The prop is part of your gear train. Engine RPM/lower unit reduction ratio x prop pitch, assuming equal prop slip and identical props other than pitch will give you the same number. I never did figure out why things like the 90 hp Merc runs a 2.33:1 gearbox and the 115 runs a 2.0 on the same lower unit with the engine RPMs spec'd the same in some cases. I know they advertise the 90 as the work engine and the 115 the play offering. Well that picture in my Avatar is a 90 triple 2 stroker and the speedometer is kissing 50 at 5600 RPM Ballistic 13+ x 24 XP 3 blade SS. I call that playing, not working!
