Maritime New Zealand Guidelines
SAFETY BULLETIN ISSUE 17 – SEPTEMBER 2008
SMALL CRAFT WIRE CABLE STEERING CONTROLS
This safety bulletin is for:
• owners, operators and crew of commercial vessels and jet boats
• ship surveyors
• safety auditors
• authorised persons
• safe ship management companies
• maritime safety inspectors
• manufacturers of vessels and jet boats with wire cable steering controls.
The purpose of this bulletin is to recommend that all operators of boats fitted with a cable and pulley type steering
system consult with the cable manufacturer or supplier of the system to ensure that the pulley (also referred to as
sheave) diameter is within acceptable limits.
Safety concern: cable/pulley diameter ratio
Following a recent steering cable failure on a commercial jet boat, Maritime New Zealand has carried out extensive
testing and analysis into the cause of that failure.
Results of these material tests and failure analysis have brought to our attention the critical importance of the correct
cable/pulley diameter ratio to be applied when employing such systems. If the pulley diameter is too small for the
cable then the risk of fatigue failure, due to higher internal stresses, is extremely high and the service life of the cable
is dramatically reduced.
Sample recommended ratios
The following table is given as general guidance and example of required pulley diameters for different sizes of AIAI
316 stainless cables, and provides a manufacturer’s minimum and recommended ratios:
CABLE DIAMETER CABLE/SHEAVE PULLEY (SHEAVE) CABLE/SHEAVE PULLEY (SHEAVE)
(d) RATIO DIAMETER (D) RATIO DIAMETER (D)
AISI 316 MANUFACTURER’S MINIMUM SIZE MANUFACTURER’S RECOMMENDED
STAINLESS STEEL MINIMUM RATIO RECOMMENDED SIZE
(7 X 19) RATIO
3 mm 34 : 1 102 mm 51 : 1 153 mm
4 mm 34 : 1 136 mm 51 : 1 204 mm
5 mm 34 : 1 170 mm 51 : 1` 255 mm
MNZ GUIDELINES - SAFETY BULLETIN ISSUE 17 – SEPTEMBER 2008
The above minimum and recommended ratio examples are given by a particular manufacturer and apply to AISI 316
Stainless Steel 7 x 19 Marine Wire Rope. Manufacturers may have different recommendations depending on the
specific type and properties of their cable and should be consulted in each case.
Note that the diameter “D” is measured at the innermost profile or root of the sheave and the measurement “d” is
the maximum external diameter of the cable. The repairer or operator should physically measure the diameter of the
cable before applying the relevant formula.
When assessing the suitability of a wire for installation, and when sizing the pulleys to be used with this wire, the
recommended sheave/cable ratio for the wire involved should, where possible, be used. The minimum ratio is also
acceptable, however, should only be applied where the design of the controls prevents the recommended values
Reference should also be made to the recommended specifications regarding the width of the pulley or sheath
groove. A manufacturer’s recommendation is 1.5 times the diameter of the cable. Example: 4.34 mm diameter
times 1.5 = 6.51 groove width.
It is also recommended that the alignment of the pulleys or sheaves is within 2 degrees of alignment of the cable.
Where a pulley is hinged the tension of the wire is important to ensure the wire is aligned properly with the groove of
the pulley; this should be checked during the inspection of the cable and steering arrangement.
Inspection of cable and pulley steering arrangements
Wires are consumable items of equipment. They have a limited lifespan and owner is responsible for undertaking
regular inspection of the wires to check for any wear, kinks, compressions, abrasion or corrosion to the wire.
Pulleys also have a limited lifespan and their alignment, sheave wear, balanced rotation and secure foundations are
very important. Checks must be completed on the jet boat’s full steering system prior to operations, especially
behind any pulleys. If any broken wire strands are discovered, the cable must be replaced immediately.
Cable and pulley steering arrangements should be closely inspected at regular intervals paying particular attention to
the length of wire that works with the pulley, the tension in the wire and how securely the wires are terminated. Any
cable showing signs of significant wear, damage or corrosion should be replaced at the earliest opportunity.
Importance of lubrication
Corrosion from water, and abrasion from dirt or dust contamination, can greatly reduce the lifespan of both the wire
and the pulley. A suitable lubricant can also significantly prolong the fatigue life of a cable that is subjected to
cyclical stress reversal (as is experienced in a cable/pulley application). It is therefore important that cables are
lubricated in accordance with manufacturers’ recommendations. Note: in some operating areas and applications
lubrication will cause dust or dirt to stick to the cables, potentially leading to rapid wear and cable drag. These
conditions should be taken into account prior to determining appropriate lubrication practices in each case.
MNZ GUIDELINES - SAFETY BULLETIN ISSUE 17 – SEPTEMBER 2008
• All operators of boats fitted with a cable and pulley type steering system should consult with the cable
manufacturer or supplier of the system to ensure that the pulley (also referred to as sheave) diameter is within
acceptable limits for the wire being used.
• Wherever possible, the recommended sheave/cable ratio should be applied, in preference to the minimum
ratio, when sizing the pulleys for a given wire.
• Cables should be lubricated in accordance with manufacturers’ recommendations.
• Any cable showing signs of significant wear, damage or corrosion should be replaced at the earliest
• Regular inspection of steering gear arrangements is critical to boat safety.
For further information, please contact:
Manager, Safety Management Systems, Maritime New Zealand
Phone: 0508 22 55 22
Fax: (04) 494 1263