. See Figure 8-6, phase 3. To waive
                                                       the 1 meter minimum GM, the asset must be
       Submersible barges that are used                hard on the blocking before the phase of min-
       for FLO/FLO lifts rely on bottom
                                                       imum stability and the minimum GM (not ac-
       contact of one end of the barge to
       ensure sufficient stability until the           counting for the list) must meet or exceed
       cargo has landed on the blocks                  regulatory body requirements of 0.5 feet
       and stability can be increased                  (0.15m) in all phases of the operation, includ-
       through added waterplane. Prob-                 ing the free surface effect. This is the mini-
       lems with exact positioning and                 mum GM required by regulatory agencies. A
       high knuckle block loading add to
                                                       detailed stability analysis for all operations
       an already difficult procedure.
       When one end of the cargo has                   must be included with the waiver request. In
       landed, the barge must rely on the              no case should a GM below 0.5 feet be ac-
       cargo staying in position and con-              cepted. Efforts should be made to meet the
       tributing to the stability of the               Navy requirements.
       barge/cargo system until more of
       the barge's cargo deck comes out                8-5.4   Stability of the Heavy Lift Ship with
       of the water.                                           the Asset Secured Aboard during
                                                               Transit
A thorough study of the changing conditions            The heavy lift ship with the asset aboard,
of the heavy lift vessel must be completed for         must be able to withstand beam winds as de-
the entire loading and unloading process. The          scribed in paragraph 8-5.1.1. The contractor
point of minimum stability should be known             must present a stability analysis (righting arm
to compare to the minimum stability condi-             curve) meeting these criteria in the Transport
tions already calculated for the asset. Review         Manual.
the stability of the heavy lift ship to ensure
that it is not at the point of minimum stability       The dynamic stability under the righting arm
at the same time that the asset assumes its            curve at a given angle of heel is a measure of
draft-at-instability. If this happens, the asset       the amount of energy that has to be put into


                                                                                                   8-29
DRAFT                                 U.S. Navy Towing Manual


the ship to give it that angle of heel. This        downflooding angle (whichever is less) is not
heeling or overturning energy can be supplied       less than 40 percent in excess of the area un-
by wind, waves, or a combination of these           der the wind lever curve to the same limiting
and other forces. This quantity can be mea-         angle. Figure 8-10 demonstrates this graphi-
sured by making a plot of the righting arm          cally.
(GZ). See Figure 8-10. The area under this
GZ curve from zero degrees (or where the            The angle of heel at which the cargo deck
curve first crosses the x-axis) to the angle in     edge is submerged must also be indicated.
question, multiplied by the displacement, is        8-5.4.1 Damage Stability
equal to the amount of energy that is available
to return the vessel to the original static heel    The damage stability requirements of MIL-
angle (most likely zero degrees).                   STD-1625C Safety Certification Program for
                                                    Drydocking Facilities and Ship building
The righting arm curve should also display          Ways for U.S. Navy Ships, (Ref. T) were de-
the wind lever curve (see Figure 8-10). This        veloped to ensure the safety of a vessel in a
curve represents the heeling energy devel-          dry dock. If a floating drydock is subjected to
oped by the wind acting on the sail area of the     damage to two main watertight subdivision
vessel (with the asset aboard). The area will       groups, adherence to these guidelines ensures
change as the vessel heels which gives this         the safety of the docked vessel. These re-
curve a downward arc. This curve is depen-          quirements may not be stringent enough to
dent on wind velocity and represents only one       ensure survival and safety of an asset on the
particular speed. For the purposes of analysis,     heavy lift ship if it is damaged in a seaway.
the maximum wind including gusts expected
during transit should be used.                                           NOTE
The American Bureau of Shipping (ABS) us-                 The current commercial fleet of
es this GZ curve to establish their require-              heavy lift ships and barges does
ments for dynamic stability. The area under               not meet these requirements. The
these two curves are then compared to deter-              vessels should meet the damaged
                                                          stability requirements of their clas-
mine adequate dynamic stability. The area for
                                                          sification society. These require-
the GZ curve is computed from the first inter-            ments should be studied to under-
cept (where it first crosses the x-axis) to the           stand the limits and risks involved
second intercept or the downflooding angle                in regard to damaged stability.
whichever is less (see ship’s stability book for
downflooding angle). If both the downflood-
                                                    8-6 Blocking
ing angle and the second intercept are greater
than 50 degrees, then 50 degrees is used. The
area under the wind lever curve is taken from       Unlike normal dry-docking operations, heavy
0 degrees to the same limiting angle. The           lifted assets are subjected to significant mo-
range of dynamic stability, from the intersec-      tions caused by exposure to an ocean environ-
tion of the wind lever and righting arm curves      ment. This section will discuss the design of
to the point of zero righting moment must not       the blocking which will be the support system
be less than 36 degrees (see Figure 8-10). Ad-      for the asset. This structure will carry the en-
ditionally, the second intercept point must be      tire weight of the asset as well as protect the
greater than 36 degrees.                            asset from potential damage from the motions
                                                    of the heavy lift ship. The proper design of
ABS requires that the area under the righting       this system will ensure a safe transit for the
arm curve at or before the second intercept or      asset.

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                                U.S. Navy Towing Manual
                                                                                                  DRAFT



            Levers (M )




                                                                     R ighting A rm (G Z)
                                            A




                                                                            W ind Lever
             C
                                    B


                                                                                            If over 50°,
                                                                                            use 50°




        0            10    20             30              40           50            60
First                                                                                     S econd intercept
Intercept                       A ngle of H eel (deg rees)
                                                                                          (m ust be greater
                                                                                          than 36°)
                                   N ot le ss th an 36°




                           For Ad equ ate D ynam ic S tability:
                           A R E A (A +B ) ≥ 1.4 ⋅ A R E A (B +C )




                                            NOTE

                          Downflooding Angle not shown.
                          See ship’s stability book for appro-
                          priate downflooding angle.


                                 Figure 8-10. GZ Curve




                                                                                                              8-31
DRAFT                                    U.S. Navy Towing Manual


8-6.1   Preparing the Docking Plan                     sets dry-docking plan should be used as a pre-
                                                       liminary plan. This will be sufficient to sup-
                      NOTE                             port the weight of the vessel with minimal
                                                       dynamic loadings. The loading due to dynam-
        Check with the planning yard to be             ic motions, the gravity component at the max-
        sure that the latest revision of the
        docking plan is being used.
                                                       imum angle of inclination, and the effects of
                                                       wind loading must all be included in the anal-
                                                       ysis.
Most vessels, and certainly all active Navy
vessels, have a dry-docking plan. This plan            Check with the planning yard to be sure that
contains information that shows the correct            the latest revision of the docking plan is being
placement of docking blocks to provide a               used. This plan, along with any previous
proper fit to the hull and to eliminate damage         FLO/FLO docking plans for the asset, should
of underwater projections as well as distribute        be provided to the lift contractor early in the
the docking loads. This serves as a first esti-        process and, if possible, in the RFP.
mate in preparing a docking plan for a FLO/
                                                       8-6.2   Docking Blocks
FLO operation, but preparation of the dock-
ing and seafastening plan requires consider-           The contractor should provide a proposed
ation of factors other than what is normally           docking plan in the Transport Manual. It
considered in a dry docking.                           should contain descriptions of all the docking
                                                       blocks. It should provide the physical charac-
A FLO/FLO is not an ordinary dry docking in
                                                       teristics of the blocks, including material and
terms of operations and loading. For a FLO/
                                                       dimensions. Keel block height should be kept
FLO operation, the keel block height must be
                                                       to a minimum to reduce the required depth for
minimized. The keel block height will affect
                                                       float on and float off. As such, the use of con-
the stability of the lift ship by dictating the re-
                                                       crete base blocks, common in dry docking,
quired depth of water and depth of submer-
                                                       should be avoided.
gence of the heavy lift ship. This will proba-
bly determine acceptable loading sites.                The Transport Manual also provides calcula-
Additionally, tall keel blocks will require            tions to verify that the blocks are stable and
even taller side blocks. These blocks may be           structurally adequate to withstand the loading
subject to loads caused by currents, waves or          used in lifting capacity calculations and that
motions of the asset. Precaution must be tak-          the number of side blocks (and shores) is ade-
en to be sure that these blocks do not tip over        quate to provide sufficient bearing area to re-
during loading. Transporting across the ocean          sist overturning. Due to the dynamic nature of
subjects the docking blocks to higher dynam-           the operation, all blocks and shores should be
ic loading and, at the same time, requires the         secured to the cargo deck of the heavy lift
docking blocks to absorb the flexing of the            ship. Prior to loading, these blocks should be
heavy lift ship under the asset.                       inspected to be sure that they are the same di-
                                                       mensions, in proper location and in the same
A docking plan should be prepared for every
                                                       material condition as that reported by the
operation. Previous plans can be used as a
                                                       Transport Manual.
guide, but each FLO/FLO presents unique
concerns. The asset's loading condition,               The keel blocks are subject to both the static
weather, and expected sea conditions will              weight of the asset and the dynamic loads in
likely be different and must be accounted for          the vertical direction imposed by the action of
in the plan. Careful preparation is essential          the sea. The side blocks and sea fasteners will
for a successful operation. As a start, the as-        bear some of the assets weight and also be


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                                      U.S. Navy Towing Manual
                                                                                                         DRAFT
subject to dynamic loading in both the verti-       heavy lift ship, taking into account the loca-
cal and transverse directions. To find the total    tion of the asset.
force on the blocks it is necessary to calculate
                                                    The formulas for computing the acceleration
the effects of dynamic motion.
                                                    (a) in the vertical direction (z direction) are as
8-6.2.1 Dynamic Loading                             follows:

The commercial industry designs sea fasten-                           0.0214Px 0.0214Ry
                                                                                           -                         -
                                                         az = 1 + h + ---------------------- + -----------------------
                                                                                    2                        2
ers using load forces based on equations from                                 Tp                       Tr
the Principles of Naval Architecture (PNA)
The Society of Naval Architects and Marine
                                                    where:
Engineers (Ref. U) for roll and pitch. The US
Navy developed their own series of equations        az    = vertical acceleration factor (g)
for determining these dynamic forces. An ex-
planation of these equations is contained in        h     = heave acceleration (g) (Table 8-4)
DOD-STD-1399-301A, Interface Standard               P     = Maximum angle of pitch (degrees)
for Shipboard Systems Section (Ref. V). The                 (Table 8-5)
two equations produce very similar results for
a similar set of conditions. The equations          x     = distance of center of gravity of asset
from PNA are used to calculate the forces due               forward or aft from center gravity of
to dynamic motions separately from the static               heavy lift ship (ft)
force due to weight then they are combined.         R     = Maximum angle of roll (degrees)
The Navy equations combine these effects in-                (Table 8-6)
to a load factor. Both sets of equations use as-
sumed values for maximum roll and pitch. Ei-        y     = distance of asset off centerline of
ther approach can be used but only the Navy                 heavy lift ship (ft)
approach is demonstrated here. See Princi-          Tp    = Period of pitch (sec) (Table 8-5)
ples of Naval Architecture (PNA) The Society
of Naval Architects and Marine Engineers            Tr   = Period of roll in (sec) (Table 8-6)
(Ref. U) more information about the commer-
                                                    The first term in this formula accounts for the
cial approach.
                                                    static force due to the weight, gravity compo-
The keel blocks will bear most of the weight        nent. This factor represents the portion of the
of the asset with the side blocks and seafas-       asset’s weight to be borne by the blocking. A
teners taking only a partial load. To be con-       value of 1.0 is used for keel blocking to be
servative, the keel blocks will be designed to      conservative.
support the entire weight. The weight of the        Values for pitch and roll amplitudes and peri-
asset is equal to its displacement at the time      ods are given in Table 8-5 and 8-6. The com-
of loading. As discussed earlier, an account of     mercial industry typically uses the following
the dynamic loading must also be included           values as a first estimate to develop their load
for accelerations in the vertical direction.        factors:
To determine the design loads (forces) that         R     = 20 degrees of roll, one direction
must be resisted to hold the asset on the cargo
                                                    P     = 15 degrees of pitch, one direction
deck of the heavy lift ship, multiply the
weight of the asset (w) by an acceleration fac-     Tr    = roll period of 10 seconds, port to
tor (a) determined from the motions of the                  starboard back to port


                                                                                                                         8-33
DRAFT                                  U.S. Navy Towing Manual


Tp     = pitch period of 10 seconds, bow to          az    = vertical acceleration factor (g)
         stern back to bow
                                                                           NOTE
The load factors which have proved effective
on past lifts, are approximately equal when                 The vertical acceleration factor, az,
                                                            represents an increase in the
compared to those developed in DOD-STD-
                                                            downward force. It is essentially a
1399 Section 301A, provided that the loading                multiplier to increase the effect of
is similar to those in the past. Namely, GM and             gravitational acceleration and the
roll/pitch periods must be in the same range.               units used, g, reflect this.
The ship motions observed during actual op-
erations indicate that both approaches are           8-6.2.3 Keel Block Loading Distribution
comparable for a sea state 7 analysis.
                                                     Using the Navy docking drawing as a guide
Using these commercial standards or the US           in placement of keel and side blocks will as-
Navy approach is acceptable, however, they           sure alignment with ship’s structure, omis-
should not take the place of a detailed motion       sions for hull penetrations and appendages
study. This commercial industry approach             and a proper fit to the curvature of the hull.
does not include an additional heave accelera-       Specifics of the blocking must be provided
tion. If the results from the motion analysis in-    for the proposed blocking arrangement to
dicate that the heavy lift ship with assets          make sure that the asset’s hull is properly
aboard will roll or pitch at periods greater than    supported. To ascertain that structural re-
this rule of thumb, further evaluation by            quirements are not violated, the loading dis-
NAVSEA is required. These values should be           tribution of the asset on the blocking must be
compared with the expected values from               calculated. The distribution of the asset’s
route planning. If larger angles of heel or          weight, as shown on the longitudinal strength
pitch are expected, those values should be           drawing (20 station weight breakdown), indi-
used.                                                cates how the asset’s weight is distributed on
                                                     the asset’s hull structure and thereby onto the
Information concerning acceleration factors          blocking as shown on the Navy docking
along with the angles or roll, pitch, heave, and     drawing.
surge should be presented in the Transport
Manual.                                              The weight of the asset and the location of the
                                                     asset’s centers of gravity (vertical, transverse,
8-6.2.2 Loading of Keel Blocks                       and longitudinal) must be accurately predicted
                                                     in order to avoid overloading the blocks. Keel
To find the total force on the blocks, multiply      bearing should be uniform and continuous. If
this acceleration factor by the weight of the        keel bearing is non-uniform, as in the case of a
asset.                                               asset with a partial bar keel, long overhangs,
                                                     highly concentrated weights or excessive hull
                 DL k = wa z
                                                     projections, special considerations must be
                                                     given to further spread the load over individual
where:                                               keel blocks. For loadings that are not continu-
                                                     ous and uniform, a more rigorous method may
DLk = total dead load on the keel blocks             be required to determine the load distribution.
      (tons)                                         As an example, MSOs and MCMs require ad-
                                                     ditional shores to be placed under the stern due
w      = weight of the asset (tons)                  to long overhangs.
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                                     U.S. Navy Towing Manual
                                                                                        DRAFT
8-6.2.4 Distribution of Asset’s Weight               Survey the asset to find information on all
                                                     variable weight and abnormalities (trim
Once the total force on the keel blocks is
                                                     weights added, hull damage, cargo, etc.)
known (DLk from 8-6.2.2), an assessment of
the loading distribution should be conducted             • Record vessels drafts
to ensure that the blocking is not overloaded.           • Calculate the asset’s expected drafts at
This loading distribution should be compared               the time of docking
to the docking drawing that was used as a first
estimate of required blocking. To conduct a              • Calculate the asset’s displacement and
loading distribution:                                      centers of gravity

Examine ship data for the latest information.        Compare the expected values with the record-
                                                     ed values and resolve any discrepancies.
Sources include docking drawings, curves of
form, and full load weight distribution.

         Table 8-4. Heave and Surge Motion Parameters for Calculation of Loading Factors for
                                   Conventional Surface Ships.

       Sea                   LBP                         Heave                    Surge
       State              meters (feet)              acceleration (g)         acceleration (g)

         4           Less than 46 (150)                    0.10                     0.06
                     46-76 (150-250)                       0.10                     0.05
                     76-107 (250-350)                      0.10                     0.05
                     107-152 (350-500)                     0.06                     0.04
                     152-213 (500-700)                     0.06                     0.04
                     Greater than 213 (700)                0.04                     0.02

         5           Less than 46 (150)                    0.17                     0.10
                     46-76 (150-250)                       0.17                     0.10
                     76-107 (250-350)                      0.17                     0.10
                     107-152 (350-500)                     0.14                     0.05
                     152-213 (500-700)                     0.10                     0.05
                     Greater than 213 (700)                0.07                     0.05

         6           Less than 46 (150)                    0.27                     0.15
                     46-76 (150-250)                       0.27                     0.15
                     76-107 (250-350)                      0.27                     0.15
                     107-152 (350-500)                     0.21                     0.10
                     152-213 (500-700)                     0.16                     0.10
                     Greater than 213 (700)                0.11                     0.05

         7           Less than 46 (150)                    0.4                      0.25
                     46-76 (150-250)                       0.4                      0.20
                     76-107 (250-350)                      0.4                      0.20
                     107-152 (350-500)                     0.3                      0.15
                     152-213 (500-700)                     0.2                      0.15
                     Greater than 213 (700)                0.2                      0.10




                                                                                                 8-35
DRAFT                                    U.S. Navy Towing Manual



         Table 8-4. Heave and Surge Motion Parameters for Calculation of Loading Factors for
                                   Conventional Surface Ships.

        Sea                    LBP                        Heave                     Surge
       State                meters (feet)             acceleration (g)          acceleration (g)

         8            Less than 46 (150)                     0.6                        0.35
                      46-76 (150-250)                        0.6                        0.30
                      76-107 (250-350)                       0.6                        0.30
                      107-152 (350-500)                      0.5                        0.25
                      152-213 (500-700)                      0.4                        0.25
                      Greater than 213 (700)                 0.2                        0.10

                                                       8-6.2.5 Calculation of the Asset’s Loading on
                                                       the Docking Blocks by the Trapezoidal
                                                       Method
                      NOTE
                                                       It is important to get an estimate of the load
       In the event of lifting a damaged               on the blocks to ensure that they are not over-
       asset the displacement at the time
                                                       stressed. To do this, start by examining the re-
       of lift may differ from the displace-
       ment to be transported. This is due             quired blocking of the docking drawing. This
       to the entrained water in the dam-              will provide the first estimate of the longitu-
       aged area that will run out during              dinal location of the asset with respect to the
       and after the lifting operation.                blocking and its center of gravity with respect
                                                       to the center of blocking (see Figure 8-11).



        Table 8-5. Pitch Motion Parameters for Calculation of Loading Factors for Conventional
                                           Surface Ships.

                             Length between
              Sea                                         Pitch angle*         Pitch period
                           perpendiculars (LBP)
             State                                          degrees              seconds
                               meters (feet)

               4         Less than 46 (150)                   2.0                 3.5
                         46-76 (150-250)                      2.0                 4.0
                         76-107 (250-350)                     1.0                 5.0
                         107-152 (350-500)                    1.0                 6.0
                         152-213 (500-700)                    1.0                 7.0
                         Greater than 213 (700)               1.0                 8.0




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                                      U.S. Navy Towing Manual
                                                                                         DRAFT

        Table 8-5. Pitch Motion Parameters for Calculation of Loading Factors for Conventional
                                           Surface Ships.

                           Length between
          Sea                                             Pitch angle*         Pitch period
                         perpendiculars (LBP)
          State                                             degrees              seconds
                             meters (feet)

            5          Less than 46 (150)                    3.0                 3.5
                       46-76 (150-250)                       3.0                 4.0
                       76-107 (250-350)                      2.0                 5.0
                       107-152 (350-500)                     2.0                 6.0
                       152-213 (500-700)                     2.0                 7.0
                       Greater than 213 (700)                1.0                 8.0

            6          Less than 46 (150)                    5.0                 3.5
                       46-76 (150-250)                       4.0                 4.0
                       76-107 (250-350)                      4.0                 5.0
                       107-152 (350-500)                     3.0                 6.0
                       152-213 (500-700)                     3.0                 7.0
                       Greater than 213 (700)                2.0                 8.0

            7          Less than 46 (150)                    7.0                 3.5
                       46-76 (150-250)                       6.0                 4.0
                       76-107 (250-350)                      6.0                 5.0
                       107-152 (350-500)                     5.0                 6.0
                       152-213 (500-700)                     4.0                 7.0
                       Greater than 213 (700)                3.0                 8.0

            8          Less than 46 (150)                   11.0                 3.5
                       46-76 (150-250)                      10.0                 4.0
                       76-107 (250-350)                      9.0                 5.0
                       107-152 (350-500)                     7.0                 6.0
                       152-213 (500-700)                     6.0                 7.0
                       Greater than 213 (700)                5.0                 8.0

         *Note: Pitch angle is measured from horizontal to bow up or down.

