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Subject: Re: Silicon Sea: Leg 80
From: Noyce, Bill (William.Noyce@XXX.XXX)
Date: Wed Oct 17 2001 - 08:35:19 EDT
> 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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