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From: Bob Young (no email)
Date: Mon Apr 02 2001 - 19:16:47 EDT
I like to keep things as simple as possible. Assuming that distance traveled
and speed over ground are best handled by a GPS receiver, all we need is a
simple way to measure speed through water. The taffrail log (Walker) only
measures distance run, and I do not like trailing a line continuously, as it
will sooner or later inevitably get tangled in something. The through-hull
paddle wheel type devices are expensive and I have never found one to work
well. They operate in the boundary layer of water, which is influenced by the
proximity of the hull, so a correction factor must be applied. This factor
changes with varying speed, sea state, and angle of heel, and all it takes is
one piece of weed and the thing stops turning altogether.
Since a nautical mile is about 6000 feet, at a speed of one knot we travel 100
feet in one minute. So, all that is required is 100 feet of light line, and an
object which floats in water, but well submerged so it exerts reasonable drag.
A block of heavy wood works fine as a drogue. Tie one end of the line to the
boat, and the other to the drogue, and flake or coil the line loosely on deck.
As a timing device, the stop watch built into most cheap digital wristwatches
works well.
Drop the drogue from the stern into the water, and simultaneously start the
timer. When all the line has run out, and it snatches taut, stop the watch.
Now simply divide 60 by the elapsed time in seconds, and the result is your
speed in knots. For example, if all the line runs out in 12 seconds, your
speed is 60/12=5 knots.
Recovering this device is easy enough, but one might employ an old fishing reel
mounted on the stern pulpit to make it even easier.
-- Bob Young, on 04/02/2001
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