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post #6 of Old 04-04-2008
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Speed through the water is an unnecessary and potentially misleading input when calculating true wind speed, just as Negrini is arguing.

Take this example: You're entering the San Francisco Bay with a true following wind of 20 knots. Your boat speed though the water, as derived by a paddle-wheel, is 5 knots. You have a flood current of 5 knots behind you. The apparent wind reading is 10 knots (20 knots minus 5 knots boat speed minus 5 knots current speed).

If you're just reading boat speed through the water, you'd conclude -- incorrectly -- that the true wind speed was 15 knots (10 knots apparent wind plus 5 knots boat speed).

The only way to calculate true wind speed is to know the speed of the boat over ground (SOG) and the true course over ground (COG), and to reconcile this with the apparent wind speed and direction. SOG could be derived from, e.g., a GPS input but certainly not from a paddle-wheel device.

And, yes, in a lake or low-current situation it wouldn't matter much.

And, yes, if the helmsman had both GPS speed info and apparent wind info he/she could calculate true wind speed easily enough. I was in just such a situation last week, when singlehanding with a following wind of up to 28 knots true and a following current of just over a knot. Example: with an apparent wind speed of 18 knots behind me, and a GPS-derived speed over ground of 8.5 knots (including an estimated following current of 1 knot), I could easily estimate the true wind: 18 knots apparent plus 8.5 knots COG = 26.5 knots true.

Why did I care about true wind speed? Easy. Because I was alone, sailing in relatively restricted waters, having to make significant course changes which involved gybing. Jibing a full main on a 40+ foot vessel in 26 knots of wind is no fun, and must be done very carefully if you're not going to break something. So it was very important to me to know that although my apparent wind was, e.g., only 18 knots, the true wind speed was over 26 knots.


Last edited by btrayfors; 04-04-2008 at 10:29 AM.
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