B&G H2000 Pilot *DISCONTINUED* - Manual (Page 90)

HB-0844-04
3-25
3.6.1 True Wind Direction Calibration
Why you need to have further calibration for True Wind Direction will become
clear when you go sailing. The true wind might vary in direction from tack to
tack, independently of any wind shifts. This phenomenon has come to be
known as the true wind “tacking”. This happens because the true wind angle
that the instruments are calculating is wrong.
Fig 3.4 - True Wind Direction
The reason for this is a variety of errors when calculating the true wind angle
from the apparent wind angle, one of the largest of which is Upwash, an
aerodynamic effect which is shown in Fig 3.5
The apparent wind that the instruments measure is actually deflected by the
rig and sails from the 'real' apparent wind angle that we need to use to
calculate the true wind. Add to this the various twisting effects of the mast
and the Masthead Unit.
It is easy to see the true wind direction 'tack' as little as 2-3 degrees, which
would mean the correction factors being as accurate as 0.5°, or about 1%.
The Hydra 2000 has a simple, easy to use method for correcting these
errors.
The problem stems from the true wind direction 'tacking' as the boat
manoeuvres from tack to tack. We need to know the error that the true wind
suffers in any manoeuvre, be it a normal upwind tack, a reach to reach tack,
or a downwind gybe.