1) Determine the Max RPM of you Cummins 370
2) Take the boat out and get up on plane
3) Determine if you can reach the engine's max rpm.
Going over max RPM indicates that you are under pitched.
Failing to reach max RPM indicates that you are over pitched.
Admittedly, diesels make this a little weirder since the max rpm range for a engine can vary greatly.
The pitch is stamped on the prop hubs.
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