Join Date: Sep 2003
Location: Auckland New Zealand
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Re: Calculating true wind
The reality is that the speed and course over ground will give you a closer to real True wind, in other words the wind that you would feel if you are anchored.
But for most sailing applications, and more specifically oddly enough for most racing applications, the instruments produce a "true wind" which is really a wind velocity that is not corrected for current, in other words, the wind that you would feel if you were drifting with the current.
Both are useful tactically but neither are extremely useful for sail trim or even for sail selection.
Sorry, a bit confused.
If the wind instrument is working on its own, then the wind speed you are reading is pure apparent
speed, nothing to do with true. The only time the wind in this condition can be considered true
is if the boat is guaranteed to be stationary i.e. anchored. The true and apparent become one. As soon as the boat begins to move in any direction, the wind is apparent and not true.
Only if a boat speed indicator is working in the same NMEA system with the wind speed and direction can the readings be vectored to produce true wind speed. And the calculation of true wind speed done by the wind instrument and the boat speed indicator working together has no reference to course or heading or any other such dimension.
How do I know this? I had a protracted argument years ago with Sailing Dog who successfully proved that I was wrong and the correct answer is as outlined above.
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