Gill,

Checking VMG is probably not really interesting, during a regatta, like Matt said already. Of course the GPS doesn't know the changes in wind direction.
You would need a wind indicator on you boat and feed that information to the GPS. That could make it accurate and is what they do on the "big" boats. Still waiting for someone to do this for a beach cat, and affordable <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Ncik’s way of getting VMG is quite good, I think, but will take some time.
An other thing we can do is get a track, pick a part with just one tack and 2 legs , with as little changes in direction as possible. GpsAR should calculate the right VMG. Then take the average VMG of both tacks and you should be very close.(if there is no current)

GpsAR takes an average wind direction for the complete track, so VMG can be a fair bit off when the wind shifts during a regatta. So better not take the average of a complete track.

When someone is interested, I’ll take an extra GPS with me to Mumbles.

And while we’re at it, I wouldn’t be too happy if a GPS would be banned. In my case (Garmin foretrex 101) it’s also my starting watch and that works pretty well. And the GPS won’t help you winning a regatta anyway. Just nice for analyzing what happened, after the race and for training purposes.

Geert
Blade 9