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    Re: Silicon Sea: Leg 80
    From: Bill Noyce
    Date: 2001 Oct 17, 6:35 AM

    > OK, but Great Circle is not of much use since on a small sailboat it
    is just
    > about impossible to sail it. But you have an idea of how many days to
    go. Try
    > using Mercator/Rhumbline calculations, the distance is a bit far for
    accuracy
    > from Mid-Latitude.
    
    Here's an attempt at Mercator sailing.  I used a spherical
    approximation,
    not an ellipsoid:  MP = 7915.7 * log10( tan( Lat/2 + 45d ) )
    
    From   4d  9.6' N   MP =  250.2       111d 34' W
    To    19d 30.0' N   MP = 1193.6       154d 45' W
      dLat = 920.4' N  dMP =  943.4 N  dLo = 2591' W
    
    TC = atan(dLo/dMP)  =  290.0d
    Dist = dLat/cos(TC) = 2690.1 nmi
    
    Compared to the GC distance of 2687.4, I'll agree it's not worth the
    aggravation to try to save 3 miles.  I assume this is because we're
    at a fairly low latitude.
    
    If we were far enough from the equator for it to make a difference,
    I get the impression that the normal approach is to plot a few
    waypoints along the GC course, then sail the rhumbline from point to
    point.  With waypoints plotted a day or two apart, we would get
    essentially all the benefit of the shorter distance, while still
    being able to sail a "constant" compass course for long stretches.
    And we'd be adjusting our heading based on new fixes from time to
    time anyway. I assume a wobbly approximation to a GC course is still
    shorter than an equally wobbly approximation to the rhumbline...
    

       
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