Will Prop size change rpm as well as pitch?

Hey ya'll. It's me again. 96 evinrude 112spl. My old aluminum prop is pretty beat up been like that since I bought it. It's a 14 x 19 I found a 13x18 stainless in pretty good shape. Right now im running 5200rpm top end . I was wondering if this prop would still be ok?

Typically you'll see about 200RPM change for every 1" difference in pitch. Diameter of the prop doesn't seem to have much of an effect. Quite possibly you'll be fine with the SS 13x18.

Since your original prop was so beat up it's really hard to tell what the true difference will be. If the blades were in really bad shape you could have been cavitating which will make RPM higher than it really should be. That will go along with less bite on the water and reduced performance.

Also, aluminum props flex more than SS, so under load stress they will act like a slightly lower pitch. The SS prop will keep it's shape better and therefore maintain thrust under load.

Your aluminum probably wasn't cupped (??) and the SS probably is (??) Cupping adds the effect of more pitch, so your end result might be about the same RPM, with maybe more speed (another ??). Only way to know is to try it.

The stainless is also heavier you should be fine. For effeciency and performance you dont want to run Wot for extended periods anyways. Find the point on the control handle where the spark advance stops moving and make a mark on the control box. You will extend engine life just running at max spark advance.

My guess is the 18 will run you to 5500. But what do I know? Just a wild guess. I only repair props and lower units, don't really build many from scratch. Let us know. You are gonna see more snap, but just trim and back down the throttle. No more "balls to the walls", Tim. Tim

timguy said:

My guess is the 18 will run you to 5500.

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I was thinking around the same range. The performance numbers with the beat up old prop do muddy the waters a bit.

I've found when it comes to props, guess as close as you can, then slap it on, go for WOT run and see what it does.