It is important to use the weight distribution        (see Figure 8-12), the loading distribution
of the asset at the time of its loading onto the      may be approximated by using a trapezoidal
heavy lift ship. For the current condition of         approximation.
loading, the displacement and centers of              If the asset’s longitudinal center of gravity
gravity are calculated using the current draft        (LCG) aligns vertically with the center of
readings and the weight distribution as de-           blocking (Cb), the forward and after blocks
scribed above. In the case where the blocking         will share the load fairly equally. In practice,
can be assumed to be continuous and uniform           this is rarely the case. When the asset’s LCG




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               Table 8-6. Roll Motion Parameters for Calculation of Loading Factors for Conventional
                                                 Surface Ships.1

        Sea                      Beam                        Roll angle 2
                                                                                            Roll period
       State                  meters (feet)                   degrees

           4            Less than 15 (50)                         7            See note3 for determination of roll
                        15-23 (50-75)                             6            period
                        23-32 (75-105)                            6
                        Greater than 32 (105)                     5

           5            Less than 15 (50)                        12            See note3 for determination of roll
                        15-23 (50-75)                            10            period
                        23-32 (75-105)                           10
                        Greater than 32 (105)                     9

           6            Less than 15 (50)                        19            See note3 for determination of roll
                        15-23 (50-75)                            16            period
                        23-32 (75-105)                           15
                        Greater than 32 (105)                    13

           7            Less than 15 (50)                        28            See note3 for determination of roll
                        15-23 (50-75)                            24            period
                        23-32 (75-105)                           22
                        Greater than 32 (105)                    20

           8            Less than 15 (50)                        42            See note3 for determination of roll
                        15-23 (50-75)                            37            period
                        23-32 (75-105)                           34
                        Greater than 32 (105)                    31
       1
           Excludes multi-hulls, surface effect ships, and all craft supported principally by hydrodynamic lift.
       2
           Roll angle is measured from vertical to starboard or port
       3
           Full roll period is to be calculated from:


                                                     1⁄2
                        T r = ( C c × B ) ⁄ ( GM )


                      Where:
                       Tr - is the full roll period (seconds)
                       Cc - is a roll constant based upon experimental results from similar ships - usual rate 0.38 to
                            0.49 (sec/√ft) (0.69 to 0.89 (sec/√m)). For Heavy lift ships use Cc-0.40 unless a better
                            estimate is provided. It may be as high as 0.44. See Table 8-3 for examples of other
                            surface ships.
                       B - is the maximum beam at or below the water line (m or ft).
                      GM - is the maximum metacentric height (m or ft).


is forward or aft of the Cb, we can assume the                        is aft of Cb, the after blocking will carry more
load distribution is roughly trapezoidal.                             of the load.
Again, this is assuming that there is no signif-
icant anomalies in either the ship’s load dis-                        The amount of load supported by the after
tribution or in the blocking build. If the LCG                        blocks will depend on the distance that LCG




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             AP                                                FP
  SRP                                 LB P




                                          LC G




                                                              D e ck of H eav y Lift S hip
              OHA                         A
                                 Cb


                                      B

             Load
              M ax
                                                           Load M in

                                          Lk




 A P = A fter P erpendic ula r

LB P = Le ngth be tw een perpe ndiculars of ass et

S R P = D istance from A P to point from w hich
        dista nce to keel block s is m easure d
LC G = A ss et’s longitudinal c enter of gravity

O H A = D ista nc e from S R P to k eel block
  L k = Length of keel block ing
  C b = L k = C ente r of blocking
         2
   B = L k = A pproxim ate C ente r of Tra pezoid
         6
   A = D istance from as set’s LC G (cente r of G ra vity) to C b (ce nter of B locking)
      = C B-[LB P + SR P-LC G -O H A ] (N ote tha t if A is a ne gative num be r the
        tra pe zoid is reve rsed in the above diagram so the Loa d m ax is gre ates t at
        the forw ard e nd of the as set


                           Figure 8-11. Load Distribution.




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is from Cb. The further away from LCG that                      B    = Distance from center of trapezoid to
Cb is, the more uneven will be the distribution                        center of blocking (Lk/6) (ft)
of loading. In fact, if the LCG is outside of
                                                                Lk   = Length of keel blocking (ft)
the center third of the blocking arrangement,
the load distribution will be triangular and the                Le   = Length of effective keel blocking
difference between maximum load and mini-                              (ft) = 1.5 Lk - 3 A
mum load may be significant. See Figure                         The load distribution, as defined by Load
8-11 for an illustration of the relationship be-
                                                                Max and Load Min, is used to determine if
tween LCG and the center of the assumed                         the blocking (and in some cases the cargo
trapezoid, B. For a trapezoidal load distribu-
                                                                deck) is adequate to support the asset. Check
tion, B will be approximately 1/3 of the                        the maximum loading against the loading as-
length of the keel blocking (Lk) from the after
                                                                sumed for the ship’s docking drawing. The
end or 1/6 of Lk from the center of blocking
                                                                keel blocks should be checked to ensure that
(based purely on geometry). This is a good
                                                                they are not overstressed. Assume that the
initial estimate to determine the maximum
                                                                last block in the line will see Load Max. To
and minimum load.
                                                                find the stress on this block use:
To determine the maximum and minimum
loads, use the following equations:
                                                                         S = ------------------------ ×  -----------------
                                                                             Load Max                     2240 lb
                                                                                                                          -
If A < B, load distribution is trapezoidal:                                           Ae                 1 ton 

                                                                where:
                     DL k
          Load Max = ---------  1 + --- 
                                     A
                             -        -
                       Lk            B                         S              = Stress on block (psi)
                                                                Load Max = Maximum expected blocking
                     DL k
          Load Min = ---------  1 – --- 
                                     A                                     load (tons)
                             -         -
                       Lk           B
                                                                Ae             = Effective area of keel block
If A > B, load distribution is triangular:                                       (in2)

                            4DL k                                                         CAUTION
                                              -
          Load Max = --------------------------
                     3 ( L k – 2A )                                   These calculations assume a con-
                                                                      tinuous row of keel blocks. If this is
                       Load Max                                       not the case, increases in loading
            Load Min = ------------------------
                                 Le                                   should be made accordingly.

where:                                                          This stress should be lower than the propor-
Load Max = Maximum expected blocking                            tional limit for the material used. See Table
           load (tons)                                          8-7 and paragraph 8-6.2.7 for the allowable
                                                                stress on blocks. If it is not, consider using
Load Min = Minimum expected blocking                            more keel blocks, keel blocks with better con-
           load (tons)                                          tact area, or redistributing the asset’s load
DLk = Loading due to weight and dynamic                         more evenly.
      effects (see ) (tons)                                     8-6.2.6 Knuckle Loading

A      = Distance from center of gravity of                     Special consideration must be given to the
         asset to center of blocking (ft)                       end of the blocking arrangement as an asset


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                                      Figure 8-12. Keel Blocks.


                      Table 8-7. Allowable Block Stress (Assuming Douglas Fir).

                                                                  Allowable Unit Stress
                  Keel Width, ft                                      for Blocking
                                                                        (S), lb/in2

                      ≥3.00                                               370

                       2.50                                               323

                       2.00                                               277

                       1.75                                               254

                       1.50                                               230

                       1.25                                               207

                       1.00                                               184


makes initial contact with the blocking. The         an asset is at an even keel condition, the
individual block at each end of the keel block       heavy lift ship will likely deballast in a way to
row is referred to as the knuckle block and          expose deck area as quickly as possible. This
may be subject to high compressive stress as         trimmed condition may also cause localized
the asset first lands. Initial contact will be       loading on the knuckle block.
highly localized and may cause high stresses
and deformation of the block. For an asset           As the heavy lift ship continues to rise, the
with trim by the stern, the reactions at the af-     knuckle block will deform and the asset will
termost block should be analyzed. Even when          reduce trim. Additional support will be pro-


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vided by blocks forward of the knuckle block        8-6.2.7 Safe Allowable Compressive Stress
and contact with the asset will increase. At        of Blocking
the same time, more and more of the asset’s
weight will be supported by the blocking. The       The allowable timber compressive stress for
stress seen at the knuckle block will likely        distributed loading on keel blocks, taken as
rise at first and then decrease, reaching a         the fiber stress at the proportional limit for
maximum somewhere between initial and full          Douglas fir, is 370 psi. This assumes a uni-
contact. This maximum stress should be ana-         form pressure on a 42 by 48 inch docking
lyzed to ensure the strength limits of the ma-      block resulting in a total load of approximate-
terial is not exceeded. Naval Ship’s Technical      ly 330 tons. When computing the stress for
Manual (NSTM) S9086-7F-STM-010, Chap-
                                                    the actual condition, the weight of the asset
ter 997, Docking Instructions and Routine
Work in Drydock (Ref. W) provides a sound           and the area in contact with the blocks should
methodology for computing the knuckle               be used to determine loading. Note that this
block stress and much of the material present-      limit applies only to keel blocks. Side block
ed here is borrowed from that source.               criteria are discussed in 8-7.1.




                                  Figure 8-13. Heavy Lift Blocks.




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Table 8-7 lists allowable timber compressive        Normal dry-docking plans contain calcula-
stresses for the blocking based on the propor-      tions to analyze the block’s resistance to the
tional limit for Douglas fir. The computed          dynamic forces associated with both hurri-
stress is dependent upon the area of the keel       cane and seismic (earthquake) loading. In the
in contact with the knuckle. For vessels with       case of FLO/FLO, the dynamic loading must
keels that are narrower than 3 feet, the allow-     be based on ship motions and wind forces.
able stress has been reduced. This is neces-        This will result in greater angles of inclina-
sary because of the compression of the block        tion and significant overturning moments.
during loading. For narrower keels, there will      8-7.1   Side Blocking
be less area in contact with the blocks con-
centrating the load so that less load can be        Side blocking must be able to withstand some
supported. Appendix D of NSTM, CH-997               portion of the asset's deadweight as well as
(Ref. W) presents a detailed explanation of         dynamic loading. As the heavy lift ship rolls
total load as a function of compressive stress.     or pitches, the contribution of the side block-
                                                    ing in supporting the deadweight will in-
When docking ships with keels that are nar-         crease. There is also an increase in dynamic
row, it is advisable to use hard wood capping       loading due to the effect of rolling and pitch-
at the knuckle block. The hard wood capping         ing. The static and dynamic effects will be
will be able to carry stress concentrations that    examined separately.
would cause severe crushing of soft capping.        Side blocks are used for handling the loading
Hard caps should be used in conjunction with        up to the angle to which the heavy lift ship
a soft wood stratum below to give the same          will heel over or a heel of 15°, whichever is
overall compressive characteristics to the          greater. Roll bars or spur shores (see Figure
block. For certain vessels with bar keels (e.g.,    8-13) should be used for angles from 15° -
tugs), using caps bound with steel angles will      45°.
prevent the keel from cutting into the cap.
                                                    To be effective, side blocks should have rela-
                                                    tively planar surfaces (minimum curvature).
8-7    Seafastening Plan                            Blocks will form to the shape of the hull un-
                                                    der significant compressive loading. Attempt-
The seafastening plan is a composite arrange-       ing to shape the blocks to exactly fit the sur-
ment of side blocks, spur shores and seafas-        face of the hull will add considerable effort
teners (Figure 8-13). Side blocks will supply       and may not significantly improve their con-
some of the necessary support associated with       tact area or overall performance. It will be
the vertical loading for the initial docking        difficult to land the ship precisely in the in-
phase. They provide resistance of any over-         tended location and small variations may
turning moment resulting from ship's motions        prove to make the shaping of the blocks a
and heel angles. Spur shores, or roll bars, are     wasted effort. Contact can be improved by
installed to provide further resistance to the      using wedging material. This will likely be
dynamic overturning moments in a seaway.            easier to accomplish if the block surfaces are
Seafasteners (stopper blocks) are installed to      planar.
prevent fore and aft and athwartships sliding       The blocks should be placed under the asset's
action as the heavy lift ship pitches and rolls.    major structural members such as main trans-
Each of these will be addressed separately in       verse bulkheads and secondary frames. They
this section.                                       should also have enough effective surface



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                                                                                    M id 1/3


                                                                                P rofile Vie w




        C ribb in g                               /3
                                               d1
                                          Mi




                                                   NOTE

                                 Cribbed blocks should be arranged
                                 such that the force at loading acts
                                 through the middle one-third of all
                                 tiers.



                                    Figure 8-14. Cribbed Blocks.


area (width) to span two frames. The asset's              ty in both the transverse and longitudinal di-
docking drawing will provide recommended                  rection (see Figure 8-14). The side block caps
locations.                                                should be of a soft material such as Douglas
The transverse stability of individual side               Fir or Southern Yellow Pine. The discussion
blocks is essential and depends on overall                concerning the use of hard woods as capping
block height and hull shape. These side                   material for keel blocks does not apply to side
blocks must be of a construction and located              blocking. Side blocks will compress and form
such that the resultant force (normal to the              to the curvature of the hull and are not subject
shell at point of tangency) falls within the              to the high knuckle loading associated with
middle one-third of all horizontal layers of              initial landings. Side blocks must be securely
blocking. Using a two-tiered block with a                 fastened to the deck of the heavy lift ship to
double wide base will help ensure this stabili-           resist overturning and sliding.



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                                    Figure 8-15. Spur Shores

8-7.1.1 Stability of High Blocks                      • Side blocks over 6 feet in height (mea-
                                                        sured from the deck to the highest cor-
Heavy lifting Navy ships with large sonar               ner of the soft cap) shall be tied togeth-
domes or other underwater projection may re-            er longitudinally (steel tie rods,
quire the use of excessively tall blocks. In            cribbing, etc.)
these cases, the stability of the blocks are a
                                                      • Loading force should act in the middle
concern and special precautions must be tak-
                                                        one-third of all tiers of blocks (see Fig-
en. These blocks may require additional crib-           ure 8-14)
bing to ensure that they do not tip over. Use
the following guidelines when considering         8-7.2   Loading on Side Blocking
the stability of the blocks.                      The side blocking will support both static and
                                                  dynamic loads. The side blocking supports
   • All keel blocks over 8.5 feet in height      some of the weight of the asset in the zero
     require cribbing                             heel condition. This support increases as the
                                                  vessel heels. Side blocking is also needed to
   • Keel blocks over 6 feet but less than 8.5    resist dynamic loads that may be caused by
     feet should be cribbed when located in       ship motions and wind which are dependent
     the after one-third or forward one-third     on environmental conditions during loading
     of the block line                            and transit.



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8-7.2.1 Assessing the Loading on Side                 open ocean seafastening plan should be con-
Blocking                                              ducted. Winds and ship motions associated
There are several conditions that need to be          with this transit should be evaluated to deter-
examined in determining when to install and           mine the total number of side blocks and spur
the amount of side blocking required for a            shores.
transport. The most strenuous condition is            8-7.2.2 Loading on Side Blocks
when the asset is loaded on the heavy lift ves-
                                                      The side blocks will support a portion of the
sel and the vessel is in an open seaway. In
                                                      deadweight of the asset, both in a heeled con-
this condition, the most extreme wind and
                                                      dition as well as an upright condition. For the
motions will be experienced. A reduced con-
                                                      zero heel condition, assume that side blocks
dition of loading exists during Float On. The
                                                      will take 15 percent of the assets displace-
condition of the loading associated with the
                                                      ment (w) and that this load is evenly distribut-
float on operation should be evaluated. It may
                                                      ed between port and starboard. Therefore, the
be advantageous to not complete the contruc-
                                                      load on the side blocking for port or starboard
tion of the side blocking until after the asset is
                                                      is calculated by:
loaded. This is normally to reduce the height
of the side blocking so that less submergence                           ( 0.15 )w
of the cargo deck is required during the load-                                            -
                                                                 DL s = ------------------- = 0.075w
                                                                                 2
ing operation. If it is proposed to use fewer
side blocks for the float-on operation, this          where:
condition must be analyzed separately (see
                                                      DLs = Vertical load on side blocks for one
Section 8-7.3). As this is conducted in a rela-
                                                            side (tons)
tively sheltered place, the expected loads are
less. A third potential condition arises when         w    = Displacement of asset at time of
the load site and the build site are not the                 loading (tons)
same. That is to say that there is some transit       The number of side blocks required on one
after the loading but prior to the completion         side for supporting displacement (Nd) without
of the seafastening plan. If the location of the      considering dynamic and wind effects can be
build site requires moving the vessel with the        calculated by:
asset loaded, the environmental conditions                               DL
                                                                          s
associated with the transit should also be ana-            N d = -----------
                                                                           -
                                                                 S p Ae
lyzed as a separate evolution. This may occur
if an asset is in extremis or the port of loading
does not have deepwater and adequate indus-                               0.075w
                                                                                                    -
                                                             = --------------------------------------
                                                               S p Ae  ------------------- 
trial services located in the same area. There-                                     1 ton 
                                                                              2240 lbs           -
fore calculating the side loading of the block-
ing is a multi-step process.
                                                              1086.4 w
The approach outlined here breaks down the                                    -
                                                            = -----------------
                                                                 Sp A e
loading into static (deadweight) and dynamic
(rolling and wind). Each of these compo-
                                                      where:
nents can be considered separately and then
combined to determine a suitable blocking ar-         Nd = Minimum number of side blocks on
rangement. As a minimum, an analysis of the                one side to support displacement




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DLs = Vertical load on side blocks for one          For example, typical ship motions for ocean
      side (tons)                                   transport are:
Sp = Strength at the proportional limit of          R       = 20 degrees of roll, one direction
     the block material (lb/in2)                    P       = 4 degrees of pitch, one direction
    = 800 psi for Douglas fir                       Tr = roll period of 10 seconds, port to stbd
                                                         back to port
Ae = Effective surface area of side block
     in contact with asset (in2)                    Tp = pitch period of 7 seconds, bow to
                                                         stern back to bow
w   = Displacement of asset at time of
      loading (tons)
8-7.2.3 Dynamic Loads During Transport
                                                                                          NOTE
Side blocks and spur shores are also needed                     These values should be compared
to resist dynamic loading due to wind and                       with the expected values from
ship motions. The method for calculating the                    route planning. If larger angles of
moments associated with these forces are pre-                   heel or pitch are expected, those
sented. In performing an analysis, it is impor-                 values should be used.
tant to examine all likely conditions the ves-
sel will experience. The predicted sea state        This estimate should not be used as a substi-
during transit may be as high as sea state 7        tute for an actual route analysis.
while the conditions during loading may be
                                                    The acceleration factor is calculated by:
limited to sea state 4. Each of these scenarios
should be evaluated to ensure that the block-
                                                                  0.0107Px .0002R 2 y 0.0214Rz
ing arrangement is sufficient. These dynamic                                           -
                                                    a y = sin R + ---------------------- + ---------------------- + -----------------------
                                                                                2                        2
                                                                                                                -
                                                                                                                                  2
loads will be dependent on the environmental                              Tp                      Tr                        Tr
conditions encountered during the transport.
                                                    where:
8-7.2.4 Dynamic Loads from Ship Motions
                                                    ay = athwartships acceleration factor in (g)
Calculating overturning moments caused by
sea state dynamic forces is similar to calculat-    R = Maximum angle of roll (deg)
ing seismic overturning moments for a nor-          P = Maximum angle of pitch (deg)
mal dry-docking operation (see NSTM 997).
These calculations are modified to include the      x     = Distance of center of gravity of asset
acceleration loads associated with rolling and              forward or aft from center of gravity
pitching of the heavy lift ship. The maximum                of heavy lift ship (ft)
ship motion and roll angle is found by exam-        y     = distance of asset’s centerline off cen-
ining the expected weather during transit. For              terline of heavy lift ship (ft)
example, if the routing indicates that the max-
imum condition is sea state 7, use roll and         z     = distance of center of gravity of asset
pitch angles from Tables 8-5 and 8-6. This is               above center of gravity of heavy lift
the limit to which the heavy lift ship is as-               ship (ft)
sumed to heel during the transit. Note that
                                                    Tp = Period of pitch (sec)
this load is supported by only one side of
blocks.                                             Tr = Period of roll in (sec)


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                           Figure 8-16. Overturning Moment Due to Wind Forces.

To calculate the overturning moment (Mr)                blocks on one side required to resist the dy-
associated with athwartships motion, the                namic loads due to rolling is:
weight of the asset is multiplied by the height
of the center of gravity above the asset’s keel.                                   Mr
                                                                       N r = -----------------
                                                                                             -
This is then multiplied by an acceleration fac-                              Ae SpL2
tor similar to that calculated for keel blocks.

       M r = ( w ⋅ ay ) ( KG ) ( 2240 lbs/ton )         where:

                                                        Nr = Number of additional blocks for roll-
where:                                                       ing
Mr = Overturning moment due to rolling                  Mr = Overturning moment due to rolling
     (ft-lbs)                                                (ft-lbs)
w      = displacement of vessel at time of
         loading (tons)                                 Ae = Effective surface area of side block
                                                             in contact with asset (in2)
ay = acceleration factor for athwartships
     motion (g)                                         Sp = Strength at the proportional limit of
KG = The center of gravity of the asset                      the block material (lb/in2)
     above its keel (ft)
                                                             = (800 psi for Douglas fir)
The effort to resist this overturning moment
can be provided by the side blocks, but the             L2    = Average moment arm of side block
number of side blocks needed is dependent on                    reaction force (ft)
the size of the blocks and their distance from
the centerline (see Figure 8-13) and available          But a better solution is to resist this moment
space on the cargo deck around the asset.               with a combination of side blocks and spur
Therefore, the number of additional side                shores.


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8-7.2.5 Dynamic Loads from Winds                            Mw = Overturning moment due to wind
Additional side blocking is necessary to resist                  forces (ft-lbs)
the overturning moment associated with the                  Ae   = Effective surface area of side block
wind forces that will be encountered during                        in contact with asset (in2)
transport. Calculating overturning moments
caused by wind is similar to calculating hurri-             Sp   = Strength at the proportional limit of
cane loading in a normal dry docking situa-                        the block material (lb/in2)
tion. This is also a 2 step process, once for the                = 800 psi for Douglas fir
wind force during loading (< 25 Knots) and
once for the wind loading during the transit.               L2   = Average moment arm of side block
To determine the expected wind during tran-                        reaction force (ft)
sit, use the guidelines listed in paragraph                 8-7.2.6 Determining the total amount of side
8-5.1.1.                                                    blocking required

The overturning moment associated with the                  The total amount of side blocking required is
wind forces can be estimated by the following               a combination of the amount calculated in
equations:                                                  paragraph 8-7.2.3 for dead weight loading in
                                                            paragraph 8-7.2.4 for dynamic loading and in
                                          2
            M w = 0.004 A s L 3 V                           paragraph 8-7.2.5 for wind loading. That is

                                                                         N T = N d + N r + Nw
where:
Mw = Overturning moment due to wind
     forces (ft-lbs)                                        But a better solution may be a combination of
                                                            side blocks and spur shores. Side blocks pro-
As    = Projected sail area of the asset (ft2)              vide better support to static type loading be-
        (See Figure 8-16)                                   cause of their higher compressive strength.
L3    = Lever arm from the cargo deck to                    Spur shores may be better placed farther out
        the center of the sail area of the as-              and higher against the hull of the asset to re-
        set (ft) (See Figure 8-16)                          sist dynamic loading.

