I would not rely on a hand held GPS to be very accurate with speed. I don't know why but for some reason if I run across the street with mine, held out in front, aimed up to get a good signal, it says I did 54 mph! I ran from the house to the beach one time, 50 yards, it said 35 mph! (no, I'm not that fast)

I have a much more expensive unit for aircraft (also by Garmin, like my handheld) that is very accurate on speed, it never puts out wild numbers, but it has a much better antenae system. I think that's the problem with the hand helds, the antenae don't get good signals all the time and that is reflected in bogus speeds. We need to develop a better antenae, maybe one that you could stick to the top of the hull forward of the beam, that would eliminate the mast tip motion problems.