To go with the spirit of the GPS in Rowing series, I would like to point out another of Dr. Kleshnev's newsletters. In the April 2004 Rowing Biomechanics Newsletter he presented a simple gadget for measuring velocity - a straw on the stern of the boat! Give it a read. I think it's safe to say that GPS and Impeller both do a better job - but it wins on price! In fact it also beats GPS in a current.