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    Re: Silicon Sea: Leg 80
    From: Dan Hogan
    Date: 2001 Oct 17, 9:12 AM

    On 17 Oct 2001, at 8:35, Noyce, Bill wrote:
    
    > > 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.
    
    Also we are on a "northerly" course.
    
    > 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...
    
    The big problem for a sailboat is the wind and current. That makes it very
    hard to follow a Great Circle and many times a non-GC route is faster.
    
    
    
    
    Cheers
    -Dan-
    

       
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