True and Apparent Wind

DW_Swan42On the subject of instruments, please remember that the TRUE WIND SPEED and ANGLE on modern instruments is a mathematical result of instantaneous calculations in the onboard small computer

Remember – the only real information the instruments have to work on is

BOAT SPEED – from the impeller transducer in the hull.

APPARENT WIND SPEED – from the impeller transducer at the mast head

APPARENT WIND ANGLE – from the vane transducer at the mast head.

Therefore:

A boat motoring into no wind at 5 knots will have a AWS of 5 knots, and a AWA of 0 degrees.

A boat motoring directly into a 5 knot breeze will have a AWS of 10 knots, and a AWA of 0 degrees.

A boat motoring directly downwind from a 5 knot breeze will have a AWS of 0 knots, and a AWA of 0 degrees.

A boat motoring at 90 degrees to a breeze of 5 knots will have a AWS of 5 knots, and a AWA of 45 degrees.

Using this info, a vector calculation is done – but it is NOT always correct!

Tide can profoundly change the calculation, making the boat actually faster or slower through the wind and therefore the TRUE calculated wind speed lower or greater (respectively) and the calculated angles also.

Heeling changes the AWA progressively – if a boat was nearly flat to the water, the vane will “feel”the wind as directly ahead.

Waves speed up the masthead, and therefore the impeller, as the boat pitches forward, and slows it as the bow rises. This affects the AWS and therefore the TWS and TWA.

This gives the tactician in particular a problem in anticipating the downwind leg AWS and AWA, therefore what spinnaker is appropriate, and the likely course to sail.

TRUE WIND information is very useful, but the sails only feel the APPARENT WIND, and for this reason sail trimmers prefer to have the APPARENT WIND info as well.