generic prop question

[rockyrude]

I am looking for a new prop for my 69 evinrude 115 and finding something intersting, as the pitch goes down, the diameter goes up. Doesn't this have the effect of moving the same amount of water?

 

[kynolan2183]

Re: generic prop question

On a sump pump I would say yes and a prop is a pump..... but
it is the rate of pump we are wanting on a boat prop.

The limiting factor is WOT RPM. At 15 pitch and 5000 rpm max you can only pump X water. However your motor might be able to Over Rev.
While a 19 pitch at 5000 rpm can't Over Rev, thus you are pumping more water... and more pounds of thrust.

 

[rndn]

Re: generic prop question

The diamter does not have much of an effect on the performance. Much much more important is the rpm's your motor reaches at wide open throttle (WOT). It is imperative for the health of your motor that it be allowed to reach very near the upper rpm limit as set by the manufacturer. If the prop is a 15" pitch it moves 15" of water for every revolution, it doesn't matter if it a 12" diameter or 14" diamter. These number represent zero slip which can't happen but the theory is the same.