Only 4500 -4800 RPMS- 90 HP Johnson on 20'SF

[Bushwacker]

In addition to a 13.75 x 15P prop, I also tried 13.5 x 17P and 13.25 x 19P aluminum props on my old 90hp-at-the-prop "115", (1975 vintage) on my Seafari. It turned the 17P up to about 5200; 19P only turned about 4900-5000. Best prop was 13.75 x 15P, which turned 5500-5600 in either stainless or aluminum versions. If the blades are badly chewed up on the props your testing, you've lost a bunch of efficiency and may be getting misleading results. I know several folks running 115's on 19-20' hulls and they're all running 15" pitch props! There's probably a pretty strong message somewhere in all that data!