B&G H2000 Pilot *DISCONTINUED* - Manual (Page 99)

HB-0845-06 99
MHU
Figure 11 - Upwash
In Figure 11 the wind that the instruments measure is
actually deflected from the 'real' wind angle. Add to this the
various twisting effects of the mast and the Masthead Unit,
and we get some idea of the problems involved. The hardest
part is that 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 degree, or about 1%. For
any particular windspeed the correction needed for all these
errors have to be different from day to day, not least because
of the problems of wind gradient we discussed earlier.
As we have seen 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 tacking upwind, a reach-to-reach tack, or
gybing downwind. Once you know the error, and the
windspeed you had at the time, then we can enter it as a
correction into a table of corrections similar to that shown in
Table 4 - Example of True Wind Angle Correction Table.