B&G Hydra 2000, User Manual

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Hydra 2000 User Manual
Part 3 - Calibration
HB-0844-02
3-9
3.3 APPARENT WIND SPEED/ANGLE CALIBRATION
3.3.1 Principles of Wind Speed/Angle Calibration
Wind shear and wind gradient can be a problem when calibrating
apparent Wind Speed and Angle. It is easy to see how shear can
affect the Apparent Wind Angle; no sooner have you set it up than
the shear changes and everything is out again. This can lead to a
circular situation if you are not careful. The best solution is to do
your calibration on a day when the shear is minimal and thereafter
leave it as an indicator of the Wind Angle at the masthead, always
remembering that this is not necessarily the wind angle that you
are sailing at.
How do you know the shear is minimal? If you are finding it easier
to attain speed on one tack than the other, then there is shear. A
good look at the general weather conditions is also helpful. Do not
calibrate in building sea breezes!
What of wind gradient then? Well, this is the biggest culprit for
getting true wind speeds accused of gross inaccuracy. The
problem is that most people use the wind speed as a measure of
the pressure or force of the wind, which it is not. It is a measure of
the wind speed at the top of the mast, and that is all. If it is 12kts at
the top of the mast and only 4kts at the water, then the breeze will
feel a lot softer and provide less power for the rig than if the
breeze is twelve knots all the way down to the water.
This is why we recommend that the last thing you touch is the
Apparent Wind Speed. It is calibrated in the factory where wind
tunnel calibrated units are available, and apparent inaccuracies are
more likely attributable to effects such as wind gradient, rather
than to a basic calibration problem.