Or - just look around and see what size everybody else is using for your size boat and engine hp. Then find a propeller shop that sells used propellers and find one of about the same size and put it on the boat. Try the boat checking for ability to achieve hull speed
; ability to achieve max rated rpms; and whether the engine is lugging/smoking at cruise
to max speed. Then exchange the propeller for others one size up or down (pitch or diameter) until you find one that allows you to get hull speed
at less than full rated rpms and without smoking. It is a crude system but works if you have the time and can find a friendly prop shop.
- - If you can get a calculated prop size, that would be your starting point versus just asking or looking around. You still need to test the recommended prop for the aspects mentioned above.