Laser310
Well-known member
It's a gate mark, so the VMG position would be the middle of the gate, which would correlate with VMG reducing as they get to the mark and turn away from the centre to either of the marks.
I think I have a definitive answer - VMG is wrt the wind.
see attached screenshots from the augmented reality feed - Prada Cup round Robins, Day 1, UK vs. USA
UK is leading at the top of the beat, nearly at the starboard layline, but still on port
They are probably sailing nearly 90deg from the center of the gate.., if it were 90, their VMG on the mark would be 0
but VMG is 25 and 28 kts in the shots - consistent with measuring VMG wrt the wind, and not consistent with a measurement wrt the gate.
note that the "speed" measurements displayed on the boats don't agree with those at the bottom of the screen.., but this is a minor issue
in the first shot the bottom of the screen gives speed=33.7 and VMG=25.6 - this corresponds to a TWA of about 40deg; Cos ^-1 (25.6/33.7)=40deg