Originally Posted by BootyPirate (Post 2125245)

Well the nice thing about figuring out props is that is is all simple math. (Pitch x RPM) / (Reduction x 1056) = MPH.

I'll assume you have an Alpha 1 with a 2.0:1 Gear ratio and to make it easier we will use no prop slip. Regardless of the boat the distance traveled with is the same with each RPM, (again assuming you have no slip). You can also add +1" to pitch if the prop is cupped.

19P@4200 = 37.4 MPH
19P@4500 = 40.1 MPH
19P@4700 = 41.9 MPH

21P@4200 = 41.8 MPH
21P@4500 = 45.3 MPH

Which is why a 19P with the extra 500 RPM 4700 vs. 4200 with the 21P give you gain no increase in speed. From my experience there is no substitue for cubic inches when trying to make your boat faster. The time and money it takes trying to hot rod a small block in a boat is not worth the minimal 2 -3MPH gain in speed. Even if you go big block you are only talking about a 7-8 MPH gain. I learned to leave it alone and be content with going slow with a mostly stock 260HP engine. Sure it will only do 40 MPH, but I enjoy much more spending that $$ for gas, beer, towables for the kids and skimpy bikinis for the Mrs. :D

I'm assuming you are doing you speed readings from a GPS?