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					             Set & Drift
             Capt. Dan ASA 105




March 2006
            Two situations
   Known start point and known fix
   Known set and drift
       Calculating Set/Drift
           Plot Dead Reckoning
0815




                                 0945
         Calculating Set/Drift
   At 0850 you get a good 2-LOP fix
     You plot your position
     And you plot your DR position for the
      same time
       Calculating Set/Drift
0815

                     Plot DR for fix time

                      0850


        Plot fix   X 0850
                                            0945
       Calculating Set/Drift
0815



                       0850


                   X 0850
                                                 0945


  Measure the heading (True) from DR pt to Fix

   190º T
        That’s the set - 190º T
       Calculating Set/Drift
0815



                        0850


                    X 0850
                                      0945
Measure the distance from DR to Fix


                    .85 nm
        Calculating Set/Drift
 0815



                              0850


                      X 0850

        Calculate the speed                     0945

              0815 to 0850 = 35 min

 60x.85nm             =              51   =   1.47
Speed*35 min              Speed*35 min
                   Review
   Plot DR
   When you have a fix, also calculate your DR
    for the time of the fix - where you should
    have been
   Measure the heading from the DR to the fix
    - that’s set. It’s always TRUE heading
   Measure the distance between the DR and
    the fix
   Using 60D =ST, calculate speed - that’s
    DRIFT
Not So Hard
          Known Set & Drift
   If we know the set and drift - either
    from a source such as the Tide and
    Current tables or from our own
    calculation
   We can figure out how to get to our
    destination via the most direct route.
   I.E. we can correct for Set and Drift
    and calculate the effect on SOG.
                 Consider this
   Boat speed = 0
   Set =090
   Drift =2
                                  1                 2




        In an hour with 0 boat speed, we end up two miles East
              Consider this
   If our destination were to the East,
    we’d get there a lot faster.
   Let’s say we had a three knot boat
    speed.
   We scribe one hours worth of boat
    speed from the end of the S&D vector
          1     2                   3




      SOG is 3+2=5
   What would SOG be if we were headed
    West?
   We mark an hour’s worth of set and
    drift, here 2 nm east
                        1     2
Then we scribe boat speed starting from
 the end of the set/drift vector.

        3            1      2
   SOG is the distance we travel from
    start in one hour.

           3             1     2




               1nm
Scribe boat speed from the
  end of the S&D vector
Measure SOG from DR start
       to end of DR.
   How about when we’re not going the
    same or exactly the opposite direction
    of Set?
   Let’s say the S&D is NE and we want
    to go 090º True.
     Set 045ºTrue/Drift 1.5 kt
   Scribe an hours worth of set and drift
Set 045ºTrue/Drift 1.5 kt
     Course: 090T
    Boat Speed 3kts.

     1.5
Set Dividers at 3 (boat speed)
Scribe arc from end of S/D to
            course

       1.5
 Measure the red line hdg
That’s the corrected heading
        for this S&D

       1.5
Measure the distance from
start to the intersection of
        boat speed.
        That’s SOG
      1.5




  3.8 knots
                    So…
   We’re making about 3.8 knots.
   If we have to go 3.8 miles on this
    course, it will take an hour.
   We will steer the same corrected
    course whether it is half an hour or
    two hours…. Assuming the current
    doesn’t change.
    Review: With known set and
               drift
   Scribe the S & D vector
   From end of S & D vector, scribe Boat
    Speed line to DR course.
   Heading of line is True heading
    corrected for S & D
   Distance from start point of DR to
    intersection of True course line is SOG
            Parting comment
   Current comprises:
     Set and Drift
     Steering errors

     Leeway

     Anything else that drives us off our
      calculated DR.
 What do you figure is the
biggest culprit in driving a
boat off its intended track?

				
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