I only know enough to be dangerous. The local prop guy doesn't allow testing a prop before buying so I have to have a pretty good idea when I start out. I have a 1986 Johnson 120 hp and have the a prop that came with the boat plus one that I bought and have had repitched. The orignal is a 14-1/2x19 (unknown brand) that ran at 5300 WOT and the newer is a 15x17 Michigan that ran at 5500 rpm then repitched to 15x17 and ran 5600 rpm. The boat speed (gps) was 41 and 43. <br />So I know the limiter is a factor and to calculate the theoretical speed I need to know the gear ratio.<br /><br />Thanks!
