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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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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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