If I get a 25 pitch hydro thats 14.5 diameter or a 24 pitch hydro thats 15.5 diameter would the rpm be the same? I have to think it would, and the 25 would be a bit faster. Any other opinions on this one?

The RPM should not be the same. With all things equal you should expect about a 150-200 rpm difference in rpm with each 1/4" diameter change.

There are many factors that determine diameter. Each boat and set up may require a different diameter. The number of blades on the prop will also determine the diameter of the prop. The only way to find the optimum diameter is to test by reducing diameter until you have gone too far, then go back to the optimum diameter.

Matt

Matt
I'll probably need some help with my 40' Jag cat. But the thought of trimming the #6 props too far is a bit scary. Maybe your experience can help here.
Thanks
John

Matt, would changing prop diameter affect the stern/bow lift characteristics of a given prop? thanks Chuck

As you decrease diameter, you decrease stern lift.

Julie

Dueclaws,

It's very common to reduce the diameter of the speedmaster props. We take great care when doing it to make sure that the desired result is achieved. There are some calculations that help to determine if reducing the diameter is the right course of action. Other factors such as the use of the boat and current and desired handling properties also play into diameter adjustments.

Matt

you can figure 200 rpm higher in rpm per 1/4 reduction ,this would be a starting referance. This may require more than once to to hit the exact mark,But once you go to far, it can not be put back on .Darin