Online Boat Prop Calculator

[Putershark]

Can be found on this website http://www.rbbi.com/folders/prop/propcalc.htm and to help you use this program (since some many not know all the figures)

1. You can secure the final drive ratio number for "your" engine at most of the OEM's web pages for example, the final drive ratio for my Honda 150 is 2.14 to 1 thus I would enter in 2.14 in that field.
2. Prop Slippage is a rough guess pending which type, number of blades, cupping, and material you may use on your engine with Stainless Steel having the least amount, then composite and finally Aluminum having the greatest amount of slippage... So I would figure 8 to 10% for Slippage or .08, .09 Etc.
3. Then comes prop pitch which should be easy enough to find on your present prop and the really nice feature of this routine, is that you can get a "rough idea" of what a future prop "may" return by making some small changes. Just remember, that any hull will only return so much speed due to hydrodynamic drag, and short of mounting on a F4 Phantom II engine on the stern you are most of the time not going to make large jumps in Top End speed. Also each engine can only swing so much prop and with all things being constant, only by swinging a larger pitch prop at the same RPM, will you gain much Top End Speed.

With this routine though, you can try at home different combos and see if you will gain any returns over your present combo... I would say that for each increase of pitch you try, that if you reduce your WOT RPM by 200 to 500 (200 being if your engine is at or near the MAX HP rating for your hull, 500 if not) then you would have a pretty good guide "if" the change is even worth the money your are about to spend doing such

Enjoy!