RPM vs Pitch-any equation?

[newcdnboater]

Hi, I have a 20' Imperial with a 1987 3.0l Alpha one drive which currently has a 15x21 aluminum prop. The best WOT RPM I have hit is 3100rpm. The engine, ignition, and drive are all recently tuned and are finally working great. Is there an equation to determine the pitch to acquire the 4600rpm I should be able to hit? My top speed right now is about 32mph which is fine but I'm worried the engine may be labouring too much with this high of a pitch. I'm thinking a 19 pitch but should I go even lower than that? I don't ever plan to pull a skier.

 

[Dhadley]

Re: RPM vs Pitch-any equation?

Are you sure the tach is correct?

Rule of thumb - when comparing props of the same make and style you'll gain about 200 rpm for each inch of pitch you drop. If you compare different makes or styles, that rule is out the window.

That being said, you're looking at dropping 7" of pitch to get to 4500. Doesn't seem right especially given the speed you get now.

Are you sure the tach is correct?