V     = wind speed (knots)                                  8-7.3 Additional Side Block Consider-
                                                            ations
Note: The factor in this equation provides for
                                                            In some cases, the heavy lift contractor may
unit conversion.
                                                            desire to install a minimum number of blocks
The number of additional side blocks re-                    during float on and build the remainder of the
quired on one side to resist the forces due to              blocks after the asset is loaded. This is nor-
wind is:                                                    mally done to reduce the required depth of
                                                            submergence during loading. This is accept-
                           Mw                               able given that the environmental conditions
                N w = -----------------
                                      -                     during loading and transit to the build site are
                      Ae S p L2
                                                            evaluated. The above calculations should
                                                            therefore be repeated using the maximum ex-
                                                            pected weather conditions during loading and
where :
                                                            building. Generally, using a sea state 4 condi-
Nw = Number of additional blocks for                        tion is acceptable. In no case should the mini-
     wind                                                   mum number of side blocks be less than the


                                                                                                       8-49
DRAFT                                  U.S. Navy Towing Manual


number necessary to support the static load          Considering the above, and bearing in mind
and the expected roll angle. A minimum of            that side blocks work better in direct loading
five degrees of roll should be used. If the as-      and spur shores are better to resist dynamic
set has an initial list, e.g., USS COLE had a        over turning loads, we can again look at the
2.5o list on loading, that value should be add-      equation for the acceleration factor, in para-
ed to the 5 degrees to determine the number          graph 8-7.2.4 to determine the number of side
of initial side blocks for loading. The static       blocks to be used in combination with spur
load is calculated using the equations in para-      shores. The static, direct, loading for side
graph 8-7.2 and adjusting the roll angle. In no      blocks can be determined from the first term
case should the minimum number of side               (sinR) in the equation
                                                                                                                  ·
blocks be less than four (two port and two                          0.0107Px 0.0002R y 0.0214Rz
                                                                                                                  2
starboard).                                                                              -
                                                      a y = sin R + ---------------------- + -------------------------- + -----------------------
                                                                                  2                         2                           2
                                                                           Tp                         Tr                          Tr
In the past, obtaining an accurate fit of side
blocks has been a recurring problem. Contrib-        while the remainder of the equation can be
uting factors include inadequate drawings,           used as a first estimate of the number of spur
bad offsets, damage to the hull of the asset, or     shores required.
landing the asset slightly out of position due
to the dynamic environment in which these            8-7.4         Spur Shores
lifts were done. Placing side blocks in posi-
                                                     The number of side blocks needed to resist all
tions shown on the Navy Docking Drawing              of the loads experienced during a transport
offers the best possibility of a correct fit, but    will likely be too large to fit the given space.
wedging material, may still be needed. Air           In almost all cases, the number of side blocks
bags have been used on top of the side blocks        that can be placed in a given area will be less
and inflated by divers after the asset was           than what is needed. They simply may not fit
landed. This has been effective in stabilizing       on the cargo deck of the heavy lift ship, par-
the asset for the lift. With wedging or air          ticularly if more than one asset is transported.
bags, however, the blocks have an unknown            To make up for this difference and to resist
compressive flexibility due to dynamic load-         dynamic forces associated with roll angles
ing. If a significant change in the contact area     greater than 15 degrees, spur shores are used.
or the compressive flexibility is observed, the      A combination of spur shores and side blocks
number of side blocks required should be re-         will be used to resist the total load.
calculated after the asset is loaded, taking into
account the dynamic loading, maximum roll            Spur shores (roll bars) can be used in combi-
angles, and actual location and effective area       nation with side blocks to resist overturning
of the side blocks. Additional side blocks or        moments due to dynamic motions of the
shores may be necessary.                             heavy lift ship and high winds (Mr + Mw).
                                                     Spur shores are tall, column like structures
The side blocks should be positioned as              that are placed further outboard on the hull
shown on the Navy docking drawing. This              than side blocks (See Figure 8-15). They do
will ensure the best possible fit since the side     not contribute significantly to supporting the
block build has been based on hull offsets in        weight of the vessel, but they can make a sig-
these locations. This will also help to ensure       nificant contribution to resisting overturning.
stable blocks up to angles of 15°. This ap-          The number of side blocks can be reduced if
proach is somewhat conservative as the spur          spur shores are used to help resist the dynam-
shores will share some of the deadweight.            ic moments associated with transit. Consider


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                                     U.S. Navy Towing Manual
                                                                                            DRAFT
the following points when deciding to use          8-7.4.1 Loading on Spur Shores
spur shores:                                       The number of shores required is dependent
   • Spur shores are generally easy to install     on several factors. Because the shores are
     and take up less deck space.                  slender, they will fail as columns before they
                                                   will fail in direct compression. This is the op-
   • Roll angles in excess of 24 degrees           posite of the failure mode for side blocks. It is
     have been observed during this type of        necessary to determine the maximum column
     lift/transport. Therefore, supports           stress that the shore can withstand to ensure
     should be placed at various angles to         that the shores do not buckle under a com-
     encompass the total range of stability of     pressive load. The actual load that each spur
     the heavy lift ship. Spur shores are          shore will see will be dependent on the num-
     more suitable than side blocks for this       ber of side blocks that are used and the local
     duty.                                         structural load limit on the side of the asset. If
                                                   space limitations require only a few side
   • Spur shores are easily angled to resist
                                                   blocks, than a larger number of spur shores
     the highest roll the ship is likely to en-
                                                   will be needed.
     counter.
   • Thrust blocks (base plates) must be pro-                              NOTE
     vided at the base of all spur shores and
     be firmly secured to the cargo deck of               In no case shall the number of side
                                                          blocks be reduced below the mini-
     the heavy lift ship.                                 mum number required for loading.
   • The shores must be suitably secured to
     prevent the shores on one side from           The equations will be based on the actual
     falling out when those on the other side      number of side blocks used. This number will
     are compressed. This limits the bearing       likely be less than the number calculated
     load per shore to the compressive load        above and is largely dependent on deck space
     of the soft cap.                              and the hull of the asset. The docking draw-
                                                   ing and structural details should be checked
   • A higher number of shores will likely         to determine suitable locations and places
     be required to resist the same moment         where spur shores are more appropriate. The
     as fewer side blocks.                         following series of equations determine the
   • Sufficient space will be required be-         maximum column stress for the shore based
     tween multiple assets to install spur         on its geometry and material. The maximum
     shores.                                       stress for each shore is found by:

   • Shores must be secured in the fore and
                                                                                     Ls 4
                                                                       1 –  1  --------- 
     aft, as well as the athwartships direc-                   Sc = C         -
                                                                             --  d ⋅ K 
                                                                              3
     tion.
   • Because of their point loading on the
                                                   where:
     hull of the asset, the local structural
     limit must be evaluated in determining        Sc = Maximum column stress (lb/in2)
     the number of spur shores to be used
                                                   C   = Proportional limit
     and designed with a top spreader to
     spread the load to the asset's hull struc-        = 3,000 psi for Douglas fir parallel to
     ture.                                               grain


                                                                                                  8-51
DRAFT                                       U.S. Navy Towing Manual


Ls = Length of shore (ft)                                 8-7.4.2 Determining the Number of Spur
                                                          Shores
d      = Minimum dimension of shore cross
         section (ft)                                     To determine the number of shores required,
                                                          for a given number of side blocks, it is neces-
K      = Relationship between elasticity and              sary to evaluate the entire build. Information
         proportional limit                               about the spring constants of both the blocks
This constant (K) is calculated by:                       and the shores is required as well their loca-
                                                          tions. To calculate the spring constant of the
                                                          shore use:
                             E
                    K = 1.11 ---
                               -
                             C                                                              AsE
                                                                                                    -
                                                                                     K s = ----------
                                                                                           12L s
where:
K      = Relationship between elasticity and              where:
         proportional limit
                                                          Ks = Spring constant of spur shores (lb/in)
E      = Modulus of elasticity of shore (lb/
         in2)                                             As = Cross sectional area of shore (in2)
       = 1.6 x 106 psi for Douglas fir                    E = Modulus of elasticity of shore (lb/in2)
C      = Proportional limit (lb/in2)                         = (1.6 x 106 psi for Douglas fir)
       = 3,000 psi for Douglas fir parallel to
         grain                                            Ls = Length of the shore (ft)

To prevent the shore from buckling, the shore             To test the suitability of a build (column sta-
reaction (Rs) must be equal to or less than:              bility of the shore and compression of the side
                                                          block) one would use the following series of
                      R s ≤ Sc As                         equations to determine the average reactions.

where:
                                                                               ( Mw + Mr ) ( Ks L1 )
Rs = Maximum shore reaction (lb)                                                                                           -
                                                                      R s = ------------------------------------------------
                                                                                  2                           2
                                                                            L1 N s K s + L2 N b Kb
Sc = Maximum column stress (lb/in2)
As = Cross sectional area of the shore (in2)                                  ( Mw + Mr ) ( Kb L2 )
                                                                                                                           -
                                                                      R b = ------------------------------------------------
                                                                                  2                           2
                                                                            L1 N s K s + L 2 N b K b

                        NOTE

         The R s of a shore must be less                  But since we know the maximum reaction
         than the local structural limit of the           that a particular shore can handle, we can re-
         asset’s hull structure.                          work the equation to find the minimum num-




8-52
                                                                           U.S. Navy Towing Manual
                                                                                                                             DRAFT
ber of shores required. The equation then be-                                            8-7.4.3 Distribution of Spur Shores
comes:
                                                                                                              NOTE
       ( M w + M r ) ( Ks L1 ) – R s L2 N b K b
                                                                      2
 N s = -----------------------------------------------------------------------------
                                                                                   -            The locations for spur shores are
                                                2                                               estimated by hull shape and struc-
                                    R s L 1 Ks
                                                                                                tural drawing. The actual position-
                                                                                                ing of each shore is dependent on
where:                                                                                          determining local structure on the
                                                                                                ship.
Ns       = Number of shores required on one
           side
                                                                                         The above procedure determines the mini-
Mw = Moment caused by wind for transit                                                   mum number of spur shores to be used. This
     (ft-lbs)                                                                            procedure assumes the number of side blocks
Mr       = Moment caused by rolling for tran-                                            used is determined by available spacing and
           sit (ft-lbs)                                                                  for the direct support up to the maximum ex-
                                                                                         pected angle of roll. These sideblocks will
Ks       = Spring constant of spur shores (lb/
                                                                                         generally be placed to resist rolls up to at least
           in)
                                                                                         15 degrees but preferrably to the maximum
L1       = Average lever arm of the spur                                                 angle of roll. The spur shores must resist rolls
           shore's reaction forces (ft) (see Fig-                                        beyond this angle. To help resist these rolls,
           ure 8-13)                                                                     the shores should be distributed throughout
Rs       = Max allowable reaction of shores                                              the range of angles.
           (lb)
                                                                                         Since spur shores need to support the load
L2       = Average lever arm of the side block-                                          down the axis of the shore and tend to trip
           ing reaction forces (ft) (See Figure                                          out, positioning the spur shores in increments
           8-13)                                                                         of about 5 degrees should provide acceptable
Nb       = Number of side blocks required on                                             load sharing. They should be set up in pairs,
           one side of the ship                                                          fore and aft, to resist twisting. They need to
                                                                                         be positioned perpendicular to the hull on lo-
Kb       = Spring constant of side blocks (as-                                           cal structure and high enough on the hull to
           sume 200,000 lb/in)                                                           resist the overturning moment yet be short
                                                                                         enough or supported not to fail under buck-
                                       NOTE                                              ling. While effective length dictates a higher
                                                                                         angle, angles of 45 degrees or less should be
           For larger assets or to reduce the
                                                                                         chosen unless compensating for the overturn-
           number of shores required, consid-
           er using steel shores.                                                        ing moment dictates placing the shores higher
                                                                                         on the hull.

                                                                                         This method may produce a different number
                                                                                         of spur shores than previously determined. To
                                                                                         be conservative, the larger number should be
                                                                                         used and a minimum of two shores (one fore
                                                                                         and one aft) should be installed at each angle.
                                                                                         The shores should be secured to the deck at
                                                                                         the foot of the shore to ensure that they do not
                                                                                         slide when loaded. They should also be

                                                                                                                                      8-53
DRAFT                                   U.S. Navy Towing Manual


placed normal (perpendicular) to the curva-           imum loading for each spur shore through a
ture of the hull.                                     variety of roll angles. This distribution should
                                                      be similar to the distribution determined by
Final verification of the compensation of the
                                                      the above method.
overturning moment (Mo) is accomplished by
checking to see that the overturning moment           8-7.5   Seafasteners
about the attachment point of the spur shores
is less than the righting moment (Mr) pro-            Seafasteners must be designed to restrain the
duced by the spur shores, that is                     asset from movement at the high angles of
                                                      pitch and roll anticipated during transits.
Mo < Mr                                               Some resistance to these forces is provided by
Mo is created by the transverse dynamic force         the friction between the keel and the blocks.
working through the ship's center of gravity          Seafasteners must be installed at the forward
and its separation from the attachment point          and aft ends of the keel or some other reason-
of the spur shore.                                    ably accessible location of the asset to resist
                                                      longitudinal movements due to the maximum
Mo = (displacement) (ay)(Lo)                          angle of pitch of the heavy lift. They should
where Lo = distance between the line of ac-           also be installed on the port and starboard
tion of the dynamic force working transverse-         sides of the keel to help resist athwartships
ly through the ship's center of gravity and the       movement of the asset during rolling. It is
position of the shores on the hull in the verti-      prudent to install seafasteners at both the fore
cal direction.                                        and aft ends of the keel to prevent twisting. A
                                                      minimum of two seafasteners should be in-
Mr is created by the resultant force that the         stalled at each end (one port and one star-
spur shores can create in the transverse direc-       board) (see Figure 8-17). However, each
tion against the hull and its separation from         seafastener should be capable of resisting the
the line of action of the weight of the vessel        entire sliding force.
passing through the ship's center of gravity.
                                                      8-7.5.1 Dynamic Force
Mr = (displacement)( 2- az) Lr cos R
(R = maximum angle of roll)                           The dynamic force that must be restrained in
                                                      each direction is equal to the weight of the as-
where (az) is the dynamic load factor in the
                                                      set times the dynamic load factor. This is sim-
vertical direction which increased the load on
                                                      ilar to the procedure that was used to calcu-
the blocking when the heavy lift ship pitches
                                                      late the dynamic loading on the blocking (see
up and decreases the loading on the blocks
                                                      8-7.2.4). Here, the main concern is the asset
when the heavy lift ship pitches down, (2-az).
                                                      sliding off the blocking. However, for the
Lr is the distance between the line of action of      transverse direction, the dynamic load factor,
the downward force through the ship's center          ay, will be the same. Therefore, the dynamic
of gravity and the position of the shore on the       load in the transverse direction will be
hull in the transverse direction. The cos R
factor is to adjust the line of action of the                          DL t = ∆ay
weight from the vertical to account for the
maximum angle of roll.
                                                      DLt = Dynamic load in the transverse di-
In practice, the Transport Manual will recom-               rection determined by the maxi-
mend a seafastening plan that will include,                 mum angle of roll to be expected in
among other things, a plan for positioning                  route. (tons)
spur shores. It is likely that the contractor will
perform a detailed analysis to determine max-         ∆       = Displacement (tons)
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                                                         U.S. Navy Towing Manual
                                                                                                          DRAFT
ay    = Athwartship acceleration factor (g)                            relatively flat bottom vessels as long as the
        (see 8-7.2.4)                                                  vessel doesn’t lift off the blocking due to be-
                                                                       ing submerged.
Similarly, the dynamic load in the longitudi-
nal direction will be:                                                 The friction factor used for longitudinal slid-
DLl = ∆ax                                                              ing is less because there is a greater possibili-
                                                                       ty of the vessel lifting off the blocks due to
DL1 = Dynamic load in the longitudinal di-                             submergence. When a 500 foot heavy lift ship
      rection determined by the maximum                                pitches 3 degrees, the trim increases by 26
      angle of pitch to be expected in route.                          feet. Other factors contributing to these con-
      (tons)                                                           clusions include variations in materials, vari-
∆    = Displacement (tons)                                             ations in hull shape, column stability of the
                                                                       blocks, and possible overhang of the asset.
ax = longitudinal acceleration factor (g),                             These approximations should be provide rea-
                     0.0004Px 0.0214Pz                                 sonable estimates for sizing seafasteners. To
                                                                  -
       = sin P + S + ---------------------- + ---------------------    determine the amount of this load that is re-
                                  2                        2
                            Tp                       Tp                sisted by friction, multiply the weight of the
                                                                       asset by the frictional factor. For the trans-
where:                                                                 verse direction, assume a frictional factor of
P    = Maximum angle of pitch (degrees)                                0.15. Therefore, the frictional resistance (FRt)
       (Table 8-5)                                                     can be found by
S    = Surge acceleration (g) (Table 8-4)
                                                                                       FR t = 0.15∆
x    = Distance of center of gravity of asset
       forward or aft from center gravity of
       heavy lift ship (ft)                                            and
z    = Distance of center of gravity of asset
                                                                                        FR l = 0.05∆
       above center of gravity of heavy lift
       ship
                                                                       8-7.5.3 Sea Fasteners Resistance
Tp = Period of pitch (sec) (Table 8-5)
                                                                       The seafasteners must resist the force that is
Note that the commercial industry assumed a
                                                                       not carried by friction. Therefore, the force
pitch of 15o at a period of 10 seconds which
                                                                       carried by the seafastener is equivalent to:
is considerably higher than the value for pitch
in Table 8-5.
                                                                                      SF = DL – FR
8-7.5.2 Assumed Friction Factors
In practice, it has been observed that a ship                          In the transverse direction
slides transversely when heel exceeds rough-
ly 15 degrees and longitudinally when pitch                                          SF t = DL t – FR t
exceeds roughly 3 degrees. The dynamic fric-
tional resistance of steel on wet or greased
wood is approximately 22 percent. Depend-                              Where:
ing on the weight and center of gravity of a
                                                                       SFt = Transverse Seafastener force (tons)
ship on docking blocks, this equates to an an-
gle of approximately 12 degrees before slid-                           DLt = Dynamic load in the transverse direc-
ing will occur. These numbers work well for                                  tion (tons)


                                                                                                                   8-55
DRAFT                                 U.S. Navy Towing Manual




                                     Figure 8-17. Sea Fasteners.


FRt = Frictional resistance (tons)                   Where:
In the longitudinal direction, assume a fric-        SFl = longitudinal seafastener force (tons)
tion factor of 0.05. Therefore,                      DLl = Dynamic load in the longitudinal di-
                                                     rection (tons)
             SF l = DL l – FR l
                                                     FRl = Longitudinal frictional resistance
                                                     (tons)
Or,
                                                     ∆     = Displacement (tons)
            SF l = ∆a x – 0.05∆
                                                     ax    = longitudinal acceleration factor (g)




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                                          U.S. Navy Towing Manual
                                                                                           DRAFT
8-8 Surveys                                             If not, he should, as a minimum, review the
                                                        time required for a complete sequence (this
Several surveys must be conducted to ensure             information should come from an actual oper-
that the vessel and its systems can adequately          ation and not just from published capabili-
perform the FLO/FLO operation. These sur-               ties). The inspection should also include a
veys will determine that all material and pro-          general walk through of the vessel to verify
cedures are in accordance with the Transport            sea worthiness and proper adherence to class
Manual and conform to the requirements of               society regulations. First and foremost, the
this manual.                                            heavy lift ship must be in class and must
                                                        present the latest certificate of class and mate-
8-8.1 Hydrographic Survey                               rial condition survey. It is often prudent to
                                                        have the Docking Observer, IMS and Load-
                      NOTE                              master participate in these surveys as well.
      Heavy lift ships in general, are not              After the vessel has been accepted, several
      designated to make contact with                   detailed surveys should be completed to en-
      the bottom. Adequate depth of wa-
                                                        sure all systems are in good working order.
      ter must be provided so that the
      heavy lift ship does not contact the              The surveys of the heavy lift ship, assets and
      bottom. Contact with the bottom                   blocking described below are conducted by
      may require dry docking or inspec-                the Docking Observer, the IMS, and the
      tion of the heavy lift ship by divers             Loadmaster.
      in order to maintain its class certifi-
      cation.                                           8-8.3   Structural Surveys
                                                        A thorough inspection of the heavy lift ves-
The hydrographic survey must be conducted               sels primary structure should be completed.
at the proposed loading and unloading sites             The plating, strength members, joints, foun-
and in the approach channel by an adequate              dations, seachests, entire cargo deck where
number of soundings referenced to Mean                  blocking may be installed, and structure asso-
Low Water. These surveys are part of the de-            ciated with mooring must be checked. Indica-
cision making on choosing the loading and               tions of excessive corrosion or local failure
unloading sites and dictate the operating pro-          should be analyzed accordingly. In addition
cedures for each. A sounding chart must be              to this general walk around inspection, the
included in the survey results. Complete tidal          latest material condition survey, records of
ranges, approach channel width and depth                repair, and design data should also be exam-
configuration, dredging frequency, and any              ined. These may alert the inspectors to any ar-
regularities must be also noted. Where a his-           eas that might warrant a more thorough in-
tory of hydrographic data is available, rates of        spection. The information collected by the
siltation must be noted.                                visual inspection should be analyzed and
8-8.2 Acceptance Survey
                                                        compared with the information contained in
                                                        the past surveys to determine whether detail
An acceptance survey should be conducted                surveys and/or repairs are required in any ar-
by the contracting officer (generally the MSC           ea.
area representative). This is a general survey
                                                        8-8.4   Indicators and Controls
that shows that the vessel and its systems are
the same as those described in the Transport            An inspection of the heavy lift vessel’s bal-
Manual. If possible, the Contracting Officer            last/deballast control system shall be accom-
should observe a ballast/deballast sequence.            plished. This system is critical to completing


                                                                                                    8-57
DRAFT                               U.S. Navy Towing Manual


a safe operation and should be in good work-         • Effectiveness of the operation of all
ing order prior to the start of the FLO/FLO            pumps, motors, valves, and generators
procedure. In general:                                 by remote control and local control.
   • Draft indicators must be provided               • The accuracy and reliability of water
     showing the draft of the heavy lift ship          level indicators when compared with
     at all four corners of the ship and cargo         actual sounding of the water level in
     deck. Backup systems such as visual               each tank.
     observation should be addressed.                • Tightness of air-cushioned boundaries,
   • Indicators must be provided to continu-           if they are required, in the tanks.
     ously display trim and heel of the heavy
     lift ship during docking and undocking          Controls
     ballast/deballast operations.
                                                     • Control panel: Check wiring, relays,
   • Ballast tank level indicators must pro-           bulbs and lenses for dust collection and
     vide for controlling ballasting/debal-            abrasion of wires.
     last. The accuracy of these indicators
     must be adequate to prevent accidental          • Motor controls: Check contractors, re-
     overstressing of tank bulkheads by ex-            lays electrical and mechanical inter-
     cessive differential heads and acciden-           locks and manual overhauls.
     tal overstressing of the overall ship           • Limit switches: Check panel limit
     structure in shear and bending.                   switches and switch activator mecha-
                                                       nisms.
   • Ballasting system valve indicators must
     be provided that show the position of        8-8.5   Pre-loading Block Check
     the valves.
                                                  Before submerging the heavy lift ship, block-
The surveyor should observe at least one          ing should be inspected to ensure it is in ac-
complete ballasting and deballasting cycle        cordance with the arrangements in the ap-
and provide a report on the below systems. If     proved Transport Manual. As a minimum, the
possible, this survey should take place at the    inspection should concentrate on the follow-
same time that the Contracting Officer per-       ing areas:
forms the acceptance survey to avoid duplica-        • Location of first keel block (after most
tion of effort.                                        on the asset)
       Ballasting/Deballasting Systems and           • Location of the alignment marks or col-
       Gauges                                          umns for port and starboard alignment
   • Actual ballasting and deballasting                to the center of keel blocking
     times. If these times are different from        • Location    of   fore-and-aft   centering
     ballasting and deballasting times for             markers
     which the system was originally de-
     signed, reasons for this variation must         • Side clearance of the asset
     be explained in the survey results.             • Rudder, propeller, and other hull pro-
                                                       jections clearances above the cargo
   • Adequacy of the power supply, deter-
                                                       deck and blocking
     mined by operating all applicable
     pumps (and the fire pump, if installed          • Offsets from center line or from set keel
     on dock) at the same time.                        blocks and side blocks

