prop size

[Mr. BrownTrout]

On another thread some were talking about prop sizes for running over around Central Oregon. Here's my question, my boat came from right around Randall Washington, not real high up but not sea level either. Now I have the boat here in Boise, Idaho. It came with an aluminum prop 13 1/4x 15 and a 19 pitch SS prop. The motor is a 70 hp evinrude on a 15' vee hull fiberglass boat. Over here all I can get is 28mph with the aluminum prop and 25mph with the SS prop. My RPM's are 4000 with the SS prop and 4200-4400RPM with the aluminum prop. Would it be better to go down in pitch or overall diameter and of course save the props it came with in case I ever make it back over to my haunting grounds in Astoria for sturgeon.

[MattPark]

You want your wot rpm to be within the range the engine manufacturer specified. What year engine is that?

[Mr. BrownTrout]

Its a 1984 70hp evinrude. I thought I read somewhere that the max rpm was 5000-5500rpm???? I could be wrong and probably am wrong.

[Starfish]

You need to go even lower in pitch to raise your RPMs. But something doesn't sound quite right there. You're getting nearly the same speed and RPMs between a 15 pitch and 19 pitch prop. I would have guessed that a 15 would be about right, but you could try a 13 and see what that does for you.

You might also need to rejet your carburetor to run a little leaner at that elevation, although if you do make adjustments be sure to richen it up again if you return to sea level.