The best method of speed detection would be a radar gun..... as staed in previous posts, water pressure speedos and paddlewheel speedos are inaccurate at best. GPS units are accurate, but not as accurate as you may think. In order for the GPS to get an accurate speed readout, it must travel for some distance.... most GPS's are accurate to 15' (last I heard), and every time you turn, the speed that you are reading off a GPS is actually velocity, not speed..... you could be running at 50MPH in a turn, but the GPS will show far less because you are changing course. That being said, if the GPS is fluctuating a bit during your short 75 yard run, the margin of error is greater than if you were to travel a mile with it. If you have several boats and are serious about the hobby, I would reccomend investing in a radar gun. If you are part of a club where you are at, you could split the cost with other club members.