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                                     U.S. Navy Towing Manual
                                                                                      DRAFT
   • Keel blocks levels for the length of the      operation so that communications can be
     ship’s keel (checked visually) to ensure      maintained between all operating stations.
     there are no excessively high blocks
                                                   Fire Protection Systems
   • Heights of side blocks and keel blocks,
                                                   The fire protection systems intended for
     if not flat
                                                   fighting fire on the cargo deck or asset must
   • Special blocking arrangements for hull        be thoroughly checked and tested for con-
     projections, hull openings, or special        formance to all requirements of paragraph
     support blocks                                5.3.14 of MIL-STD-1625. The capacity avail-
                                                   able to serve the asset’s firemain (either per-
   • Removal of unnecessary blocks                 manent or temporary) shall also serve the fire
8-8.5.1 Wooden Blocks                              stations on the cargo deck, but in no case
                                                   shall be less than 1,000 gallons per minute.
Inspect wooden blocks for deterioration re-
                                                   The supply pressure shall be capable of pro-
sulting from excessive crushing, warping,
                                                   viding a minimum mozzle pressure of 60 psi
cracking, checking, rotting, or damage from
                                                   when supplying fire nozzles at the specific
dogging. Check for loss of contact at edges
                                                   capacities at the most remote and highest ele-
resulting from checking and unequal shrink-
                                                   vation hose connections.
age.
                                                   Block Handling Systems
8-8.5.2 Block Securing Method

All blocks must be secured in place. Secur-        The block handling system must be observed
ings, supports, nuts, boltheads, and other fas-    in operation and must be inspected.
teners should be sounded. If the blocking          Mooring and Anchoring Systems
does not land on transverse strength members
                                                   The mooring and anchoring systems must be
of the cargo deck, conduct an investigation to
make sure that adequate grillage is being used     inspected thoroughly for adequacy and for
to distribute loading to adjacent strength         signs of local buckling and excessive loading.
members. Inspect the securing and bolt con-        Electric Power Systems
nections through the wood where blocks are
                                                   Both the primary and alternate electric power
bolted to clip angles or plates that are welded
                                                   systems must be inspected. Power switches,
to the cargo deck. When blocks are set on
                                                   converting panels, and cables for providing
steel frame supports, inspect the bolts and
                                                   power to the asset must be inspected for ma-
supports as well.
                                                   terial condition and proper fit and size.
8-8.6 Additional Systems
                                                   Ship Positioning Gear
The two systems described above, structure
and ballasting, are the two systems that make      Bitts, bollards, winches and cleats must be in-
FLO/FLO vessels different from other ships.        spected for fatigue, looseness, or other signs
                                                   of excessive loading.
In addition to these systems, the surveyor
should also conduct an inspection of the more      Ship Services
traditional ship’s systems. The survey should
                                                   Compatibility of all connections (firemain,
include a review of the following:
                                                   electrical, cooling water, etc.) should be veri-
Communication Systems and Alarms                   fied as specified in the Transport Manual and
                                                   identified at the Pre-Loading conference.
The communication systems and alarms must
be checked thoroughly and tested for proper        Safety Equipment


                                                                                              8-59
DRAFT                                  U.S. Navy Towing Manual


All safety equipment necessary to comply             be taking place in open water instead of with-
with the governing regulatory agency should          in a drydock.
be inspected. While some safety equipment
from the asset can be used, most will not be                            WARNING
suited for this purpose. To avoid delays, be                 All sea suctions for the asset
sure that it is clear who is responsible for pro-            and the heavy lift vessel should
viding safety equipment for the riding crew.                 be secured during diver opera-
                                                             tions.
8-8.7   Asset Inspection
                                                                        WARNING
It should be ensured that the assets are rigged
for sea by completing a walkthrough of all                   All parties must be informed
compartments and soundings of all tanks.                     when divers are being used. Ex-
                                                             treme caution must be used to
This survey should include an inspection of                  ensure the safety of these indi-
the watertight integrity of the hull and ensur-              viduals. No deballasting or other
ing that Condition Zebra is set. The final con-              ship movements should occur
                                                             while divers are working under
dition of the asset’s loading should be in-
                                                             the asset.
spected and recorded just prior to its
departure and copies made available to all
                                                     When the divers have reported that assets
parties. All tanks and voids must be accurate-       have landed satisfactorily on the blocks, the
ly sounded and photographs should be taken           deballasting operation should continue until
of the topsides of all assets. These photos will     the cargo deck emerges from the water. A
accurately identify the nature and position of       thorough examination of the condition of the
any items that may be have been added top-           landing should be completed by the Load-
side. They will be used to verify that the as-       master, Docking Officer and the IMS. A deci-
sets condition has not changed when it is time       sion whether to continue deballasting or to re-
for off-loading. No weights, including liquids       float the assets should be made. Any
such as fuel or water, should be shifted, add-       irregularities found should be noted and cor-
                                                     rected, and any necessary wedging and/or
ed, or removed from the asset unless autho-
                                                     shoring must be placed.
rized by the heavy lift ship's master and the
OIC. A checklist is provided in Appendix H.          If the decision is made to continue with the
                                                     deballast procedure, this effort should be
8-8.8   Post Float-On Inspection                     completed immediately and the remainder of
                                                     the build should commence.
When the asset lands on the blocks as debal-
lasting begins, the condition of this landing        8-8.9   Examination of the Seafastening
should be examined. Divers should be used to         8-8.9.1 Prior to Transit
ensure that no blocks have tipped, that the as-
                                                     Following the completion of the build, all
set is in the predicted location, and that there     components, keel blocks, side blocks, and
are no interferences. Divers from the local          spur shores should be surveyed by the Load-
drydock facility should be proficient in this        master, Docking Officer, and IMS. They
type of inspection. Appropriate safety precau-       should inspect the spur shores and seafasten-
tions must be taken as these operations will         ing before departure to make sure that they


8-60
                                         U.S. Navy Towing Manual
                                                                                           DRAFT
are satisfactorily installed and in accordance         8-9.2   Arrival Activities at Off Loading
with the Transport Manual. Any agreed to                       Site
changes should be noted.                               When the heavy lift ship is safely at anchor,
8-8.9.2 During Transit                                 each asset and all cargo should be inspected
The seafastening and blocking should be in-            by Navy and contractor personnel as well as
spected daily by the OIC of the asset and the          the Independent Marine Surveyor. Any voy-
heavy lift ship’s Master, or more frequently if        age related damage should be recorded in a
rough weather is encountered during transit.           post transit report and made available to all
                                                       parties. The assets should be returned to the
8-8.9.3 Upon Arrival                                   float on conditions of loading prior to float
Upon arrival, the Heavy Lift Project Team              off. All tanks and cargos should be in the
should inspect blocking, spur shores, and              same condition as prior to float on. The OIC
seafastening and note any movement and/or              of the riding crew should prepare an updated
damage that may have occured.                          loading condition report. If this is not possi-
                                                       ble, as in the case of a damaged asset, e.g.,
8-9 Offloading Operations                              USS COLE, a deadweight survey of the asset
                                                       should be conducted.
The final phase of the operation is the offload        The heavy lift contractor should prepare to re-
of the vessels. While this is less complicated         move the seafasteners and any "Lift-On/Lift-
than the loading procedures, it is still a criti-      Off" deck cargo and prepare the ship and as-
cal phase of the operation and demands care-           sets for off loading. The seafasteners should
ful planning. Selection of an appropriate off-         not be removed until the lift ship is at the final
loading site will allow the operation to               off load site.
proceed without incident.
                                                       An off loading conference should be held as
8-9.1    Prior to Arrival at Destination
                                                       soon as practicable before or immediately fol-
All parties involved in the offload procedures         lowing arrival of the heavy lift ship. Again,
should be available at the discharge location          any unnecessary delays in releasing the heavy
at least two days prior to the arrival of the          lift ship can be very costly. All parties in-
heavy lift ship. A pre-arrival conference with         volved in the off loading operation, including
all parties represented should be held to re-          any local tug captains and pilots, should at-
view off-loading details. This meeting is in           tend this meeting. A detailed review of the off
advance of the conference held after the arriv-        loading procedure should be made and agreed
al of the heavy lift ship. Many of the off-load-       to by all parties.
ing details, including the off-loading site,
                                                       8-9.3   Off Loading
number of assist tugs, and a rough time line
can be determined or confirmed at this time.           The off loading operation proceeds in essen-
                                                       tially the reverse order of loading, with all the
Arrangements should be made for pier space
                                                       individuals performing the same tasks as at
for the assets after they are unloaded. These
                                                       loading. Any deviation from the approved
arrangements should be in place prior to the
                                                       Transport Manual should be agreed on by
arrival of the lift ship. If the assets are to tran-
                                                       representatives from all parties.
sit under their own power, sufficient manning
needs to be arranged. If the assets are to be          If the assets are to be towed to their final des-
towed, sufficient tug assets need to be provid-        tination, sufficient tugs to complete the un-
ed.                                                    loading and transport the vessels need to be


                                                                                                    8-61
DRAFT                               U.S. Navy Towing Manual


provided. Having an excess of tugs may be a
cheaper alternative to delaying the off-load-
ing procedure.




8-62
                                      U.S. Navy Towing Manual


                                                    mishap, the equipment often has failed be-
               Appendix A                           cause of an earlier human error or oversight
                                                    in design, manufacture, maintenance, or use
 SAFETY CONSIDERATIONS IN                           of the equipment.
         TOWING                                     Therefore, all personnel must be trained in
                                                    the use of, and have ready access to, appropri-
                                                    ate Navy technical manuals and other publi-
A-1 Introduction                                    cations to guide them in their operations.
                                                    Consequently, the approach to achieving
The purpose of this appendix is to supplement       safety in towing operations is to:
the specific safety precautions for towing op-
erations discussed in this manual with the             • Comply with existing Navy parent doc-
general safety precautions published in                  uments, such as the OPNAVINST
OPNAVINST 5100.19C, N45, 0579LD057                       5100.19 series for general policy and
1210, Navy Occupational Safety and Health                procedural guidelines, and refer to the
(NAVOSH) Program Manual for Forces                       pertinent technical manuals and Planned
Afloat (Ref. X).                                         Maintenance System (PMS) cards for
                                                         specific information on operation and
A-2 Scope and Applicability                              maintenance of commonly used gear
                                                         and equipment.
The safety information contained in this man-          • Comply with Navy technical manuals,
ual shall apply to all afloat Naval Commands             such as this volume on towing, and
that are involved in towing operations. It shall         manufacturers’ operating manuals for
also apply to United States Naval Ships                  more detailed information on special-
(USNS) of the Military Sealift Command                   ized operations. Use PMS cards and
(MSC) and its activities and the Marine                  data for information on gear and equip-
Corps, when embarked in the aforementioned               ment that are primarily or peculiarly as-
vessels and to the extent otherwise deter-               sociated with such specialized opera-
mined by the Commandant of the Marine                    tions.
Corps. This information, in combination with
the OPNAVINST 5100.19 series, comprises                • Encourage the use of systems safety
the Navy Occupational Safety and Health                  analyses, in which the overall system or
(NAVOSH) standards for towing operations                 activity of concern is planned and re-
as required by the OPNAVINST 5100.23C,                   viewed from the standpoint of safety.
Navy Occupational Safety and Health (NA-                 Factors such as the specific environ-
VOSH) Program (Ref. Y). For additional sal-              ment in which an operation is to be
vage safety information, consult the US Navy             conducted should be considered and ac-
Salvage Safety Manual 0910-LP-107-7600                   counted for in planning. Consequently,
(Ref. Z).                                                fewer omissions should occur and safe-
                                                         ty awareness among all personnel who
                                                         may be involved should increase. See
A-3 Basic Safety Philosophy
                                                         Section 3-4.1.5 and Table 3-2 for a dis-
                                                         cussion of factors of safety in the selec-
Many safety studies have indicated that hu-
                                                         tion of towing components.
man error is a common cause of mishaps.
Even though the failure of some item of             No list of safety precautions in towing can be
equipment may be listed as the “cause” of a         comprehensive without the principles of good


                                                                                               A-1
                                      U.S. Navy Towing Manual


seamanship. The precautions stated here and         tives or Instructions must be completed. Even
in the OPNAVINST 5100.19 series are basic           for missions that are repetitions of previous
and must be followed.                               tows, the preparation phase must be repeated
                                                    to ensure that nothing is overlooked. In both
Personnel involved in towing operations must
                                                    the preparation and operational phases of any
be thoroughly trained, disciplined, and
                                                    tow, it is essential that full and open commu-
equipped not only to perform routine duties,
                                                    nication exists between the preparing activity
but also to react appropriately to unusual or
                                                    and the towing vessel.
nonroutine situations. The officers and crew
of vessels involved in towing operations            A-4.1.3 Safety
should continuously conduct safety indoctri-
                                                    Safety is paramount in the preparation of in-
nation lectures and exercises aimed at reduc-       dividual Command Instructions and Towing
ing unsafe conditions or practices and at re-
                                                    Bills, as well as in the preparations for indi-
acting appropriately to unusual circumstances       vidual towing tasks.
through professional knowledge of their du-
ties and towing procedures.                         Appendix H includes checklists to help in the
                                                    operational planning and preparations for
A-4 Specific Safety Precautions                     tows. All hands must fully understand that
                                                    good planning and preparation for emergency
In addition to the safety precautions in the        situations are just as important for safe tow-
OPNAVINST 5100.19 series, many para-                ing as correct ship handling and good sea-
graphs within this manual also contain specif-      manship. Planning is not a simple paperwork
ic notes of safety-related information. Rather      drill. The preparation phase of a towing oper-
than repeating notes from these two sources,        ation demands the same knowledge and sea-
the following paragraphs discuss only the ap-       manship skill as the actual at-sea phase.
proaches that are recommended specifically          Past experience has amply demonstrated that,
for towing operations.                              from the very onset of the tow tasking, it is
A-4.1 Specific Approaches                           imperative that the plan for preparing the tow
A-4.1.1 General Specifications                      for the transit be thoroughly conducted and
                                                    reviewed before implementation. In some in-
The General Specifications for Ships of the         stances, such as ocean tows of complex units
United States Navy (Ref. D) mandates that           like dry docks, the plans and the tow may be
any ship that is likely to require towing, espe-    prepared by a civilian marine contractor and
cially emergency towing, should be equipped         supervised by the Supervisor of Shipbuilding
to “tow or be towed.” The equipment inven-          and Repair at an appropriate Navy facility.
tory should be such that in an emergency
nothing is required to be brought on board the      In a peacetime Navy (or in the early stages of
tow or fabricated on the tow. Each ship must        war) the availability and quality of “in-house”
be capable of receiving or rigging an emer-         expertise in the field of towing and tow prep-
gency towing rig designed so that the ship can      arations can vary widely. The towing unit
tow or be towed.                                    must therefore monitor the efforts of the ac-
                                                    tivity preparing the tow. The towing unit
A-4.1.2 Non-Emergency Towing
                                                    must make continuous inspections and take
For non-emergency situations (and for emer-         positive action immediately to correct identi-
gencies, to the extent that time permits) the       fied deficiencies. The towing unit Command-
preparation procedures outlined in this manu-       ing Officer or a representative should attend
al and in appropriate Type Command Direc-           any meeting held by the cognizant activity for


A-2
                                    U.S. Navy Towing Manual


the tow and the preparing activity and should     Few Navy tows will be exact duplicates of
make any comments or recommendations              earlier tows. Even though some tows may ap-
necessary.                                        pear to be duplicates, there will be differences
                                                  in weather, route, and configuration of the
A-4.1.4 Planning                                  towed vessel. Thus, the pre-tow planning and
Although this manual presents planning pro-       preparations must be conducted each time a
cedures in considerable detail, extreme care      towing task is undertaken to ensure a mini-
and judgment must be exercised. Blind de-         mum of oversights and mishaps.
pendence upon the results of routine calcula-     A-4.2 Contingency Planning
tion methods, especially computerized pro-
cedures, without careful cross-checking can       Contingency planning is very similar to oper-
lead to major errors and possibly extreme op-     ational planning, except that it concentrates
erational difficulties.                           on the aspects of being prepared to respond
                                                  to emergency conditions. Being prepared in-
Even a poor choice of location for conducting     cludes both knowing what to do and having
pre-tow preparations can lead to major prob-      the appropriate supplies and equipment avail-
lems. If available, the tow should be prepared    able to do it. The Navy “tow-and-be-towed”
at a full-service, easily accessible location     instructions, including individual ships’ bills
and then moved to a staging area once fully       and equipment, are one example of contin-
prepared and made ready for sea.                  gency planning.




                                                                                              A-3
           U.S. Navy Towing Manual




      This Page Intentionally Left Blank




A-4
                                      U.S. Navy Towing Manual


                                                    B-3 Strength
               Appendix B
                                                    Steel wire rope currently provides the stron-
      WIRE ROPE TOWLINES                            gest towing hawser for a given diameter and
                                                    is usually specified by the Navy as the pre-
                                                    ferred hawser for towing.

B-1 Introduction
                                                                       CAUTION

The towing hawser is the key element in the               Aramid fiber lines (Kevlar, Spectra)
                                                          have a similar strength to diameter
tug-tow connection. For Navy towing ships,                ratio as wire rope and offer a con-
the hawser is usually wire rope. It is especial-          siderable weight savings, but this
ly important to keep a wire rope hawser in ex-            light line provides no catenary and
cellent condition, to protect it against exces-           aramid fibers do not possess the
sive wear, and to inspect and lubricate it                stretch characteristics of polyester.
regularly.                                                Therefore, these lines are not well
                                                          suited for ocean towing.
To maintain a written reference of a wire rope
towline’s history, the Naval Sea Systems            Target sleds are virtually the only tows for
Command requires that all U.S. Navy and             which a synthetic fiber line hawser is current-
MSC vessels regularly engaged in towing op-         ly specified.
erations keep a Towing Hawser Log. Appen-           Wire rope strength varies with the type of
dix F includes instructions for keeping this        construction and material as well as with size.
log.                                                Consequently, it is important to be certain
                                                    that all wire ropes used in towing are of the
B-2 Traceability                                    proper construction, core, and required mate-
                                                    rial.
The ability to trace a rope’s history is an im-     B-3.1 Elongation (Stretch)
portant element in accident investigation, as
well as in general product improvement ef-                            WARNING
forts. Some of this information is maintained
                                                          Wire rope stretches under load
in the Towing Hawser Log (see Appendix F).                far less than most natural and
American made wire rope and some brands of                synthetic fiber lines and thus
foreign made rope can be identified by spe-               presents less danger to bystand-
cial core marker materials used as a part of, or          ers from loose ends “snapping
layered around, the core of the wire rope, as             back” if it fails under high loads.
well as by the metal tags and other informa-              The elongation under load is suf-
                                                          ficient, nonetheless, to be dan-
tion on the reel upon which the rope is deliv-            gerous. The recoil can be ex-
ered. Identification of manufacturing source              tremely violent and all personnel
through core markers is particularly useful in            should stay well away from any
cases where the color coding has not been ap-             potential recoil path.
plied to a strand. Additional information on a
specific domestic wire rope producer’s core         In addition to the above noted danger, the
color marking practices is available on re-         sudden release of tension can sometimes
quest from the manufacturer.                        cause a popped core or a “birdcage” in the



                                                                                                  B-1
      U.S. Navy Towing Manual




      Figure B-1. Bird Caging.




      Figure B-2. Popped Core.




B-2
                                    U.S. Navy Towing Manual


rope when a failure in the towline or its con-    inner layers can be lubricated at such oppor-
nections allows the rope to rebound from an       tune times as:
overload. These conditions also can result
                                                      • Overhauls
from operating a wire rope through an under-
sized sheave groove (see Figures B-1 and              • When the hawser is reversed, end-for-
B-2).                                                   end, on the drum
                                                      • When towing in good weather, at which
B-4 Maintenance, Cleaning, and                          time extra line may be run out to ex-
    Lubrication                                         pose the inner layers for lubrication.
                                                  The Navy procedures for wire rope lubrica-
Wire rope, like a machine, is made up of          tion are currently being modified. The most
many moving parts. The individual steel           recent guidance is contained in NAVSEA In-
wires slide independently and must be kept        terim MRC for ARS 50 Class Running Rig-
clean and protected against the effects of        ging (Ref. AA).
movement and pressure by adequate lubrica-
tion.                                             B-5 New Hawsers
Corrosion damage is also a danger. The exact
                                                  Wire rope for towing hawser is shipped in cut
loss of strength resulting from corrosion of
                                                  lengths on reels.
wire rope cannot be estimated. Washing the
tow hawser down with fresh water and lubri-       B-5.1 Unreeling
cating it during retrieval after each use can
help retard corrosion. This, however, is not a                       CAUTION
“cure-all” since the core remains saturated
                                                         Remove rope from the shipping
with salt water.
                                                         package very carefully. Improper un-
                                                         reeling can cause permanent dam-
Properly specified and procured wire rope is
                                                         age, such as kinks and hockles (see
lubricated during manufacture. Since the time
                                                         Figure B-3).
in storage may not be known, the towing ship
should clean and relubricate a new towing
hawser upon receipt. Relubrication will be re-    Unreeling wire rope requires careful and
quired, based on frequent inspection, and may     proper procedures. Mount the reel on a hori-
be required as often as after each use of the     zontal shaft supported high enough for the
hawser. Procedures for inspecting and lubri-      reel to clear the deck so the reel is free to ro-
cating wires are detailed in NSTM CH-613          tate. To begin the unreeling process, hold the
(Ref. F).                                         rope end and walk away from the reel as it
                                                  unwinds. Use a braking device to keep the
A pressure lubricator has been developed for      rope taut and prevent the reel from overrun-
wire rope and is the preferred method of lu-      ning the rope. This is particularly necessary
brication. Grease (MIL-G-18458) is currently      with powered reeling equipment.
specified. This product contains a corrosion      B-5.2 Reeling
preventive and can be thinned with solvents
such as JP5 or turbine oil 2190 (MIL-L-           When reeling a wire rope hawser from a reel
17331) for cold application.                      to a towing machine drum, it is best for the
                                                  rope to travel from the top of the reel to the
Take care that all sections, including dead       top of the drum, (see Figure B-4). This meth-
layers on the drum, are kept lubricated. These    od avoids putting a reverse bend into the rope


                                                                                                B-3
        U.S. Navy Towing Manual




      Figure B-3. Kinks and Hockles.




         Figure B-4. Re-reeling.



