maybe dialed in right..you did not list your boat make and model or your engine..but a 21 pitch prop would probably drop you about 400 RPMs..maybe pick up a few MPH, but lose some hole shot.
50 MPH would be nice, the 19 is right where i need to be for a 5 blade. Wondering where a 3 Blade would put me as far as RPM! its a 99 bayliner 2050 bowrider with a 5.0. mercruiser. Im in the middle of doing a 4BBL conversion on it. At about 6200ft above sea, so the more air i can get into it the better im thinking we will see i like the hole shot of the 5 blade no doubt about that
seems like you would need to go up 2 pitchs to go from 5 blade to 3 blade..maybe a 21 and you should pick up a few MPH. but you will have to experiment..almost impossible to say for sure.
we will see 
