This seems like the only reliable way to go. Speed traps are subject to wind shifts, radar guns are fine, but expensive and not so easy to come by. Why is the data so hard to compile with a GPS? Even a cheap one will give accurate averages and max speeds. Mine read 29 knots max in a big puff on my Hobie 20. We were going up wind, but not too close hauled, on a mountain lake with very flat water. The speedy stretch didn't last long - maybe a quarter mile - but was very wild.