B-4
                                    U.S. Navy Towing Manual


as it is being installed. A reverse bend can      is first tied down to a strong point aft of the
make a rope less stable and, consequently,        drum. In the case of a towing machine or
more difficult to handle.                         winch, there is usually a strong point on the
B-5.3 Installing New Wire Rope                    fantail such as an H-bitt or a heavylift roller.
                                                  These devices are not intended to be pulled
                  CAUTION                         on in the forward direction, but they are built
                                                  for much heavier loads than they will be re-
     Rapid acceleration can cause                 quired to withstand while supporting a cable
     significant stress on a wire rope.
     Avoid such stress on the rope by
                                                  brake.
     accelerating gradually.
                                                  To install the wire, pass the bitter end through
                                                  the brake and onto the winch or open the
Wire rope should be installed on a towing
                                                  brake by removing the spring assemblies and
machine drum under a tension of at least five
                                                  the top plate. Place the wire to be loaded on
percent of its breaking strength. Each wrap
must be positioned tightly against the neigh-     the bottom plate of the brake and reinstall the
boring wrap. A tight fit will help prevent the    top plate and spring assemblies. Next, tighten
wire rope from becoming buried between            the spring assemblies with the clamp nuts un-
wraps when used under heavy loading. Bury-        til the proper tension is reached. Once the ca-
ing the wire between wraps is likely to result    ble brake has been properly adjusted, wind
in serious damage. Loose or poorly spaced         the rope onto the winch in a continuous man-
wires may cause movement in underlying            ner until all the wire is on the winch drum.
layers during towing. In practice, the wire
rope is initially installed on the towing ma-     Take care to keep the wraps tightly together.
chine drum under as high a tension as practi-     Wind the first layer slowly, using a heavy
cal.                                              maul or hammer to obtain a tight fit. Protect
                                                  the wire as necessary during any hammering
                    NOTE                          by using soft-faced hammers or wooden
                                                  blocks. Once the first layer is installed it
     For both smooth and grooved drums,
     the towing hawser must be wound on           should be retained as the foundation for sub-
     the drum under fairly high tension,          sequent layers and not disturbed during tow-
     approximately 5 percent of the               ing operations.
     breaking strength.
                                                  If a Wallis Brake is not available, or if the
Using stoppers to load the wire bight by bight    wire rope could not be initially installed un-
is one way to maintain tension, but it is cum-    der sufficient tension even with the brake, it
bersome and time consuming. During the            can be shackled to a bollard or a mooring
construction of the first four ARS-50 Class       buoy, payed off the drum, and then hauled in
ships, a cable brake called a Wallis Brake was    under the correct tension.
used to help install the wire rope towing haw-
sers (see Figure B-5). This cable brake is de-    When new wire ropes are put in service as
signed for the continuous loading of the wire     towing hawsers or pendants, record their
rope under tension.                               identification (see Section B-2 for Identifica-
NAVSEA 00C has detailed plans for con-            tion Markings) in the Towing Hawser Log
struction of a Wallis Brake. The Wallis Brake     (See Appendix F).


                                                                                              B-5
              U.S. Navy Towing Manual




                      NOTE

      Wallis Brake sized for a family of wire
      rope sizes. The approximate dimensions
      for a 2-inch Wallis Brake are shown.
      NAVSEA 00C has details for construction.




              Figure B-5. Wallis Brake.



B-6
                                     U.S. Navy Towing Manual


B-6 Stowing                                           • Evidence of heat.

When the towing hawser is removed from the         B-7.2 Specific Steps
drum, wind it neatly on a reel and store it in
                                                   Detailed steps for inspection and maintenance
an acid free, dry, protected location. Whenev-
er a wire rope towing hawser is to be stored,      of wire rope are specified in NSTM 613. The
lubricate it first with MIL-G-18458 grease         principal steps in wire rope inspection are:
(preferably with a pressure lubricator) and
                                                       a) Clean the rope by wire brushing and
then keep the outer layer lubricated with the
same grease throughout the storage period.               wiping with rags.

                                                       b) Inspect wire rope for rust, deteriora-
B-7 Inspection
                                                         tion, corrosion, wear or flattening, bro-
                                                         ken strands, and weakened splices.
                  CAUTION
                                                       c) Count number of broken or protruding
      In general, wear gloves when han-
      dling wire rope, except when it is                 wires in each wire rope lay length.
      moving under load. In this case,
      the gloves can get snagged and                   d) Measure wire rope diameter with ver-
      can drag the hands into danger.                    nier calipers.
      Wire rope should not be handled
      when it is moving under load.                Replace wire rope when one or more of the
                                                   following conditions exists:
B-7.1 General Criteria
                                                      • The nominal rope diameter is reduced
Inspect the rope thoroughly as it is being              by more than the amount shown in Fig-
wound after each use. Refer to Figure B-6 for
                                                        ure B-7 for the applicable size rope for
nomenclature of wire rope and Figure B-7 for
measuring guidelines.                                   measuring rope diameter

The inspection criteria for general usage run-        • Six wires are broken in one rope lay
ning rope are as follows:                               length or three wires are broken in one
                                                        strand lay length
   • Reduction of nominal rope diameter
     due to loss of core support, internal or         • One wire is broken within one rope lay
     external corrosion, or wear of individu-
                                                        length of any end fitting (cut wire and
     al outside wires
                                                        replace with new fitting)
   • Number of broken outside wires and
     degree of distribution or concentration          • The original diameter of outside indi-
     of broken wires                                    vidual wires is reduced by one-third
   • Corroded, pitted, or broken wires at end         • Pitting due to corrosion is evident
     connection
                                                      • Heat damage is evident
   • Corroded, cracked, bent, worn, or im-
     properly applied end connections                 • Kinking, crushing, or any other damage
   • Severe kinking, crushing, or distortion            resulting in distortion of the rope struc-
     of rope structure                                  ture is evident.


                                                                                              B-7
             U.S. Navy Towing Manual




      Figure B-6. Nomenclature of Wire Rope.




B-8
                               U.S. Navy Towing Manual




 Actual
Diam eter




                                   Correct                                Incorrect




                                                Maximum Allowable
                  Rope Diameter
                                             Nominal Diameter Reduction
                    (Inches)
                                                      (Inches)

            5/16 and smaller                             1/64

            3/8 to 1/2                                   1/32

            9/16 to 3/4                                  3/64

            7/8 to 11/8                                  1/16

            11/4 to 11/2                                 3/32

            19/16 to 2                                   1/8

            2 to 2-1/2                                   5/32




                           Figure B-7. Measuring Wire Rope.




                                                                                      B-9
                                           U.S. Navy Towing Manual



                      Table B-1. Wire Hawsers Carried by U.S. Navy Towing Ships.

                                                                        Wire Rope
                       Ship Class                                     Hawser Diameter
                                                                         by Length

                      T-ATF 166*                                     2¼″ x 2500′ 6 x 37
                        ARS 50                                       2¼″ x 3000′ 6 X 37
 * T-ATFs are being refitted with wire core rope when hawsers are due for replacement.


B-8 Special Precautions                                  It is important to maintain minimum and even-
                                                         ly distributed wear. Pay special attention to
                     WARNING                             possible chafing points where the wire rope
                                                         passes over chocks, bitts, stern rollers, and so
       Proper maintenance is extremely im-               forth. Even though no particular wear may be
       portant for wire rope used in critical or         noticed, it is advisable to freshen the nip at
       potentially dangerous applications
                                                         least once per watch to change the location of
       such as towing.
                                                         possible wear.

Wire rope must be properly maintained when               B-9 Wire Rope Hawsers for Navy Tow
used in critical or potentially dangerous situa-             Ships
tions. It should not be subjected to any of the
following common abuses:                                 Navy towing hawsers are of two types:
   • Chafing                                                • 21/4-inch diameter, fiber core
   • Impact loads or rapidly changing loads                 • 21/4-inch diameter, Independent Wire
   • Incorrect size of groove on drum or                      Rope Core (IWRC).
     sheave                                              Table B-1 lists the wire hawsers carried by
   • Drum or sheave grooves that have be-                each Navy towing ship class. T-ATF-166 class
     come rough or corrugated through wear               vessels are replacing fiber core wire with
                                                         IWRC wire during normal replacement cycles.
   • Inadequate diameter of drum or sheave               Table B-2 provides the strength and weight
   • Improper winding on drum                            per foot of 6 x 37 class IPS marine ropes.
   • Improper or insufficient lubrication                B-10 Wire Rope Terminations
   • Exposure to corrosive fluids
                                                         Wire rope towing hawsers are terminated
   • Exposure to excess heat or electric arc-            with a closed, poured socket. The dimensions
     ing                                                 and weights of four common sizes of open
   • Lack of protection against moisture and             and closed Spelter sockets are shown in Fig-
     salt water                                          ure B-8. The strength of these sockets, when
                                                         properly made, exceeds the strength of the
   • Kinks or hockles.
                                                         wire rope for which they are designed. The
If wire rope is struck by lightning, inspect it          dimensions are given in detail to assist in se-
and consider replacing it                                lecting the appropriate mating jewelry.


B-10
                                            U.S. Navy Towing Manual



  Table B-2. Nominal Breaking Strength of Wire Rope 6x37 Class, Hot-Dipped Galvanized.
                     2                                                                                3
           Fiber Core                                                       Independent Wire Rope Core
                                            Nominal
                                            Diameter
Weight in Air      Improved Plow            (inches)           Weight in Air        Improved Plow        Extra Improved
  (lbs/ft)*          Steel (lbs)**                               (lbs/ft)             Steel (lbs)**     Plow Steel (lbs)**

 0.11                      4,932          1/4                     0.12                    5,292                 6,100
 0.16                      7,668          5/16                    0.18                    8,240                 9,500
 0.24                     10,980          3/8                     0.26                   11,800                13,600
 0.32                     14,886          7/16                    0.35                   16,000                18,400

 0.42                     19,260          1/2                     0.46                   20,700                24,000
 0.53                     24,300          9/16                    0.59                   26,100                30,250
 0.66                     30,060          5/8                     0.72                   32,200                37,100
 0.95                     42,840          3/4                     1.04                   48,100                53,000

 1.29                     57,960          7/8                     1.42                   62,300                71,100
 1.68                     75,240          1                       1.85                   80,800                93,000
 2.13                     94,680          1 1/8                   2.34                  101,700               117,000
 2.63                    116,280          1 1/4                   2.89                  125,000               144,000

 3.18                    139,860          1 3/8                   3.50                  150,300               172,800
 3.78                    165,600          1 1/2                   4.16                  178,000               205,200
 4.44                    192,600          1 5/8                   4.86                  207,000               237,600
 5.15                    223,200          1 3/4                   5.67                  239,400               275,400

 5.91                    253,800          1 7/8                   6.50                  273,600               313,200
 6.72                    288,000          2                       7.39                  309,600               356,400
 7.59                    322,000          2 1/8                   8.35                  345,600               397,800
 8.51                    360,000          2 1/4                   9.36                  387,000               444,600

 9.48                    339,600          2 3/8                 10.4                    430,200               493,200
10.5                     439,200          2 1/2                 11.6                    471,600               543,600
11.6                     482,400          2 5/8                 12.8                    518,400               595,800
12.7                     525,600          2 3/4                 14.0                    565,200               649,800

13.9                     570,600          2 7/8                 15.3                    613,800               705,600
15.1                     619,200          3                     16.6                    666,000               765,000
16.4                     687,800          3 1/8                 18.0                    718,200               824,400
17.7                     718,200          3 1/4                 19.5                    772,200               885,600

                                          3 1/8                 21.0                    826,200               952,200
                                          3 1/2                 22.7                    883,200             1,015,206
                                          3 5/8                 24.3                    941,400             1,083,600
                                          3 3/4                 26.0                  1,002,600             1,153,800
  * Weights are given in air. To obtain net weight in water, multiply air weights by 0.87.
  ** Nominal breaking strength in pounds.
  NOTES:
  1. All data shown is for hot-dipped galvanized wire. Bright (uncoated) wire strengths are 10% higher and are listed
     in the same tables in Notes (2) and (3). Drawn galvanized wire rope has the same strength as bright wire.
  2. Data for fiber core wire rope is taken from RR-W-410D, Table X.
  3. Data for Improved Plow Steel IWRC wire rope is taken from RR-W-410D, Table XI. Data for Extra Improved
     Plow Steel IWRC galvanized wire rope is taken from RR-W-410D, Table XII.




                                                                                                                        B-11
                                        U.S. Navy Towing Manual


                                                      Manuals
                                                          Naval Ship’s Technical Manual S9086-
                   WARNING                                UU-STM-010, Chapter 613, “Wire and
        When using a termination of less                  Fiber Rope and Rigging,” S9086-UU-
        than 100 percent efficiency, the                  STM-010/CH613, Second Revision, 1
        base strength to which the factors                May 1995.(Ref. F)
        of safety are applied must be ad-
        justed accordingly.                           Copies of Military and Federal Specifications
                                                      and Standards may be obtained from the fol-
See Table B-3 and NSTM 613 for efficiency             lowing facility:
of wire rope terminations.                            Commanding Officer
Poured socket wire terminations are not tested        Naval Publications and Forms Center
because they are presumed to be stronger than         (NPFC)
the safe working strength of the wire. Instead,       5801 Tabor Avenue
reliance is placed on the skill of the operator,      Philadelphia, PA 19120
who is initially qualified and maintains that         Tel: (215) 697-2179
qualification as described in NSTM 613. Fac-
tors of safety listed in Table 3-2 and discussed      B-12 Requirements
in Appendix M are applicable to the nominal
breaking strength of new wire. If, under an           B-12.1 Wire Rope Characteristics
emergency towing situation, a termination oth-
er than a poured socket is used, the reduced ef-      Independent wire rope core may be substitut-
ficiency of the termination must be included in       ed for fiber core and Extra Improved Plow
the allowable load calculations. Furthermore,         Steel (EIPS) for Improved Plow Steel (IPS) in
if the reason for alternate termination is to re-     any of the cases below if deemed prudent by
place a failed termination or a parted wire, it       the purchasing activity. The information be-
must be assumed that the balance of the haw-          low may reflect the original configuration,
ser has been overstressed as well. If it is neces-    but availability at the time of replacement
sary to continue using the questionable hawser,       may dictate an IWRC.
doubling the factor of safety against the low-        B-12.2 Wire Towing Hawsers for
ered system strength would be appropriate.                   T-ATF 166 Class Ships

B-11 Wire Rope Procurement                            Wire rope shall be 2 1/4-inch diameter cut to
     Requirements                                     2500-foot lengths (see 3-4.1.3 for tolerances
                                                      in lengths), IPS (or EIPS), drawn galvanized,
This section discusses the applicable specifi-        preformed, regular (R.H.) lay, polypropylene
cations for the purposes of procuring wire            fiber core (or Independent Wire Rope Core
hawsers for ARS 50 and T-ATF 166 class                (IWRC)), Type I, Class 3, Construction 6, 6 x
vessels. For detailed information, consult the        37 (Warrington Seale) IAW Specification
below list of documents.                              RRW410. Documentation of all test results
                                                      (as required by RRW410) from each Master
Federal Specifications                                Reel used in fabrication of wire lengths shall
                                                      be submitted for the production assemblies
       RRW410 Wire Rope and Strand
                                                      (one data set included with the report in Sec-
       RS550 Sockets, Wire Rope                       tion 4-2.2)


B-12
                                           U.S. Navy Towing Manual



                            Table B-3. Efficiency of Wire Rope Terminations.

                            Type Terminations                                            Efficiency*

   Poured Spelter Socket                                                                      100 percent
   Wire Rope Clips (See Table 4-1 for number)                                                  80 percent
   Swaged Socket**                                                                            100 percent
   Eye splice (hand-spliced)
      2 1/4″ and larger wire                                                                   70 percent
      1 5/8″ to 2″ wire                                                                        75 percent
      1 1/8″ to 1½″ wire                                                                       80 percent
      7/8″ to 1″ wire                                                                          80 percent
   Flemish Eye (“Molly Hogan”) (with sleeve and thimble)                                       90 percent
       * Efficiency is the strength of the termination divided by the nominal breaking strength of the wire.
       ** Not recommended for fiber core ropes.


Each of the 2500-foot lengths of 2-1/4-inch                  be submitted for the production assemblies
wire rope shall have a closed zinc-poured                    (one data set included with the report in Sec-
socket on one end and a permanent seizing on                 tion 4-2.2)
the other end (See Section B-13).
                                                             Each of the 3000-foot lengths of 2 1/4-inch
Wire rope shall be wound on reels, closed                    wire rope shall have a closed zinc-poured
socket first. Reel drums shall be modified as                socket on one end and a permanent seizing on
required to allow the closed socket to be in-                the other end (See Section B-13).
serted into the drum and held so wire can be
                                                             Wire rope shall be wound on reels, closed
uniformly wound and tightly secured. Pres-
ence of the closed socket must be verifiable                 socket first. Reel drums shall be modified as
by visual examination without disturbing the                 required to allow the closed socket to be in-
stowage of wire on the reel. Marking for ship-               serted into the drum and held so wire can be
ment and storage shall be in accordance with                 uniformly wound and tightly secured. Pres-
best commercial practices. Each reel shall be                ence of the closed socket must be verifiable
clearly marked on each side with the diameter                by visual examination without disturbing the
and length of wire in a three-inch size letters              stowage of wire on the reel. Marking for ship-
as follows: “2 1/4-in x 2500-ft w/closed sock-               ment and storage shall be in accordance with
et termination.”                                             best commercial practices. Each reel shall be
                                                             clearly marked on each side with the diameter
B-12.3 2-1/4-Inch Towing Hawsers for                         and length of wire in three-inch size letters as
       ARS-50 Class Ships                                    follows: “2 1/4-in x 3000-ft w/closed socket
Wire rope shall be 2 1/4-inch diameter cut in-               termination.”
to a 3000-foot length, EIPS, drawn galva-
nized, preformed, regular (R.H.) lay, IWRC,                  B-13 Sockets
Type I, Class 3, Construction 6, 6 x 37 (War-
rington Seale) procured IAW Specification                    Each towing hawser shall have a closed zinc-
RRW410. Documentation of all test results                    poured socket on one end and a permanent
(as required by RRW410) from each Master                     seizing on the other end. Closed sockets shall
Reel used in fabrication of wire lengths shall               be Type B, procured IAW Specification

                                                                                                               B-13
                                                      U.S. Navy Towing Manual




                                                                 DIMENSION IN INCHES
             WIRE                                                                                                       WEIGHT
             ROPE
              DIAM              A             B            C              D            F          J      K       L      POUNDS
            INCHES                                                                                                       EACH

              1 5/8          15/1/8        2 1/8        5 3/4          3 1/4        1 3/4      6 1/2   2 3/4   6 1/2      36

            2 - 2 1/8        19 1/2     2 7/18          7 5/8       3 25/32         2 1/4      8 1/2   3 1/4   8 9/16     80

          2 1/4 - 2 3/8      21 1/8        2 5/8        8 1/2       4 9/32          2 1/2         9    3 5/8   9 1/2      105

          2 1/2 - 2 5/8      23 1/2        3 1/8        9 1/2          5 1/2        2 7/8      9 3/4     4     10 5/8     140




                                                                DIMENSION IN INCHES
         WIRE                                                                                                           WEIGHT
         ROPE                                                                                                           POUNDS
          DIAM              A          C            D              L            M             N         O       P        EACH
        INCHES

          1 5/8           16 1/4       3            3             6 1/2        5 3/4        1 5/16     6 5/8   1/2        55

         2 - 2 1/8        21 1/2       4           3 3/4           9            7           1 13/16    8 3/4   1/2       125

       2 1/4 - 2 3/8      23 1/2      4 1/2        4 1/4           10          7 3/4         2 1/8      10     1/2       165

       2 1/2 - 2 5/8      25 1/2       5           4 3/4         10 3/4        8 1/2         2 3/8      11     1/2       252




                          Figure B-8. Poured Sockets FED Spec. RR-S-550D Amendment 1.




