I have a 1982 Howard whaler with the 5.7 mercruiser not sure the ratio of the drive , and it currently has a 15.25 x 15p aluminium prop on it , i can get it to turn 4500 -4600 rpm and thats about 41 mph (per gps ) and cruises about 31- 33 mph at about 3600 rpm , seems a little high to me ...
