NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Silicon Sea: Leg 80
From: Dan Hogan
Date: 2001 Oct 17, 9:12 AM
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-