B-14
                                     U.S. Navy Towing Manual


RRS550. Documentation of results of tests          or minus five feet from the center of socket
required by RRS550 shall be delivered with         eye to the bare end of the wire rope.
each wire rope assembly. Closed zinc-poured
sockets shall be attached to the wire in accor-    B-14 Lubrication
dance with the NSTM, CH-613 (Ref. F).
Testing and proof of personnel qualifications      All wire towing hawsers shall be lubricated
shall be as required by the Naval Ships Tech-      with MIL-G-18458 grease in accordance with
nical Manual. A report of tests and personnel      the NSTM CH-613 (Ref. F) prior to being
qualification documents shall be provided          placed on the towing machine drum. The use
with the wire rope assembly.                       of a pressure lubricator is preferable when
                                                   one is available.
Tolerances on 2 1/4-inch wire rope lengths
after sockets have been attached shall be plus




                                                                                          B-15
            U.S. Navy Towing Manual




       This Page Intentionally Left Blank




B-16
                                      U.S. Navy Towing Manual


                                                    in the Towing Hawser Log. American-made
               Appendix C                           fiber line and some brands of foreign-made
                                                    rope can be identified by special marker tapes
      SYNTHETIC FIBER LINE                          inserted into the fiber lines, special-colored
           TOWLINES                                 monofilaments and metal tags, and other data
                                                    on the reel upon which the line is delivered.
                                                    Identification of manufacturing source
                                                    through the marker coding is particularly use-
C-1 Introduction
                                                    ful in cases where the reel markings have
                                                    been lost. Additional information on a specif-
The material presented in this appendix does
                                                    ic domestic rope producer’s identification
not supersede any Fleet or NAVSEA direc-
                                                    marking practices is available on request
tives on the operational use or care of synthet-
                                                    from the Cordage Institute, Suite 115, 350
ic towlines. The use of single- and double-
                                                    Lincoln Street, Hingham, MA 02043. Tele-
braided polyester is approved for all routine
                                                    phone (617) 749-1016.
and emergency towing applications. Nylon
line is only approved for operations with craft
                                                    C-3 Strength and Lifetime
of less than 600 tons displacements, or other
unique or special tows as approved by
NAVSEA on a case-by-case basis.                                       WARNING

Existing nylon line should be replaced on a               The failure of synthetic fiber
size for size basis with double or single-braid-          lines under high tension loads
                                                          can be extremely dangerous.
ed polyester. This includes emergency tow
                                                          Synthetic lines, particularly poly-
and be towed hawsers.                                     e ste r an d ny lo n , re ta in h igh
Fiber lines, either natural or synthetic, can be          amounts of energy when under
                                                          tension. These lines will have se-
found serving two functions in towline sys-               vere snapback if they fail under
tems. In some systems the main towing haw-                loa d. Pers on n el sh o uld sta y
ser is made of fiber line. In other systems the           clear of areas through which the
hawser is wire rope and fiber lines are used as           end of a failed line may whip.
springs to provide relief from dynamic ten-
sion loads. In both uses, the fiber line should     C-3.1 General
be kept in excellent condition, protected
against wear, and inspected regularly.              Most synthetic fiber lines are stronger than
                                                    natural fiber (manila) lines, and they usually
When fiber line is used as the main towing          have longer lifetimes because of their resis-
hawser or as a spring, a written record of its      tance to rot and other forms of environmental
history is required by the Naval Sea Systems        deterioration.
Command in the form of the Towing Hawser
Log (see Appendix F).                               C-3.2 Specific
                                                    The primary type of fiber line currently used
C-2 Traceability                                    by the Navy for towing is polyester. The use
                                                    of nylon in towing is currently restricted in
The ability to trace a line’s history is an im-     towing applications. Polypropylene is used in
portant element in accident investigation as        some applications but does not have the supe-
well as in general product-improvement ef-          rior characteristics of polyester or nylon. Ta-
forts. Some of this information is maintained       ble C-1 presents a qualitative summary of


                                                                                                  C-1
                                             U.S. Navy Towing Manual



                                       Table C-1. Fiber Comparisons.
                                                       2              2
                                             1   Cyclic       Bending       Abrasion       Heat
           Fiber Type             Strength                                                                  Creep
                                                 Fatigues      Fatigue     Resistance    Resistance

  Nylon (dry)                        VG            VG             G            E             G               G

  Nylon (wet)                         G             F             F            F             —               G

  Polyester (dry)                    VG            VG            VG            VG            G               VG

  Polyester (wet)                    VG            VG             G            G             —               VG

  Polypropylene (dry)                 F             F             P            P             P               F

  Polypropylene (wet)                 F             F             P            F             —               F

                        E = Excellent, VG = Very Good, G = Good, F = Fair, P = Poor

        NOTES:
        1. Tensioned between two limits without bending.
        2. Usually running over pulleys. Some line wears out before failing from fatigue because of abra-
           sion.


pertinent characteristics of the three types of              C-4 Elongation
fiber lines.
                                                             The elongation or stretch of fiber line under
As one may note from Table C-1, nylon’s wa-
                                                             tension has both advantages and disadvantag-
ter-absorption characteristic changes its com-
                                                             es. Elongation tends to greatly reduce dynam-
parative rating from best to intermediate in
                                                             ic loads in the towline such as shock loads
nearly every category. Consequently, the Na-
                                                             and wave-induced loads. Unfortunately, elon-
vy has phased out the use of nylon in favor of
                                                             gation also stores a great deal of energy in
polyester. Where springs are required in tow-
                                                             ropes under tension and the release of this en-
line systems, polyester fiber will be used.
                                                             ergy when a rope fails causes a very danger-
Polypropylene will also continue to be used
                                                             ous whipping or “snap back” of the line. The
for certain purposes because it is the only one
                                                             stored energy, and potential danger, is much
of the three fiber lines that floats.
                                                             greater in the case of synthetic lines than for
The Navy also employs synthetic lines in                     wire rope under the same load. For this rea-
some of its lifting operations. These applica-               son, extreme caution is required when work-
tions demand lightweight, high strength,                     ing near fiber lines that are under load. Under
small diameter lines in very long lengths. Ar-               heavy tension loads, nylon line can snap back
amid fibers such as Kevlar, Spectra, and Vec-                at speeds up to 700 feet per second (500
tran are well suited for this need. These types              m.p.h.). Braided fiber lines tend to stretch
of fiber are not approved for Navy towing,                   about one-half to two-thirds as much as plait-
however. They have extremely low elonga-                     ed or stranded ropes of the same size.
tion and, because of their light weight, do not
provide the catenary of wire rope. These fi-                 C-5 Maintenance and Cleaning
bers, therefore, do not provide the same ex-
treme tension mitigation as the other hawser                 Although fiber lines are not subject to corro-
types.                                                       sion as wire ropes are, they still require care-



C-2
                                       U.S. Navy Towing Manual


ful maintenance and cleaning. If the line be-            • Drum or sheave grooves that have be-
comes oily or greasy, scrub it with fresh                  come rough or corrugated through wear
water and a paste-like mixture of granulated
                                                         • Inadequate radius on fairlead or stern
soap. For heavy accumulations of oil and
                                                           roller
grease, scrub the line with a solvent such as
mineral spirits, then rinse it with a solution of        • Rough or abrasive surfaces on fairlead
soap and fresh water.                                      or stern roller
The three different synthetic fibers show dif-           • Improper winding on drum
ferent responses to various chemicals. In                • Exposure to excessive heat
brief:
                                                         • Kinks or hockles.
    • Nylon weakens if exposed to acids, par-
      ticularly mineral acids. Its resistance to
                                                     C-6 Stowing
      alkalis is good at normal temperatures.
    • Polyester line will deteriorate with ex-       Stow synthetic line away from strong sun-
      posure to hot, strong alkali solutions. It     light, heat, and strong chemicals, and cover it
      is particularly vulnerable to very strong      with tarpaulins. If the line becomes iced over,
      acid solutions; therefore, even diluted        thaw it carefully and drain it before stowing.
      acid solutions should not be allowed to        If feasible, store the line on appropriately
      dry on the rope.                               treated wooden dunnage. Nylon is susceptible
    • Polypropylene is resistant to both acids       to a rapid reduction in strength when exposed
      and alkalis at normal temperatures, but        to rust; make sure that it is not exposed to
      is affected by some organic solvents           rust-prone bare steel surfaces.
      such as xylene and metacresol and by
      coal tar and paint-stripping compounds.        C-7 Uncoiling or Unreeling New
      These types of chemicals are most like-            Hawsers
      ly to be found in the paint locker in
      thinners and cleaning compounds.               Synthetic line is shipped in cut lengths, either
                                                     in coils or on wooden reels. It must be uncoiled
All synthetics are weakened by exposure to           or unreeled very carefully to avoid abrasion
strong sunlight and should therefore be stored       and permanent damage to the fibers. Looping
out of the sun. Polyester has the best resis-        the line over the head of the reel or pulling the
tance to ultraviolet rays.                           line off a coil while it is lying on the deck may
To extend the life of synthetic line, maintain       create kinks or hockles in the line. Never allow
minimum and evenly distributed wear. Pay             synthetic line to drag over rough surfaces since
special attention to possible chafing points         this will tend to abrade and cut the outer fibers.
where the line passes over chocks, bitts, stern
rollers, and so forth. Even though no particu-                           CAUTION
lar wear may be noticed, it is advisable to
                                                            A common method of uncoiling
freshen the nip at least once per watch to
                                                            wire rope by rolling the coil along
change the location of possible internal wear.              the deck is not recommended for fi-
Do not subject fiber lines to any of these other            ber lines because of the potential
                                                            for abrading or cutting the outer fi-
common abuses:
                                                            bers, and also because the coil will
    • Incorrect size of groove on drum or                   collapse when the bands are re-
      sheave                                                moved.



                                                                                                    C-3
                                       U.S. Navy Towing Manual


Synthetic lines are unreeled the same way            Keep in mind that no matter what has weak-
that wire ropes are unreeled (see Section            ened the line, the effect of the same injury
B-5).                                                will be more serious on a small line than a
                                                     large line. Therefore, always consider the re-
C-8 Breaking in New Hawsers                          lationship of the surface area of the line to its
                                                     cross section.
                   CAUTION                           Examining the line about one foot at a time is
                                                     usually practical. Turn the line to reveal all
       New synthetic hawsers should not
                                                     sides before continuing. At the same inter-
       be subjected to heavy strain prior
       to breaking them in. Limit the tow-           vals, untwist the rope slightly to examine be-
       ing loads applied to a new hawser             tween the strands of three-strand and plaited
       until it has been cycled up to its            rope.
       working load.
                                                     Synthetic lines should be inspected after each
                                                     use. Look for broken fibers in the outer layer
NSTM CH-613 (Ref. F) suggests that a syn-            and for discoloration or appearances of melt-
thetic hawser is adequately “broken in” after        ing. When examining between the strands,
five cycles of loading/unloading up to its           look for these same evidences of wear and
working load or to within 20 percent of              look also for any appearance of a powdery
breaking strength, whichever is less. This           substance between the strands. Broken outer
works the construction stiffness out of the          fibers may indicate that the line has been
line. When new lines are strained, they some-        dragged over sharp or rough surfaces. Discol-
times produce a sharp crackling sound. This          oration or melting may indicate excessive
is the result of readjustment of the line’s          frictional heat from either dynamic loads or
strands to stretching and should not be cause        from rubbing over smooth surfaces. Internal
for alarm.                                           wear, sometimes indicated as a fuzzed or
                                                     fused condition between strands, may indi-
It is not always possible to get new line to lay
                                                     cate fatigue damage from repeated or cyclic
flat due to turns set into the line during stor-
                                                     loads and overloading.
age on a reel. Never tow with a synthetic
hawser just to get the hockles or kinks out.         If the examination raises any doubts about the
Stream the line, controlling its payout with a       safety of the line, discard it. Again, keep in
capstan, until the bitter end is reached. Re-        mind that the effects of wear and mechanical
trieve it with the aid of the capstan and it will    damage are relatively greater on smaller lines
then lay flat as the excess turns will run out of    which, therefore, require more stringent stan-
the line as it is being hauled in. The ship          dards of acceptance.
should be stopped during retrieval of the line.
                                                     The following section on types of wear
When a new line is put into service as a tow-        should be helpful during the inspection of
ing hawser or spring, its identification infor-      synthetic lines.
mation (see Section C-2) should be recorded
in the Towing Hawser Log (see Appendix F).           C-10 Types of Wear or Damage

C-9 Inspection                                       The usual types of wear exhibited by synthet-
                                                     ic lines are as follows:
Regular inspection is essential to ensure that       • General external wear. External wear due
synthetic lines remain serviceable and safe.           to dragging over rough surfaces causes sur-


C-4
                                     U.S. Navy Towing Manual


  face chafing. In the extreme, the strands be-      occasionally wetted, but generally exposed
  come so worn that their outer faces are flat-      to the cold air.
  tened and the outer yarns are severed. In
  ordinary use some disarrangement or                                WARNING
  breakage of the fibers on the outside of the
  line is unavoidable and harmless if not ex-            Surging of synthetic line under
                                                         tension can cause sufficient fric-
  tensive. Generally, nylon and polyester fil-
                                                         tional heat at the contact surfac-
  ament lines have very good abrasion resis-             es to melt the surface of the line.
  tance.                                                 The melting point of polypropy-
                                                         lene line, for instance, is 320°F
• Local abrasion. Local abrasion, as distinct
                                                         to 340°F, while the softening
  from general wear, is caused by the passage            point is around 300°F. Compara-
  of the line over sharp edges while under               ble temperatures for polyester
  tension and may cause serious loss of                  are only moderately higher.
  strength, especially if accompanied by                 These temperatures are quite
  fused areas signifying high heat generated             quickly produced when a line is
                                                         surged on a winch or capstan.
  by rope surges under heavy load. Slight
  damage to the outer fibers and an occasion-
  al torn yarn may be considered harmless,         • Repeated loading. Although polyester fila-
  but serious reduction in the cross-sectional       ment line resists damage from repeated
  area of one strand or somewhat less serious        loading, permanent elongation will occur
  damage to more than one strand should              over time in heavily loaded ropes. If the
  warrant rejection. When such damage is             original length of the rope is known exact-
  noticed, preventive measures should be             ly, remeasuring under exactly the same
  taken. Typical protective steps are to             conditions indicates the total extension of
  smooth and round off all rough or sharp ar-        the rope. This method, however, may not
  eas on the surface that are chafing the line       reveal severe local permanent elongation
  and apply chafing gear such as rubber or           that may cause breaking on subsequent
  plastic sleeves or cloth material secured by       loading. Measuring the distance between
  small stuff around the line.                       regularly spaced indelible markers on the
                                                     rope can help reveal this problem.
• Cuts and contusions. Cuts and contusions
  are caused by rough or sharp surfaces. Such      • Heat. Heat may, in extreme cases, cause
  careless use may cause internal as well as         melting. Any signs of melting should obvi-
  external damage. This may be indicated by          ously warrant rejection, but a line may be
  local rupturing or loosening of the yarns or       damaged by heat without any such obvious
  strands.                                           warning. The best safeguard is proper care
                                                     and storage. A synthetic line should never
• Internal wear. Internal wear may be indi-
                                                     be dried in front of a fire or stored near a
  cated by excessive looseness of the strands
                                                     stove or other source of heat.
  and yarns or the presence of fuzzed or
  fused internal areas. It is caused by repeat-    • Strong sunlight. Strong sunlight causes
  ed flexing of the line and by particles of         weakening of synthetic fibers, but is un-
  grit that have been picked up. Ice crystals        likely to penetrate beneath the surface. Un-
  can also cause internal wear. This condition       necessary exposure should be avoided,
  results from towing in very cold weather           however. Solar degradation should be
  and will most likely occur at the stern of         checked by rubbing the surface of the line
  the tug and at the tow where the hawser is         with the thumb nail. If degradation has tak-


                                                                                               C-5
                                                   U.S. Navy Towing Manual


  en place, the surface material will come off                               ferred. This is particularly important on
  as a powder. In addition, the surface of the                               the tow, as the conditions of the tow’s
  line will feel dry, harsh and resinous.                                    chocks, bitts, etc. may be unknown and
                                                                             contact with these may cause extensive
C-11 Special Precautions                                                     chafing. Barges usually have very
                                                                             rough chocks caused by previous repet-
                                                                             itive use of wire rope or chain. Special
                        WARNING
                                                                             attention should be paid to where the
         Listed below are three precau-                                      hawser crosses the stern of the tug.
         tions to be considered when us-
         ing synthetic tow hawsers. They                                   • Since their coefficient of friction is be-
         should be taken as warnings as                                      low that of manila, synthetic lines may
         they are critical to safety of per-
                                                                             slip when eased out under heavy loads,
         sonnel.
                                                                             causing personal injury. Make sure that
                                                                             personnel are thoroughly instructed in
      • When using heavily loaded synthetic                                  these lines’ peculiarities. Take two or
        lines, the major precaution to be taken                              three turns on a bitt before you “figure
        is to be constantly alert to the potential                           8” the line; this provides closer control.
        danger of line “snap back” during fail-                              Stand well clear of the bitts.
        ure. Personnel must remain clear of the
        areas through which the ends of a failed                        C-12 Fiber Rope Characteristics
        line may whip or snap.

      • To avoid damage from rough surfaces,                            • Table C-2 provides the strength and weight
        synthetic line should not be used in ar-                          of several sizes and types of fiber ropes.
        eas where chafing potential is high. Use                          See NTSM CH-613 (Ref. F) for additional
        of a wire rope or chain pendant is pre-                           data on fiber lines.

                             Table C-2. Synthetic and Natural Line Characteristics.
                          Dry Nylon Double-Braid               Polyester Double-Braid          Polyester Single Braided 12-Strand
                              (MIL-R-24050 D)                     (MIL-R-24667 A)                        (MIL-R-24750)
  Size (Inches)
                        BS (lbs)          WT/100 ft          BS (lbs)          WT/100 ft          BS (lbs)           WT/100 ft

              3            27,825               24.3            29,480                  31.9         25,600                    30
              5            78,110               67.6            74,000                    84         67,200                    78
              6           109,675               97.1           105,000                   128         96,000                   112
              7           149,800                132           133,600                   161        131,200                   153
              8           192,600                173           180,000                   220        172,000                   200
              9           243,000                219           232,000                   287        215,200                   253
             10           284,840                270           277,000                   337        264,800                   312
             11           351,000                327           335,000                   419        319,200                   378
             12           415,800                389           396,150                   510        376,800                   449
             13           475,200                450           446,500                   576        440,800                   527
             14           548,640                524           500,650                   646        508,800                   612


                            BS = Breaking Strength                           WT = Weight

       Strength shown for nylon is for new dry nylon. Nylon wet strength is about 15% less. Multiply figures listed by 0.85
       to obtain the new breaking strength of wet nylon.



C-6
                                     U.S. Navy Towing Manual


                                                   link” chain for the sake of simplicity. Navy
               Appendix D                          stud link chain is slightly stronger than stan-
                                                   dard Type 1 die lock chain; they may be used
       CHAINS AND SAFETY                           interchangeably.
           SHACKLES                                Until recently, commercial “DiLok” chain
                                                   was made by one manufacturer, Baldt. It is
                                                   slightly stronger and heavier than Type 1
D-1 Introduction                                   standard Navy die lock chain. Section D-11
                                                   discusses the strengths of the various chains
Chain is an important component in the con-        that may be used in towing.
nection between the towed vessel and the tug.
It usually appears in the form of pendants or      D-2 Traceability and Marking
bridles at the towed-vessel end of the towline.
The chain components serve one or more of          D-2.1 Traceability
the following purposes:
                                                   The ability to trace a chain’s history is an im-
   • A chafing-resistant strong terminal con-      portant element in accident investigation as
     nection to the towed vessel                   well as in general product-improvement ef-
   • An equalizing device (bridle) to share        forts. For identification, a corrosion-resistant
     the towing load between two strong            metal tag is attached to the end link at each
     points located port and starboard of the      end of each shot or length of Navy chain. In-
     towed vessel’s bow (or stern)                 cluded among data plainly marked on the tag
                                                   is a manufacturer’s serial number, which per-
   • A means of absorbing dynamic loads in         mits tracing the chain back to its manufactur-
     the towline, by virtue of its weight,         ing source. The manufacturers also provide
     which increases catenary in the towline.      information with new chain regarding size,
Chain, like other marine tension members,          type, material, proof tests, certification, and
has evolved over the years. The Boston Naval       so forth. This information should be main-
Shipyard led U.S. Navy chain development           tained in the Towing Hawser Log (see Ap-
and manufacture for many years. Two major          pendix F) and updated as necessary for chain
developments and manufacturing responsibil-        that is used as an integral part of the towline
ities at the Shipyard were die lock chain and      connection.
the Navy detachable link. With the deactiva-       D-2.2 Marking
tion of the Boston Naval Shipyard in 1972,
                                                   Navy chain, whether die lock or stud link, is
this capability was lost to the Navy, although
                                                   marked in accordance with MIL-C-24633A
similar products were commercially manu-
                                                   Notice 1, Chain, Stud Link, Anchor, Low Al-
factured until the mid-1980s. Nonetheless,
                                                   loy Steel, Flash Bolt Welded (Ref. AB).
large amounts of die lock chain remain
throughout the Fleet and this type chain is        Commercial chain used in marine service, in-
perfectly acceptable for all uses for which it     cluding DiLok, is controlled and certified by
was designed. The Navy now purchases               various marine classification societies such as
“flash butt welded stud link” chain that is        the American Bureau of Shipping (ABS),
similar in appearance to high quality, com-        which certifies all U.S. flag vessels and many
mercial anchor chain, usually referred to as       foreign ships. Marine stud link chain is made
“welded” or “stud link” chain. In this appen-      in three grades. Grade 2 is most prevalent.
dix, this new Navy chain will be called “stud      ABS requires chain to be marked on the end


                                                                                               D-1
                                      U.S. Navy Towing Manual


link of each shot, or every 15 fathoms if the       constantly changing tension, is minimized.
chain is continuous (without connecting             Additionally, the weight and flexibility of the
links). The markings include:                       chain promotes the towline catenary and miti-
                                                    gates the effects of dynamic loading on the
      • Certificate number
                                                    rest of the towing system.
      • Chain size
      • Classification society stamp (such as a     D-5 Maintenance and Cleaning
        Maltese Cross for ABS)
      • Designation of the grade of chain, for      As with other elements of the towline, chain
        example: AB/1, AB/2, or AB/3. The           must be properly maintained and cleaned.
        other classification societies have         Perhaps the most important element of chain
        marking requirements and grading sys-       maintenance is corrosion prevention. Corro-
        tems that are similar to those of ABS.      sion leads directly to loss of chain strength by
                                                    reducing the diameter of the load-carrying
When towing a commercial ship, if it is in-         rods that form the links. In stud link chains,
tended to use the ship’s anchor chain for a         corrosion can also loosen the studs and even-
bridle or pendant, the chain should be careful-     tually lead to their loss.
ly inspected in accordance with the require-
ments of Section D-8. If the classification so-     Corrosion prevention is best achieved by a
ciety grade marking cannot be determined,           fresh-water washdown of the chain after each
the chain should be assumed to be Grade 1,          use, coupled with visual inspection for initial
which is roughly one-half as strong as stan-        signs of corrosion. During the required annu-
dard Navy chain.                                    al inspection, the chain should be carefully
Chain from unknown or non-marine sources            cleaned, inspected, and re-preserved as neces-
that is unmarked or cannot otherwise be iden-       sary; see Naval Ship’s Technical Manual
tified should not be used in towing.                (NSTM) S9086-TV-STM-010, Chapter 581,
                                                    Anchoring (Ref. AC).
D-3 Strength and Lifetime                           Cleaning should be done by scaling, sand-
                                                    blasting, or wire brushing. Penetrating oil
Chain, properly used, should be the strongest       should not be used to loosen the rust because
and longest-lived element in the towing sys-        it is difficult to remove and may reduce the
tem. Because of its construction and general-       effectiveness of corrosion prevention coat-
ly rugged configuration, chain is considerably      ings. After cleaning, a careful inspection
stronger than wire or fiber rope of the same        should be made in accordance with Section
nominal size.                                       D-8. All suspected links should be checked
                                                    by non-destructive test methods, careful mea-
D-4 Elongation                                      surement, sounding, and so forth.

The rugged, large-diameter, individual              Preservation after cleaning and any necessary
strength members of chain give it the least         repairs should be performed in accordance
elongation, or stretch, under load of any tow-      with Section D-8 and with NSTM CH-074
line component. This characteristic of chain        (Ref. K). For most chain, the use of TT-V-51
is one of the prime reasons it is used as an el-    paint (asphalt varnish) or MIL-P-24380 paint
ement in the towline system. Because it does        (anchor chain gloss black solvent type paint)
not stretch, working at chafing points, under       is satisfactory.


