I have a 19' aluminum dual console boat with a 02 Merc 125 2 stroke on it. The recommended max rpm is 5250 for the motor, I can definitely get a ways past that with my current prop. It's a 3 blade 19 pitch. It's running about 35-37 mph on flat water @5,000. Could probably get 6,000 out of it if I pegged the throttle... Would a 4 blade 21 be to much of a jump? Same diameter,which I think is 13 I believe.