Attached is an example of the process for doing the prop calculations.
So if you wanted to have your boats top speed be 20 knots, then with a 1.5:1 ratio, you would need a 3 bladed 19x16 prop.
So at idle (say it is 600 RPM) your prop would be turning 400 RPM.
Or if you decide you want to go up to 30 knots at top speed the calculations show you would need a 17x22 size prop.
On our web page there is a nice write up under "The Express" our newsletter on the relationship between weight, HP, RPM, and prop size.
Attached files: AAAD2F0.PDF