D-2
                                    U.S. Navy Towing Manual


D-6 New Chain and Links                              • Surface cracks or sharp gouges: attempt
                                                       to eliminate by light grinding. If the
New or reissued chain or links that will be            chain diameter is reduced to less than
used as components of towline connections              90 percent of the nominal diameter af-
should be treated in the same manner as new            ter grinding: discard link.
towing hawsers. The chain and links should
                                                     • Excessively loose stud: since it is diffi-
be inspected and pertinent data entered in the
                                                       cult to quantify excessive looseness of
Towing Hawser Log (see Appendix F).
                                                       chain studs, the decision to reject or ac-
                                                       cept a link with a loose stud depends on
D-7 Stowing                                            the experience and judgment of the in-
                                                       spector. Consider discarding a link if:
No special stowing precautions are needed
beyond attempts to prevent corrosion, such as            — The stud can move more than 1/8
trying to avoid moisture and salt. Again, oil              inch (3 mm) axially or more than
and grease should be avoided.                              3/16 inch (5 mm) laterally in any
                                                           direction, or
D-8 Inspection                                           — A gap of more than 1/8 inch exists
                                                           between the stud end in a link with
D-8.1 General                                              a stud welded only on one end.
Annual inspection of chain components of a           • Cracks detected by magnetic particle
towline system should follow the Navy prac-            inspection in the internal locking area
tices for anchor chain detailed in NSTM 581.           of detachable link: discard link. Exter-
After cleaning by scaling, wire-brushing, or           nal surface defects in detachable links
sandblasting, each link should be checked by           are not cause for rejection if they can be
sounding with a hammer. Give particular at-            eliminated by grinding to a depth of no
tention to locating possible loose studs, bent         more than 8 percent of the nominal di-
links, excessive corrosion, and sharp gouges.          ameter of the chain.
D-8.2 Specific                                       • Length over six links exceeding 26.65
Proper reactions to various conditions noted           times nominal chain diameter or length
in the inspection are indicated in the follow-         of individual link exceeding 6.15 times
ing notes, most of which apply to stud link            nominal chain diameter: discard links.
chain:                                               • Excessive wear or deep surface crack
   • Missing stud: discard link.                       on shackles, open links, or swivels: At-
                                                       tempt to eliminate by light grinding. If
   • Out-of-plane bending of more than
                                                       the cross-section area, diameter or criti-
     three degrees: discard link.
                                                       cal thickness in any direction is reduced
   • Average of the two measured diameters             more than 10 percent by wear and
     at any point less than 95 percent of              grinding: discard the chain.
     nominal diameter, or a diameter in any
                                                  If a substantial number of adjacent links in a
     direction less than 90 percent of nomi-
                                                  chain section meet the criteria for discarding,
     nal diameter: discard link.
                                                  the chain section should be removed and the
   • Crack at the toe of the stud weld ex-        chain joined again by detachable links that
     tending into the base material: discard      have been examined and found to be in ac-
     link.                                        ceptable condition.


                                                                                             D-3
                                        U.S. Navy Towing Manual


If a large number of links meet the criteria for            have been excessively torqued when
discarding and these links are distributed                  traversing a sharp, curved surface or the
throughout the chain’s whole length, replace                chain may have jumped over the wild-
the chain with a new one.                                   cat, making point contacts between the
                                                            link and the wildcat.
Rewelding of loose studs in the field is unde-
sirable for the following reasons:                       • Corrosion. Excessive corrosion reduc-
      • Welding in the field may produce hard              es the cross sectional area of the link,
        heat-affected zones that are susceptible           increasing the possibility of chain fail-
        to cold cracking.                                  ure from corrosion fatigue or overload-
                                                           ing.
      • Hydrogen brittleness may occur from
        absorption of moisture from the atmo-            • Sharp gouges. Physical damage to the
        sphere or welding electrodes.                      chain surface, such as cuts and gouges,
                                                           raises stress and promotes fatigue fail-
Weld repairs on loose studs should be de-                  ure.
layed as long as possible. Where a few links
are found with loose studs in a short section            • Loose studs. Loose studs, caused by
of a chain, it is recommended that this portion            abusive handling or by excessive
of the chain be cut out and a detachable link              stretching of chain, result in lower
inserted. If the major portion of the chain has            bending strength of the chain.
loose studs, the chain should be scrapped.               • Cracks. Surface cracks, flash weld cra-
Any grinding to eliminate shallow surface de-              cks, and stud weld cracks propagate un-
fects should be done parallel to the longitudi-            der cyclic loading and result in prema-
nal direction of the chain, and the groove                 ture chain failure.
should be well rounded and should form a                 • Wear. Wear between links in the grip
smooth transition to the surface. The ground               area and between links and the wildcat
surface should be examined by magnetic-par-                reduces the chain diameter. The diame-
ticle or dye-penetrant inspection techniques.              ter reduction decreases the load-carry-
                                                           ing capacity of the chain and invites
D-9 Types of Wear                                          failure.

The rough treatment to which chain items of              • Elongation. Excessive permanent elon-
towing gear are exposed can lead to various                gation may cause the chain to function
chain problems. Eight common problems for                  improperly in the wildcat, resulting in
which towing personnel should be alert are                 bending and wear of the links. Wear in
described below:                                           the grip area of the chain as well as
                                                           working loads in excess of the original
      • Missing studs. The stud contributes                proof load will result in a permanent
        about 15 percent of the chain’s                    elongation of chain.
        strength. A chain link without a stud
        may significantly increase the possibili-     D-10 Special Precautions
        ty of link failure. High bending stresses
        and low fatigue life in links are predict-
                                                      Because chain is generally the most rugged
        able consequences of missing studs.
                                                      component of the towline system, there is a
      • Bent links. A bent link is the result of      tendency to become overconfident in its capa-
        chain handling abuse. The link may            bility and somewhat less rigorous in inspec-


D-4
                                       U.S. Navy Towing Manual


tion. Avoid overconfidence when using                D-4 and D-5 describe detachable links and an
chain.                                               improved locking system for use with the ta-
Personnel tend not to check carefully enough         pered link pins. End links (see Table D-6) are
on such items as:                                    special studless links 1/8 inch to 1/4 inch
                                                     larger than the chain size. They are larger
    • Adequate radius of curvature on surfac-        than the chain size to compensate for the lack
      es of fairleads, chocks, and so forth. A       of a stud. They have the same strength as the
      ratio of 7:1 is generally accepted as the      parent chain system.
      minimum D:d ratio of bearing surface
      to chain size for heavy loads when the
      chain direction is changed significantly       D-13 Safety Shackles
      over the surface.
    • Wear in the grip (partially hidden con-                          CAUTION
      tact) area between chain links.                      Screw-pin shackles, other than the
    • Looseness from excessive wear in                     special forged shackles for stop-
                                                           pers, must never be used for con-
      shackles, swivels, and detachable links.             nections in towing rigs. The pin
    • Presence of detachable links that are                could back out due to the constant
                                                           vibration on the towline.
      not equipped with safety-lock hairpins.

D-11 Chain Specifications                            A safety shackle is characterized by a pin that
                                                     is secured by a bolt on the outside of the
Navy die lock chain characteristics are in-          shackle. For towing use, the bolt itself is se-
cluded in Table D-1. The similar Baldt               cured by a small machine bolt with two nuts
“DiLok” chain is 11 percent stronger and 1           jammed together to prevent rotation of the
percent heavier. Table D-2 provides the char-
                                                     large nut. Screw-pin shackles, which use a
acteristics of Navy stud link chain. Navy stud
                                                     threaded pin that screws into the body of the
link chain is equivalent to commercial Grade
                                                     shackle, are not approved for Navy towing.
3 as shown in Table D-3. Commercial Grade
3 chain is about 3 percent stronger than Navy        Some deck layouts present no alternative due
standard die lock. Grade 2 is only about 70          to location and size of attachment padeye.
percent as strong as Navy standard die lock          Contact NAVSEA 00C for further guidance.
and Grade 1 is only about 50 percent as
                                                     Navy shackles are manufactured in two types,
strong.
                                                     two grades, and three classes of shackles. Me-
D-12 Connecting Links                                chanical properties can be obtained from Fed
                                                     Spec RR-C-271D (Ref. E). Tables D-7
Detachable chain connecting links are fre-           through D-9 provide the physical dimensions
quently used in lieu of more traditional shack-      and strengths of safety shackles. Note the sig-
les, because they will pass through a smaller        nificant difference in strength between Grade
space and are less likely to “hang up” during        A and Grade B shackles. The shackle size and
the rigging process. Pear-shaped detachable          safe working load will be shown in raised or
links fit two chain sizes. The strength of this      stamped letters on the shackle. The pins and
link is identical to the breaking strength of the    bolts of Grade A - Regular Strength shackles
larger chain size that it is designed to accom-      are unmarked, but Grade B pins and bolts are
modate. Figures D-4 through D-5 and Tables           marked “HS.”


                                                                                                D-5
                                     U.S. Navy Towing Manual


D-14 Proof Load, Safe Working Load,                predict, than for simple rigging purposes. Ap-
     and Safety Factor                             plying the safety factors from Table 3-2 in ad-
                                                   dition to SWL, however, is overly conserva-
Calculated or predicted design loads are com-      tive and will result in unacceptably large
pared to a baseline strength in computing the      components. Therefore, when designing tow-
safety factor. Conversely, the baseline            ing systems for strenuous conditions, the
strength is divided by the recommended safe-       safety factors listed in Table 3-2 for shackles
ty factor to determine the allowable design        should be applied to proof loads listed in Ta-
load. Table 3-2 provides the recommended           ble D-9.
factors of safety for use in designing towing
                                                   Consider, for example, a predicted steady
systems. Note that safety factors, for a given
                                                   state tow resistance of 80,000 pounds. This is
type design and service, are referenced to dif-
                                                   appropriate for a 2-inch fiber core towing
ferent baselines such as breaking strength,
                                                   hawser under automatic towing machine con-
yield strength, or proof load.
                                                   trol. Table 3-2 requires a safety factor of 3 for
For chain, safety factors are referred to as       shackles. If this factor is applied to SWL, 3
“proof load,” a load demonstrated as part of       1/2-inch Grade B safety shackles, weighing
the manufacturing process, which intentional-      310 pounds, would be required in the rig. Ap-
ly introduces a permanent stretch that im-         plying the required factor of safety to proof
proves the strength of the chain. Proof load       load requires more reasonable 2 1/4-inch
for chain is 66 percent of minimum break           Grade B shackles.
strength.
For other forged-type hardware, such as            D-15 Plate Shackles
shackles, proof load is a load at which no per-
manent deformation is observed after the load      Plate shackles are frequently used in salvage
is released. This is important where the com-      and towing operations because they are sim-
ponent must mate with other components or          ple, efficient, and easily fabricated from com-
where the component has parts that must fit        monly available materials. Plate shackles are
together. In the case of shackles, it is impor-    efficient because many connections of chain
tant to be able to remove the pin after use.       to wire and chain to chain would require two
Unlike chain, however, there is no consistent      safety shackles, back-to-back, whereas one
relationship between proof load and breaking       plate shackle will accomplish the task. The
load. The relationship depends upon the met-       cheeks of towing plate shackles are fabricated
allurgical properties of the material.             from “medium” (ABS Grade A or ASTM
                                                   A-36) steel, the most readily available classi-
Safe Working Load (SWL) is frequently used
                                                   fication, and the pins are fabricated from
for rigging components and systems includ-
                                                   150,000 psi minimum yield strength bar
ing such material. The concept of SWL is
                                                   stock, also readily available. Appendix I in-
similar to the use of a “safety factor” and is
                                                   cludes drawings of plate shackles for use in
appropriate where the load is fairly well
                                                   towing. Certain salvage ships can be outfitted
known and dynamic loads are limited. The
                                                   for heavy-lifting operations. In this case,
typical use of SWL is for lifting purposes.
                                                   stronger plate shackles than shown in Appen-
The safety factor inherent in SWL for Navy
                                                   dix I may be required. Check the specific rig-
safety shackles, compared to proof load, is 2
                                                   ging plans for the specified shackles for
for Grade A and 2.5 for Grade B shackles.
                                                   heavy lifting.
This is insufficient for use in towing systems,
where the dynamic loads are more difficult to


D-6
                                          U.S. Navy Towing Manual



                    Table D-1. Die Lock Chain Characteristics (MIL-C-19944).




                                                                  Approx.
  CHAIN SIZE                                 Length    Number      Weight
                      Link       Link       Over Six   of Links    Per 15-   Approx.
                     Length     Width        Links     Per 15-     Fathom     Weight      Proof
                    (Inches)   (Inches)     (Inches)   Fathom       Shot     Per Link     Test      Break Test
 Inches   mm           A          B            C         Shot     (Pounds)   (Pounds)   (Pounds)     (Pounds)

TYPE I: STANDARD

3/4        19       4–1/2      2–5/8       19–1/2        359         490        1.4        48,000      75,000
7/8        22       5–1/4      3–1/8       22–3/4        305         680        2.2        64,000      98,000
1          25       6          3–3/16      26            267         890        3.3        84,000     129,000
1–1/8      29       6–3/4      4           29–1/4        237       1,130        4.8       106,000     161,000
1–1/4      32       7–1/2      4–1/2       32–1/2        213       1,400        6.6       130,000     198,000
1–3/8      34       8–1/4      4–13/16     35–3/4        193       1,690        8.8       157,000     235,000
1–1/2      38       9          5–3/8       39            177       2,010       11.4       185,000     280,000
1–5/8      42       9–3/4      5–7/8       42–1/4        165       2,325       14.1       216,000     325,000
1–3/4      44      10–1/2      6–3/16      45–1/2        153       2,695       17.6       249,000     380,000
1–7/8      48      11–3/4      6–3/4       48–3/4        143       3,095       21.6       285,000     432,000
2          51      12          7–3/16      52            135       3,490       25.9       289,800     439,200
2–1/8      54      12–3/4      7–5/8       55–1/4        125       3,935       31.5       325,800     493,200
2–1/4      58      13–1/2      8–1/8       58–1/2        119       4,415       37.1       362,700     549,000
2–3/8      60      14–1/4      8–3/16      61–3/4        113       4,915       43.5       402,300     607,500
2–1/2      64      15          9           65            107       5,475       51.2       442,800     669,600
2–5/8      67      15–3/4      9–3/16      68–1/4        101       6,050       59.9       486,000     731,700
2–3/4      70      16–1/2      9–7/8       71–1/2         97       6,660       68.7       531,000     796,500
2–7/8      73      17–1/4      10–3/8      74–3/4         93       7,295       78.4       576,000     868,500
3          76      18          10–13/16    78             89       7,955       89.4       623,700     940,500
3–1/8      79      18–3/4      11–1/4      81–1/4         87       8,700      100.0       673,200   1,015,200
3–1/4      83      19–1/2      11–11/16    84–1/2         83       9,410      113.4       723,700   1,089,000
3–3/8      86      20–1/4      12–1/8      87–3/4         79      10,112      128.0       776,000   1,166,400
3–1/2      90      21          12–5/8      91             77      10,900      141.6       829,800   1,244,800
3–3/4      95      22–1/2      13–3/8      97–1/2         71      12,500      176.1     1,008,000   1,575,000
4–3/4     121      28–1/2      17–1/8      122–1/2        57      20,500      359.7     1,700,000   2,550,000

TYPE II: HEAVY DUTY

 2–3/4     70      16–1/2      9–7/8       71–1/2         97       7,000      72.2       584,100      882,900
 3         76      18          10–13/16    78             89       8,100      91.0       685,800    1,035,000
 3–1/2     90      21          12–5/8      91             77      12,000     155.8       972,000    1,530,000

TYPE III: HIGH STRENGTH

 3/4       19      4–1/2       2–5/8       19–1/2        359          550       1.5       67,500      91,100
 1         26      6           3–3/16      26            267        1,000       3.8      116,100     156,700
 1–1/8     29      6–3/4       4           29–1/4        237        1,270       5.4      145,000     195,000
 1–3/8     34      8–1/4       4–15/16     35–3/4        193        1,900       9.9      211,500     285,500
 1–1/2     38      9           5–3/8       39            177        2,260      12.8      252,000     340,200
 1–5/8     42      9–3/4       5–7/8       42–1/4        165        2,620      15.9      292,500     395,000




                                                                                                                D-7
                                                   U.S. Navy Towing Manual



                          Table D-2. Navy Stud Link Chain Characteristics (MIL-C-24633).




                 Link         Link            Length Over 6 Links (C)        Number of                               Nominal
   Chain        Length       Width                   (Inches)                 Links per     Proof        Break      Weight per
    Size       (Inches)     (Inches)                                         15-Fathom    Test Load    Test Load    15-Fathom
  (Inches)        (A)          (B)     Minimum       Nominal      Maximum       Shot      (Pounds)     (Pounds)     Shot (lb.)*

      3/4       4-1/2      2-5/8       19-3/8       19-1/2        19-13/16      359           48,000       75,000           480
      7/8       5-1/4      3-1/8       22-5/8       22-3/4        23-1/16       305           64,400       98,000           660
      1         6          3-9/16      25-7/8       26            26-3/8        267           84,000      129,000           860
      1-1/8     6-3/4      4           29-1/16      29-1/4        29-5/8        237          106,000      161,000         1,080
      1-1/4     7-1/2      4-1/2       32-5/16      32-1/2        32-15/16      213          130,000      198,000         1,350
      1-3/8     8-1/4      4-15/16     35-9/16      35-3/4        36-1/4        193          157,000      235,000         1,630
      1-1/2     9          5-3/8       38-13/16     39            39-1/2        177          185,000      280,000         1,940
      1-5/8     9-3/4      5-7/8       42           42-1/4        42-7/8        165          216,000      325,000         2,240
      1-3/4     10-1/2     6-5/16      45-1/4       45-1/2        46-1/8        153          249,000      380,000         2,590
      1-7/8     11-1/4     6-3/4       48-1/2       48-3/4        49-1/2        143          285,000      432,000         2,980
      2         12         7-3/16      51-11/16     52            52-3/4        135          318,800      454,000         3,360
      2-1/8     12-3/4     7-5/8       54-15/16     55-1/4        56-1/8        125          357,000      510,000         3,790
      2-1/4     13-1/2     8-1/8       58-3/16      58-1/2        59-3/8        119          396,000      570,000         4,250
      2-3/8     14-1/4     8-9/16      61-7/16      61-3/4        62-3/4        113          440,000      628,000         4,730
      2-1/2     15         9           64-11/16     65            66            107          484,000      692,000         5,270
      2-5/8     15-3/4     9-7/16      67-7/8       68-1/4        69-1/4        101          530,000      758,000         5,820
      2-3/4     16-1/2     9-7/8       71-1/8       71-1/2        72-9/16        97          578,000      826,000         6,410
      2-7/8     17-1/4     10-3/8      74-3/8       74-3/4        75-7/8         93          628,000      897,000         7,020
      3         18         10-13/16    77-5/8       78            79-3/16        89          679,000      970,000         7,650
      3-1/8     18-3/4     11-1/4      80-13/16     81-1/4        82-1/2         87          732,000    1,046,000         8,320
      3-1/4     19-1/2     11-11/16    84-1/16      84-1/2        85-3/4         83          787,000    1,124,000         9,010
      3-3/8     20-1/4     12-1/8      87-5/16      87-3/4        89             79          843,000    1,204,000         9,730
      3-1/2     21         12-5/8      90-9/16      91            92-5/16        77          900,000    1,285,000        10,500
      3-5/8     21-3/4     12-15/16    93-13/16     94-1/4        95-5/8         73          958,000    1,369,000        11,300
      3-3/4     22-1/2     13-3/8      97-1/16      97-1/2        98-7/8         71        1,019,000    1,455,000        12,000
      3-7/8     23-1/4     14          100-1/4      100-3/4       102-3/16       69        1,080,000    1,543,000        12,900
      4         24         14-3/8      103-1/2      104           105-1/2        67        1,143,000    1,632,000        13,700

         * Not mandatory, for information only.




D-8
                                                              U.S. Navy Towing Manual



                                        Table D-3. Commercial Stud Link Anchor Chain.




                                         Length
                                          Over                 Approx.     No. of         ABS Grade 1              ABS Grade 2               ABS Grade 3
   Chain Size      Link        Link       Five       Grip      Weight       Links
                  Length      Width       Links     Radius     per 15-     per 15-
                 (Inches)    (Inches)   (Inches)   (Inches)    Fathom      Fathom     Proof       Break        Proof        Break        Proof        Break
 Inches    mm       (A)         (B)        (C)        (D)     Shot (lbs)    Shot     Test (lb)   Test (lb)    Test (lb)    Test (lb)    Test (lb)    Test (lb)

