I have used a Garmin Foretrex GPS for a number of years. The top speed I get is around 18mph in Una Rig, F5, Flat sea and normally on the reach, current is variable but then I am a mite heavy
. Waves on very strong wind days normally prevent too high a speed being attained before sailor talent runs out
. I think in the very strong wind gusts (F6+) on a broad reach during a mostly uncontrolled sleigh ride the top speed edges close to 22mph but I don't look at the device then
My feeling is the top sailors tend to travel 1-2mph faster per leg of the course combined with a very smooth sailing style to keep speed more constant and that the hull speed of the boat is approx. 21mph.
Martin S.