wouter,
gps instantanious speed readings are not as accurate as you would think. And I don't think that a 500m average distance has anything to do with the instantanious speeds that the boats can achieve. So the truth is probably somewhere between 33knots and 25 knots.

If you _really_ want to measure instantanious speed then you need a radar gun. We bought one for our regatta and anyone wishing to test their speed is welcome.
Sandbanks Open Aug 6-7 2005. Prince Edward County, Ontario, Canada. 1 hour drive west of CORK - Kingston.
http://www.westlakesailingclub.com