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

[Bushwacker]

I ran a 1975 "115" V-4 (really about 90 hp at prop)on my Seafari with a 13 3/4 x 15" Pitch OMC SST prop that would turn 5400-5500 with a heavy load. Cav plate was 1" above bottom. That same prop should work for you unless motor is mounted too low or is way down on compression/power.