On Fri, 08 Apr 2005 07:17:11 +1000, Who Me
wrote:

Kees Verruijt wrote:


The ST40 computes true wind angle/speed when it knows speed thru water,
not when it knows speed over ground.

I did fear this was the case. Why do they make the distinction? Surely
if one is not available, but the other is, it could use boat speed over
ground to make the same computation.


The solution is simple (but probably not what you want to hear): install
an STx0 speed/log instrument.

You're right, I didn't want to hear this, as I know no way of making my
GPS output speed over water, rather than speed over ground, and non GPS
based speed/log information is affected by tidal flows, etc.

The usual reason for the true wind calculation is to let you know
where the wind will be after the next mark rounding. You want the true
wind relative to the water for this purpose, and that is why the
instruments use the signals they do.

You could certainly retag the gps word to simulate a log, but the
result is only equivalent when there is no current.

The calculation is extremely sensitive to wind speed and boat speed
calibration, which is difficult to do well enough to make sensible
results where I live (NYC). There is never a total slack long enough.



Rodney Myrvaagnes NYC J36 Gjo/a


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