3/4        19    4-1/2      2-5/8       16-1/2     1/2              480      357       23,800        34,000      34,000        47,600      47,600       68,000
13/16      20    4-7/8      2-7/8       17-7/8     17/32            570      329       27,800        39,800      39,800        55,700      55,700       79,500
7/8        22    5-1/4      3-1/8       19-1/4     37/64            660      305       32,200        46,000      46,000        64,400      64,400       91,800
15/16      24    5-5/8      3-5/16      20-5/8     5/8              760      285       36,800        52,600      52,600        73,700      73,700      105,000
1          25    6          3-9/16      22         21/32            860      267       41,800        59,700      59,700        83,600      83,600      119,500
1-1/16     27    6-3/8      3-3/4       23-3/8     11/16            970      251       47,000        67,200      67,200        94,100      94,100      135,000
1-1/8      28    6-3/4      4           24-3/4     25/32          1,080      237       52,600        75,000      75,000       105,000     105,000      150,000
1-3/16     30    7-1/8      4-1/4       26-1/8     25/32          1,220      225       58,400        83,400      83,400       116,500     116,500      167,000
1-1/4      32    7-1/2      4-1/2       27-1/2     25/32          1,350      213       64,500        92,200      92,200       129,000     129,000      184,000
1-5/16     33    7-7/8      4-3/4       28-7/8     7/8            1,490      203       70,900       101,500     101,500       142,000     142,000      203,000
1-3/8      34    8-1/4      4-15/16     30-1/4     7/8            1,630      195       77,500       111,000     111,000       155,000     155,000      222,000
1-7/16     36    8-5/8      5-3/16      31-5/8     15/16          1,780      187       84,500       120,500     120,500       169,000     169,000      241,000
1-1/2      38    9          5-3/8       33         63/64          1,940      179       91,700       131,000     131,000       183,500     183,500      262,000
1-9/16     40    9-3/8      5-5/8       34-3/8     1-1/32         2,090      171       99,200       142,000     142,000       198,500     198,500      284,000
1-5/8      42    9-3/4      5-7/8       35-3/4     1-1/16         2,240      165      108,000       153,000     153,000       214,000     214,000      306,000
1-11/16    43    10-1/8     6-1/16      37-1/8     1-3/32         2,410      159      115,000       166,500     166,500       229,000     229,000      327,000
1-3/4      44    10-1/2     6-5/16      38-1/2     1-5/32         2,590      153      123,500       176,000     176,000       247,000     247,000      352,000
1-13/16    46    10-7/8     6-1/2       39-7/8     1-3/16         2,790      147      132,000       188,500     188,500       264,000     264,000      377,000
1-7/8      48    11-1/4     6-3/4       41-1/4     1-1/4          2,980      143      140,500       201,000     201,000       281,000     281,000      402,000
1-15/16    50    11-5/8     7           42-5/8     1-9/32         3,180      139      149,500       214,000     214,000       299,000     299,000      427,000
2          51    12         7-3/16      44         1-5/16         3,360      133      159,000       227,000     227,000       318,000     318,000      454,000
2-1/16     52    12-3/8     7-7/16      45-3/8     1-3/8          3,570      129      168,500       241,000     241,000       337,000     337,000      482,000
2-1/8      54    12-3/4     7-5/8       46-3/4     1-27/64        3,790      125      178,500       255,000     255,000       357,000     357,000      510,000
2-3/16     56    13-1/8     7-7/8       48-1/8     1-15/32        4,020      123      188,500       269,000     269,000       377,000     377,000      538,000
2-1/4      58    13-1/2     8-1/8       49-1/2     1-1/2          4,250      119      198,500       284,000     284,000       396,000     396,000      570,000
2-5/16     59    13-7/8     8-5/16      50-7/8     1-17/32        4,490      117      209,000       299,000     299,000       418,000     418,000      598,000
2-3/8      60    14-1/4     8-9/16      52-1/4     1-9/16         4,730      113      212,000       314,000     314,000       440,000     440,000      628,000
2-7/16     62    14-5/8     8-3/4       53-5/8     1-5/8          4,960      111      231,000       330,000     330,000       462,000     462,000      660,000
2-1/2      64    15         9           55         1-5/8          5,270      107      242,000       346,000     346,000       484,000     484,000      692,000
2-9/16     66    15-3/8     9-1/4       56-3/8     1-11/16        5,540      105      254,000       363,000     363,000       507,000     507,000      726,000
2-5/8      67    15-3/4     9-7/16      57-3/4     1-11/16        5,820      103      265,000       379,000     379,000       530,000     530,000      758,000
2-11/16    68    16-1/8     9-11/16     59-1/8     1-3/4          6,110       99      277,000       396,000     396,000       554,000     554,000      792,000
2-3/4      70    16-1/2     9-7/8       60-1/2     1-13/16        6,410       97      289,000       413,000     413,000       578,000     578,000      826,000
2-13/16    71    16-7/8     10-1/8      61-7/8     1-27/32        6,710       95      301,000       431,000     431,000       603,000     603,000      861,000
2-7/8      73    17-1/4     10-3/8      63-1/4     1-7/8          7,020       93      314,000       449,000     449,000       628,000     628,000      897,000
2-15/16    75    17-5/8     10-9/16     64-5/8     1-7/8          7,330       91      327,000       467,000     467,000       654,000     654,000      934,000
3          76    18         10-13/16    66         2              7,650       89      340,000       485,000     485,000       679,000     679,000      970,000
3-1/16     78    18-3/8     11          67-3/8     2              7,980       87      353,000       504,000     504,000       705,000     705,000    1,008,000
3-1/8      79    18-3/4     11-1/4      68-3/4     2-1/16         8,320       85      366,000       523,000     523,000       732,000     732,000    1,046,000
3-3/16     81    19-1/8     11-1/2      70-1/8     2-1/16         8,660       85      380,000       542,000     542,000       759,000     759,000    1,084,000
3-1/4      83    19-1/2     11-11/16    71-1/2     2-1/8          9,010       83      393,000       562,000     562,000       787,000     787,000    1,124,000
3-5/16     84    19-7/8     11-15/16    72-7/8     2-1/8          9,360       81      407,000       582,000     582,000       814,000     814,000    1,163,000
3-3/8      86    20-1/4     12-1/8      74-1/4     2-3/16         9,730       79      421,000       602,000     602,000       843,000     843,000    1,204,000
3-7/16     87    20-5/8     12-3/8      75-5/8     2-3/16        10,100       77      435,000       622,000     622,000       871,000     871,000    1,244,000
3-1/2      90    21         12-5/8      77         2-5/16        10,500       77      450,000       643,000     643,000       900,000     900,000    1,285,000
3-5/8      92    21-3/4     12-15/16    79-3/4     2-5/16        11,300       73      479,000       685,000     685,000       958,000     958,000    1,369,000
3-3/4      95    22-1/2     13-3/8      82-1/2     2-15/32       12,000       71      509,000       728,000     728,000     1,019,000   1,019,000    1,455,000
3-7/8      98    23-1/4     14          85-1/4     2-15/32       12,900       69      540,000       772,000     772,000     1,080,000   1,080,000    1,543,000
4          102   24         14-3/8      88         2-5/8         13,700       67      571,000       816,000     816,000     1,143,000   1,143,000    1,632,000
4-1/8      105   24-3/4     14-7/8      90-3/4     2-11/16       14,600       65      603,000       862,000     862,000     1,207,000   1,207,000    1,724,000
4-1/4      108   25-1/2     15-5/16     93-1/2     2-3/4         15,400       63      636,000       908,000     908,000     1,272,000   1,272,000    1,817,000
4-3/8      111   26-1/4     15-3/4      96-1/4     2-7/8         16,200       61      669,000       956,000     956,000     1,338,000   1,338,000    1,911,000
4-1/2      114   27         16-3/16     99         2-15/16       17,100       59      703,000     1,000,400   1,000,400     1,405,000   1,405,000    2,008,000




                                                                                                                                                                 D-9
                                                          U.S. Navy Towing Manual



                                  Table D-4. Commercial Detachable Chain Connecting Link.




               Chain Size                                                                                                  Weight
                                                                                                   Proof       Break       per Link
          Inches               mm        A         B           C          D        E         F     Test        Test         (lbs.)

           3/4              19         4-1/2    3            1-3/64    3/4     27/32     1/2          67,500      91,100        2.1
         13/16 - 7/8        21-22      5-1/4    3-1/2        1-7/32    7/8     63/64     19/32        88,200     119,000      3.4
         15/16 - 1          24-25      5        4            1-25/64   1       1-1/8     21/32       116,110     156,700      5.1
        1-1/16 - 1-1/8      27-28      6-3/4    4-1/2        1-9/16    1-1/8   1-17/64   47/64       145,000     195,000      7.2
        1-3/16 - 1-1/4      30-32      7-1/2    5            1-47/64   1-1/4   1-13/32   13/16       178,200     240,600      9.9
        1-5/16 - 1-3/8      33-34      8-1/4    5-1/2        1-29/32   1-3/8   1-35/64   29/32       211,500     285,500     13.3
        1-7/16 - 1-1/2      36-38      9        6            2-5/64    1-1/2   1-11/16   83/84       252,000     340,200     17.3
        1-9/16 - 1-5/8      40-42      9-3/4    6-1/2         2-1/4    1-5/8   1-63/64   1-1/16      292,500     395,000     22.0
       1-11/16 - 1-3/4      43-44     10-1/2    7-1/2        2-7/16    1-3/4   2         1-3/16      352,000     476,000     27.5
       1-13/16 - 1-7/8      46-48     11-1/4    7-1/4         2-1/2    1-7/8   2-5/32    1-1/4       285,000     432,000     32
       1-15/16 - 2          50-51     12        7-3/4         2-1/2    2       2-5/16    1-5/16      322,000     488,000     36
        2-1/16 - 2-1/8      52-54     12-3/4    8-1/4        2-21/32   2-1/8   2-1/2     1-13/32     362,000     548,000     44
        2-3/16 - 2-1/4      56-58     13-1/2    8-23/32      2-13/16   2-1/4   2-5/8     1-1/2       403,000     610,000     52
        2-5/16 - 2-3/8      59-60     14-1/4    9-7/32       3-1/16    2-3/8   2-3/4     1-9/16      447,000     675,000     61
        2-9/16 - 2-5/8      66-67     15-3/4   10-3/16        3-1/4    2-5/8   3-1/16    1-3/4       540,000     813,000     82
       2-11/16 - 2-3/4      68-70     16-1/2   10-13/16      3-11/16   2-7/8   3-1/4     1-13/16     649,000     981,000    100
       2-13/16 - 2-7/8      71-73     17-1/4   11-1/8        3-19/32   2-7/8   3-11/32   1-29/32     640,000     965,000    107
       2-15/16 - 3          75-76     18       11-5/8         3-3/4    3       3-17/72   1-31/32     693,000   1,045,000    120
        3-1/16 - 3-1/8      78-79     18-3/4   12-1/8           4      3-1/8   3-5/8     2-3/64      748,000   1,128,000    138
        3-3/16 - 3-1/4      81-83     19-1/2   12-5/8        4-1/16    3-1/4   3-5/8     2-5/32      804,100   1,210,000    161
        3-5/16 - 3-3/8      84-86     20-1/4   13-3/32       4-7/32    3-3/8   3-15/16   2-1/4       862,200   1,296,000    177
        3-7/16 - 3-1/2      87-89     21-1/8   13-25/32      4-13/16   3-3/4   4-1/8     2-13/32   1,080,000   1,700,000    205
        3-9/16 - 3-5/8      90-92     21-3/4   14            4-9/16    3-5/8   4-3/16    2-5/16    1,021,100   1,566,000    215
       3-11/16 - 3-3/4      94-95     22-1/2   14-1/2        4-11/16   3-3/4   4-11/16   2-7/16    1,120,000   1,750,000    256
       3-11/16 - 3-7/8      97-98     23-1/4   15               5      3-7/8   4-1/2     2-5/8     1,205,000   1,863,400    271
       3-17/16 - 4          100-102   24       15-1/2        5-3/16    4       4-5/8     2-11/16   1,298,000   1,966,000    288
         4-1/8              105       24-3/4   16-1/2         5-7/8    4-1/8   5         2-25/32   1,347,000   2,062,500    384
         4-1/4              108       25-1/2   17-3/8         6-1/2    4-3/8   5-1/4     2-7/8     1,393,700   2,134,000    422
         4-3/8              111       26-1/4   18-3/8         7-1/4    4-1/2   5-5/8     2-15/16   1,569,700   2,398,000    460
         4-1/2              114       27       19-3/8           8      4-5/8   6         3         1,672,000   2,508,000    500

           All specifications in pounds and inches, unless otherwise stated.
           See Figures D-2 and D-3 for hairpin locking details.




D-10
                                                         U.S. Navy Towing Manual



                          Table D-5. Commercial Detachable Anchor Connecting Link.




           Small End Chain Size

No.             Inches               mm          A              B              C              D                E       F                   G

2       3/4 - 15/16               19-24     7-5/8           5-3/16         1-1/2          15/16         1-1/4      2-1/4         15/16
3       1 - 1-3/16                25-30     9-3/8           6-9/16         1-13/16        1-3/16        1-1/2      2-19/32       1-5/16
4       1-1/4 - 1-9/16            32-40     11-3/4          8-1/8          2-5/16         1-9/16        1-7/8      3-1/4         1-9/16 x 1-3/4
5       1-5/8 - 2                 42-51     14-7/8          10-1/4         3              2             2-1/2      3-15/16       2-15/16 x 2-3/8
6       2-1/16 - 2-3/8            52-60     17-7/8          12-5/16        3-5/8          2-3/8         3          4-3/4         2-7/16 x 2-7/8
7       2-7/16 - 3-1/8            62-79     22-1/8          14-13/16       4-5/8          3-1/8         3-3/4      5-7/8         3-3/8 x 3-1/8
8       3-3/16 - 3-5/8            81-92     25-3/4          16-1/2         5-1/4          3-5/8         4-7/8      5-7/8         4-3/8 x 4
9       3-11/16 - 3-3/4           94-95     27-1/4          17-1/8         5-3/4          3-7/8         5-1/8      6-1/4         4-7/8 x 5-3/8
10      3-13/16 - 4               97-102    35              22-1/2         7-1/2          4-3/4         6-1/2      7-1/2         5-1/8
11      4-1/16 - 4-1/4            103-108   37              24             8              5             6-7/8      8             6-1/8




           Small End Chain Size                                                                                                                Weight
                                                                                                         Proof               Break             per Link
No.             Inches               mm              H                 J             K                   Test                 Test               (lbs)

2       3/4 - 15/16               19-24     1-3/8                21/32          1-3/16             74,000           113,500                7
3       1 - 1-3/16                25-30     1-3/4                3/4            1-3/8              118,000          179,500                14
4       1-1/4 - 1-9/16            32-40     2-7/32               1-1/32         1-11/16            200,500          302,500                28
5       1-5/8 - 2                 42-51     2-29/32              1-1/4          2-1/16             322,000          488,000                60
6       2-1/16 - 2-3/8            52-60     3-15/32              1-15/32        2-17/32            447,000          675,000                107
7       2-7/16 - 3-1/8            62-79     4-3/8                1-29/32        3                  748,000          1,128,000              208
8       3-3/16 - 3-5/8            81-92     5-1/8 x 5-1/4        2-1/8          3-1/8              1,021,000        1,566,000              328
9       3-11/16 - 3-3/4           94-95     5-9/16               2-1/4          3-1/4              1,120,000        1,750,000              520
10      3-13/16 - 4               97-102    7-1/8                2-7/8          4-1/4              1,298,000        1,996,500              850
11      4-1/16 - 4-1/4            103-108   7-7/8                3              4-3/8              1,440,000        2,220,000              920


      All specifications in pounds and inches, unless otherwise stated.
      See Figures D-2 and D-3 for hairpin locking details.




                                                                                                                                                     D-11
                                          U.S. Navy Towing Manual



                                      Table D-6. Commercial End Link.




                Chain Size               Link       Link       Link
                                       Diameter    Length     Width     Weight      Proof
                                       (Inches)   (Inches)   (inches)   per Link    Test
             Inches           mm          A          B          C         (lbs)     (lbs)
       11/16 - 3/4            17-19     13/16      5-5/8     2-7/8         1.8        48,000
       13/16 - 1              21-25     1-1/16     7-1/2     3-3/4         4.0        84,000
       1-1/16 - 1-1/4         27-32     1-3/8      9-3/8     4-7/8         8.0       130,000
       1-5/16 - 1-1/2         33-38     1-5/8      11-1/4    5-3/4        14.2       185,000
       1-9/16 - 1-3/4         40-44     1-7/8      13        6-5/8        21.6       249,000
       1-13/16 - 2            46-51     2-1/8      15        7-5/8        34.2       322,000
       2-1/16 - 2-1/4         52-58     2-1/2      16-7/8    8-3/4        45.4       403,000
       2-5/16 - 2-1/2         59-64     2-3/4      18-3/4    9-3/4        62.0       492,000
       2-9/16 - 2-3/4         66-70     3          20-1/2    10-3/4       81.0       590,000
       2-13/16 - 3            71-76     3-1/4      22-1/2    11-5/8      105.0       693,000
       3-1/16 - 3-3/8         78-86     3-5/8      25-1/4    13          148.0       862,000
       3-7/16 - 3-3/4         87-95     4          28        14-1/2      202.0     1,120,000
       3-13/16 - 4           97-102     4-1/4      30        15-1/4      258.0     1,298,000




D-12
                                           U.S. Navy Towing Manual



          Table D-7. Type I, Class 3 Safety Anchor Shackle (MIL-S-24214A (SHIPS)).




          Diameter   Diameter   Width between eyes (W)     Length inside (L)               Diameter   Approx.
  Size      Bolt      Inside                                                      Width     Outside    Weight
  (D)        (P)      Eye (E)               Tolerance                Tolerance   Minimum    Eye (R)   per 100
Minimum   Minimum    Maximum    Nominal        (+)       Nominal        (+)        (B)     Maximum    Shackles

 Inches    Inches     Inches     Inches       Inches     Inches        Inches     Inches    Inches    Pounds

1/2       5/8        23/32       13/16         1/16       1-7/8         1/8       1-3/16    1-3/8          82
5/8       3/4        27/32       1-1/16        1/16       2-13/32       1/8       1-1/2     1-7/8         158
3/4       7/8        31/32       1-1/4         1/16       2-27/32       1/4       1-3/4     2-1/8         280
7/8       1          1-3/32      1-7/16        1/16       3-5/16        1/4       2         2-3/8         395
1         1-1/8      1-7/32      1-11/16       1/16       3-3/4         1/4       2-5/16    2-5/8         560
1-1/8     1-1/4      1-11/32     1-13/16       1/16       4-1/4         1/4       2-5/8     2-7/8         785
1-1/4     1-3/8      1-15/32     2-1/32        1/16       4-11/16       1/4       2-7/8     3-1/4       1,120
1-3/8     1-1/2      1-5/8       2-1/4         1/8        5-1/4         1/4       3-1/4     3-1/2       1,520
1-1/2     1-5/8      1-3/4       2-3/8         1/8        5-3/4         1/4       3-3/8     3-3/4       1,950
1-5/8     1-3/4      1-7/8       2-5/8         1/8        6-1/4         1/4       4         4-1/8       2,410
1-3/4     2          2-5/32      2-7/8         1/8        7             1/4       4-1/2     4-1/2       3,130
2         2-1/4      2-13/32     3-1/4         1/8        7-3/4         1/2       5-1/4     5-1/4       4,630
2-1/4     2-1/2      2-21/32     3-7/8         1/8        9-1/4         1/2       5-1/2     5-3/4       5,650
2-1/2     2-3/4      2-29/32     4-1/8         1/8        10-1/2        1/2       6-3/4     6-1/4       9,400
3         3-1/4      3-13/32     5             1/8        13            3/4       7-3/8     6-3/4      14,500
3-1/2     3-3/4      3-29/32     5-3/4         1/4        15            3/4       9         8-1/2      25,000
4         4-1/4      4-13/32     6-1/2         1/4        17            3/4       10-1/2    9-1/2      35,800




                                                                                                                 D-13
                                          U.S. Navy Towing Manual



               Table D-8. Type II, Class 3 Safety Chain Shackle (MIL-S-24214A(SHIPS)).




                    Diameter   Diameter                                                    Diameter   Approx.
                                          Width between eyes (W)     Length inside (L)
         Size         Bolt      Inside                                                      Outside    Weight
         (D)           (P)      Eye (E)                Tolerance               Tolerance    Eye (R)   per 100
       Minimum      Minimum    Maximum     Nominal        (+)      Nominal        (+)      Maximum    Shackles

        Inches       Inches     Inches      Inches      Inches     Inches        Inches     Inches    Pounds

       1/2          5/8        23/32       13/16         1/16      1-5/8          1/8       1-3/8          76
       5/8          3/4        27/32       1-1/16        1/16      2              1/8       1-7/8         156
       3/4          7/8        31/32       1-1/4         1/16      2-3/8          1/4       2-1/8         262
       7/8          1          1-3/32      1-7/16        1/16      2-13/16        1/4       2-3/8         365
       1            1-1/8      1-7/32      1-11/16       1/16      3-3/16         1/4       2-5/8         535
       1-1/8        1-1/4      1-11/32     1-13/16       1/16      3-9/16         1/4       2-7/8         727
       1-1/4        1-3/8      1-15/32     2-1/32        1/16      3-15/16        1/4       3-1/4       1,020
       1-3/8        1-1/2      1-5/8       2-1/4         1/8       4-7/16         1/4       3-1/2       1,335
       1-1/2        1-5/8      1-3/4       2-3/8         1/8       4-7/8          1/4       3-3/4       1,850
       1-5/8        1-3/4      1-7/8       2-5/8         1/8       5-1/4          1/4       4-1/8       2,310
       1-3/4        2          2-5/32      2-7/8         1/8       5-3/4          1/4       4-1/2       2,850
       2            2-1/4      2-13/32     3-1/4         1/8       6-3/4          1/2       5-1/4       4,110
       2-1/2        2-3/4      2-29/32     4-1/8         1/8       8              1/2       6-1/4       8,450
       3            3-1/4      3-13/32     5             1/8       9              3/4       6-3/4      12,300
       3-1/2        3-3/4      3-29/32     5-3/4         1/4       10-1/2         3/4       8-1/2      21,800
       4            4-1/4      4-13/32     6-1/2         1/4       12             3/4       9-1/2      31,000

         See Table D-9 for shackle strengths.




D-14
                                            U.S. Navy Towing Manual



             Table D-9. Mechanical Properties of Shackles (FED SPEC RR-C-271D).




                                                        Proof Load                    Breaking Load
  Size (D)        Working Load Limit                    (Minimum)                      (Minimum)
                         Pounds                              Pounds                          Pounds
  Inches       Grade A            Grade B         Grade A             Grade B    Grade A              Grade B
3/16         650                    1,000            1,430               2,200       3,250                5,000
1/4          1,000                  1,500            2,200               3,300       5,000                7,500
5/16         1,500                  2,500            3,300               5,500       7,500               12,500
3/8          2,000                  4,000            4,400               8,800      10,000               20,000
7/16         3,000                  5,200            6,600              11,440      15,000               26,000
1/2          4,000                  6,600            8,800              14,520      20,000               33,000
9/16         5,000                  8,000           11,000              17,600      25,000               40,000
5/8          6,500                 10,000           14,300              22,000      32,500               50,000
3/4          9,500                 14,000           20,900              30,800      47,500               70,000
7/8          13,000                19,000           28,600               41,00      65,000               95,000
1            17,000                25,000           37,400              55,000      85,000              125,000
1-1/8        19,000                30,000           41,800              66,000      95,000              150,000
1-1/4        24,000                36,000           52,800              79,200     120,000              180,000
1-3/8        27,000                42,000           59,400              92,400     135,000              210,000
1-1/2        34,000                60,000           74,800             132,000     170,000              300,000
1-5/8        40,000                70,000           88,000             154,000     200,000              350,000
1-3/4        50,000                80,000          110,000             176,000     250,000              400,000
2            70,000               100,000          154,000             220,000     350,000              500,000
2-1/4        80,000               120,000          176,000             264,000     400,000              600,000
2-1/2        110,000              160,000          242,000             352,000     550,000              800,000
2-3/4        120,000              180,000          264,000             396,000     600,000              900,000
3            170,000              220,000          374,000             484,000     850,000            1,100,000
3-1/2        240,000              280,000          528,000             616,000   1,200,000            1,400,000
4            300,000              350,000          660,000             770,000   1,500,000            1,750,000




                                                                                                                  D-15
                     U.S. Navy Towing Manual




        Lok-a-Loy                                            Kenter




        Pear-Shaped                                        Navy
       Detachable Link                                Detachable Link


                                   NO TE
                     S ee F igures D -2 and D -3 for N avy
                     detachable locking hairpin details .




                               Cast Stud Link




                              W elded Stud Link


                    Detachable Link
                                                Hairpin




                               Die Lock Link


         Figure D-1. Types of Chains and Connecting Links.



D-16
                                                                          NO T ES

                                                                          1. IN ST RU C TIO N S F OR G R O O VIN G
                                                                                                                                                                                         G R O O V E FO R H A IR P IN TO B E
                                                                             A . RE PL AC EM E N T TA PE R PIN :                                                                         LO C ATE D A FT E R A S S E M B LING
                                                                                 A . AF TE R INS PE CT ING AN D C LE AN IN G DE TA CH A BLE LIN K,                                       LIN K W IT H R IG H T-H A N D
                                                                                     INS TA LL TA PE R PIN W HIC H HA S B E EN W IP E D W ITH O IL.                                      C O U P LIN G & LE FT -H A N D
                                                                                 B . D RIVE TAP ER P IN D O W N TIG H T U SIN G A H A M M ER &
                                                                                     AS SE M BLIN G P UN C H.                                                                            C O U P LIN G , A N D D R IV IN G
                                                                                 C . U SING A DR ILL 1/32-IN C H LE SS IN SIZ E T HA N TH E H AIR -                                      TH E TA P E R P IN D O W N TIG H T.
                                                                                     PIN D IAM E TE R, D R ILL M A RK TH E TAP E R P IN TH RO U G H
                                                                                     ON E O F TH E HA IR PIN H O LES IN TH E D ETAC H AB LE LIN K .
                                                                                 D . D R IVE O UT TAP E R P IN US IN G HA M M E R & D ISA SS EM B LN G
                                                                                     PU NC H . M A CH IN E T HE G R O OV E AT DR ILL M A R K