Only used the GPS a few times on a catamaran when I lost my start watch (used the timer function for racing), but got 16,4 knots downwind with kite up in 13knots of wind. Use it mostly for windsurfing, 31,24knots in 23knots of wind. It`s too embarrassing to pretend catamarans are fast <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" /> which is why I seldom use it on the cat..
I think I`ll use it more often now, see if I can get the boat over 20knots, which would be quite a feat.
And Rolf, you can download your track, and delete all spikes to get a fairly accurate reading. We use the average of our 5 best 10sec runs as a good indication, after deleting spikes. My biggest spike has been just over 100km/h (54knots), then back to 0 very quickly.. After that wipeout I started saving for an impact vest and helmet.