B&G H2000 Pilot *DISCONTINUED* - Manual (Page 95)

HB-0845-06 95
This creates a problem for the two things we are about to try
to calibrate, measured wind speed and measured wind angle.
It is easy to see how shear can affect the measured 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 one is not careful, and
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 get speed on one tack than the other for no obvious
reasons, then there is likely to be shear. A good look at the
general weather conditions is also helpful. Do not calibrate
in building sea breezes.
Wind gradient 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, 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 12 knots at the top
of the mast and only 4 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.
There are other signs that can help get a feel for the pressure
on the rig. One of the most important of these is the heel
angle; it is no bad thing to have an idea of how much heel
you normally have in any given windspeed. Target boat
speeds can also provide valuable information as to the wind
gradient. The target is read from a polar table which only
knows about one average wind condition, it does not know
if the wind has a strong gradient or none at all. So next time
you are having trouble reaching the target speeds, think
about the wind gradient and whether or not it is a soft or
heavy breeze, and use the input to help sail the boat. The
information from the instruments is generally useful - it just
needs rather more interpreting than it sometimes gets.