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    Re: Leg 83
    From: Hal Mueller
    Date: 2002 Sep 24, 14:11 -0700

    At 3:30 PM -0400 9/24/02, Arthur Pearson wrote:
    >I did use a vector diagram to figure the adjustment for current which was a
    >good dust off of something I haven't done in while.  I am curious if there
    >are formulas that could be programmed that would take as inputs the plotted
    >(or calculated) course, the speed through the water and the set and drift of
    >the current and which would then ouput the adjustment needed (or course to
    >steer) to compensate for the current and make good the plotted course.
    The vector triangle you drew has the core of the solution.  Draw something like
    where CB is your own heading/speed, BA is the current
    direction/speed, and CA is your resulting course/speed over the
    bottom (VMG).  Various applications of the law of sines and law of
    cosines will enable you to solve for the cases you run into, but
    additional logic, or a haversine solution, is needed to get the
    signs/quadrants right.
    For the typical "course-to-steer" problem, you know the length of CB
    (speed through the water), angle/length of BA (set/drift), and the
    angle of CA (bearing to destination).  You're interested in finding
    the angle of CB (course to steer), but once you've done that you may
    as well also find length of CA (speed made good over the bottom).

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