I just installed one on my daysailer and it works great. Speed is constantly updated and you can see the differences trim and weight make in your speed. Also does elapsed time, distance, and average speed. It''s easy to use and easy to read . . .
I agree with Harpooner - have used one on my Lightning for speed/trim testing. Worked great until my 4 year old dropped it off the dock (they do not float). I now use a handdheld GPS which cost less than a new Speedtech & has other features