NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Silicon Sea: Leg 80
From: Bill Noyce
Date: 2001 Oct 17, 6:35 AM
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...