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    DR thread from Nov-Dec '04
    From: Doug Royer
    Date: 2005 Jan 17, 15:47 -0800

    Being away I missed this topic and am only now going back over the archives
    and catching up.The thread discussed DR and/or EP and/or MPP.The following
    wasn't touched in the subject and I wish to bring it to the attention of
    those interested.
    The following is NOT leeway.Leeway is a whole other phenominon.The following
    is a method used by sea going personnel to "accurise" the actual trackline
    taking into consideration wind generated currents while underway and can be
    obtained with little equipment other than a pencil,parrallel rule and a
    relative motion sheet.A manouvering board makes it easier(quicker)but is not
    This is amazingly accurate in forcasting the trackline(COG) due to offshore
    wind driven currents not included on a basic DR plot trackline.This won't
    result in an EP but will give the mariner a way to keep closer to the
    calculated trackline.On vessels I've served on all have a wind speed
    indicator to at the least measure the relative wind speed.One can use the
    Beuford Scale to closely estimate the wind speed in lue of an electrical or
    mechanical indicator.
    You will need to convert relative motion to true motion.By that I mean
    convert relative wind to true wind direction using using a relative motion
    vector.The resulting drift will be a function of the true wind speed and the
    resulting set will be a function of the true wind direction.
    The drift will be 2% of the true wind speed.
    The set will be 40 * to the right of the true wind direction(Northern Hem.)
    and reversed when used in the S.H.
    The true wind speed is 20 knots sustained with the wind coming dead into the
    starboard beam.
    The ship's head is 270 * T .We'll make the true wind direction 360 *  T to
    meet the above condition.
    The wind generated drift will be 0.2 knot.
    The wind generated set will be 320 * T .  40 * to the right of the true wind
    direction: 360 * -  40 *  =  320 *
    If not taken into consideration you can see that in a few hrs run the vessel
    will not be on the trackline by a good deal.Say we run for 6 hrs under these
    conditions.The vessel will be about 1.2 nm to the leeward of the computed
    DR  with just this phenomina.Remember,this is not leeway but wind generated
    set and drift.This quantity must be added to or subtracted from the
    leeway.It must also be added to or subtracted from any major oceanic current
    the vessel is passing through.
    The above will give a more accurate DR because the mariner will be able to
    compensate for the set and drift generated.It is suggested that every 4 to 8
    hrs the mariner updates the data to correct/verify the conditions and
    recompensate as needed.
    A word about strong,sustained winds coming from either the port or starboard
    QUARTERS.While the above is still true the winds coming into the quarters
    tend to make the bow shear or pull to WINDWARD !! This can further amplify
    or decrease the amount of error off the trackline generated by wind
    depending on the ship's heading.

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