Hi Gareth,

I have used the Speedmate on my windsurfer. The impeller lasted 4 years in the Chesapeake Bay, Long Island sound, North Carolina surf and even the East River in the Bronx. It was great for getting a continuous speed readout that is not effected by current.
Never tried the catamaran version.

Some hand held GPS units can give spurious results for maximum speed. If the GPS is unable to get a good fix for a couple readings when you are sailing slowly, and then gets a good position fix when you are sailing fast it will report a false-high maximum speed reading. If you read the same speed for 10 seconds or so while sailing you can be confident that your speed (over ground at least) is correct. Just don’t trust the “max speed” stored in memory when you get to shore.

I also have a Timex GPS speed/time/distance watch that seems not to have this problem. The watch is intended for running, and the engineers must have designed in some better filtering. The unit consists of a water resistant watch portion and a (3x3x1in) receiver that is “weather” resistant. The watch only reports speed and distance – not position or velocity.

I always put my handheld GPS or the Timex receiver in a vinyl dry bag. The dry bag clips to my life jacket on the cat and to my booms on the windsurfer. Just for reference, zip lock freezer bags do not seem to be even water resistant. I killed an FRS radio that way.

Happy sailing (Tw + Ta -> 100 deg f spring is almost here!)
-colin