Originally Posted by kubcat
The ratio was from my manual. Using 0.5 instead of 2 as the gear ratio I get prop slip of 72%.
Your prop is travelling:
5050 rpm divided by reduction of 2 = 2525 rpm at the prop shaft
2525 rpm * 17 in/rev divided by 12 in/ft = 3577 ft/min (no slip accounted for here yet)
Your boat is travelling:
39 nm/hr divided by 60 min/hr = 0.65 nm/min
0.65 nm/min * 6076 ft/nm = 3949 ft/min
Slip would be about negative 10%, i.e. your boat is travelling faster than a 17" prop can possibly drive it. So it appears that one of your inputs its incorrect.
I don't really know where to start, but I would start with confirming the pin settings in the tach are correct. Then test on a second tach. Then double check the prop pitch (not just what's stamped on it). Lastly look into the gear ratio.