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    Problems with SS Leg 68
    From: Peter Fogg
    Date: 2003 Mar 16, 17:21 +1100

    Things are quiet enough at the Nav List at the moment (warm weather is
    finally returning to the frozen north!) so I would like to enlist the
    help of the collective brain's trust with problems I'm having with an
    old Silicon Sea problem Leg No 68. It begins:
    
    Sight Reduction and
    Dead-Reckoning problem Leg 68 080700 Test your knowledge!
    ---------------------- ------ ------ --------------------
    
    Our last position FIX was at (3d 00.0'N 30d 59,9'W {at} 12/05/2000 at
    20:30:00UTC). Our last DR position was at a Walker Log reading of 614.4
    mi
    (15/05/2000 at 05:30:00ZT {at}XXX.XXX We are heading for the
    island of Barbados(13d 10.0'N 59d 20.0'W). Speed 10.5Kts, Current
    Set 280.0d, Drift 0.7Kts. Var 19dW. Dev 1.5dE.
    
    The weather is deteriorating. Mares Tails clouds have changed to Cirrus
    clouds which have been thickening and lowering. The wind is increasing
    and
    is slowly backing.
    
    At a Walker Log (which reads a maximum of 999.9) reading of 919.1 mi we
    manage a quick sight of the Sun through a short break in the clouds.
    
       Body        UT             Hs
       Sun LL 10:31:12     66d 10.2'
    
    Later that evening at a Log reading of 004.2 mi we manage a quick sight
    of
    an unknown moderately bright Star through a quick break in the clouds.
    
            Body     Zone Time(ZT)   Hs              Steering Compass
    Bearing
            Unknown 18:31:12        48d 29.0'         316.0d
    
    1) What is the Running FIX(RFIX) position?
    -- ---------------------------------------
    and the answer given is:
    
    1) RFIX= 07d 39.8'N  42d 30.2'W
    
    To begin with the true course of 289.8d for Barbados was calculated.
    Then the DR position 614.4nm along that course; N06d 29' W040d 39'. This
    was cross checked by dividing the distance by elapsed time, this gave an
    actual speed of 10.24k.
    
    Then a further 304.7nm (919.1 less 614.4) later, still at 289.8dT
    (checked by calculating the course from the DR pos. to Barbados) the
    calculated DR is N08d 13'  W045d 28' for the Sun LL sight. Again the
    distance 304.7 divided by speed 10.5k gives an elapsed time of just over
    29 hours, an ETA at the new DR of 13:31 UT on 16 May 2000.
    
    First problem: the time for the Sun LL sight is given as 10:31 UT. But
    the sight reduction for this time yields an intercept of T2293. The ZT
    would be 07:31 but the sun's altitude given is much too high for this
    local time.
    So it must be 10:31 ZT, 13:31 UT which yields an intercept of T8.
    
    85.1nm further, still at 289.8dT, the DR is N08d 42'  W046d 49'. The
    unknown body seems to be Pollux, a few different methods (SHA 249d 19',
    Dec 25d 14' LHA 150d 53') all confirm that this is the body, none other
    seems suitable. However this time (18:31:12 ZT Tues 16 May 2000) the
    intercept is A227 with an azimuth of 305.5d. Working backwards in the
    direction of that intercept (227nm {at} 125.5d) gives an EP of N06d 30'
    W043d 43'.
    
    What I can't understand is the RFIX answer of 07d 39.8'N  42d 30.2'W.
    Using any position near this as a DR, let alone one 85nm to the ESE,
    gives an increasingly inaccurate Sun result. In any case, I can't see
    how so much error has crept into the DR position. Its as if since the
    sun sight the boat's direction has been reversed but there is no hint of
    this. I wonder at the coincidence of the minutes and seconds being the
    same for both sights. However using the answer as the unknown's DR gives
    an intercept of T20.
    
    Call me baffled. Does anyone else enjoy working these problems?
    
    
    

       
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