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    Re: Silicon Sea Leg 72
    From: Bill Noyce
    Date: 2001 May 29, 16:13 EDT

    > > > 4)  What is the Compass Course(CC)/Course-to-Steer for Aruba Gap
    > > >     from the DR position?
    > > > --  ------------------------------------------------------------
    > >
    > > Using Law of Sines, I find I need to adjust my course North
    > > about 1.5 degrees.
    > >   CC = 283.4 + 1.5 + 9 = 293.9d
    >
    > Mmmm..Barely OK. Try a current vector diagram.
    I drew a diagram to get the orientation of the pieces right, but I
    have a hard time measuring the resulting course adjustment, because
    it's so small.  Thus the attempt at a 'digital' method.
    TC=283.4, Set=265.0, angle between = 18.4 degrees
      sin 18.4 / speed = sin adj / drift
      sin 18.4 / 8.5 = sin adj / 0.7
    I did this with log tables before; doing it on the computer now
    says adj = 1.490 degrees.  The diagram indicates it needs to be
    added to the course.  Is there something inherently wrong with
    this approach, or have I made a slip somewhere?
    > >
    > > Mid-latitude using 13d 42'
    > > dLat = 15' N  dLon = 183d W  dep = 177.8 W
    > > TC = 274.6d  dist = 178.3 nm
    >
    > OK. Be aware that above 500 miles at 90d/270d Mid-Latitude can give an
    error.
    > Above 1200 mils it gets unreliable.
    I saw a discussion initiated by Sam Chan about Mercator sailing
    near 90 or 270, where the problem arises from rounding the course
    before using dLat/cos(C) to compute the total distance.  Is this
    a similar problem?  In Mid-Latitude sailing, where we already
    have dLat and departure, the total distance can be computed either
    as dLat/cos(C) or as departure/sin(C), so we don't have to divide
    by a tiny rounded number.
    Or are you talking about the error in distance that arises from
    assuming a spherical earth?  What method would be
    better? I assumed we weren't planning to sail a great-circle
    course here.
    

       
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