prop question.

[Streakber549]

Does anyone know how much 2 decrees of pitch might change max throttle RPM on a Yamaha F25 4 stroke on a 14.5 foot aluminum boat? I have a brand new F25 that misfires at full throttle/max speed. I'm pretty dang sure its hitting the rev limiter. If I back the throttle off a wee bit it goes away. Want just enough more pitch so it doesnt hit the rev limiter, but not so much it affects performance too much. It currently has a 9 7/9 X 10 1/2 prop. They offer a 9 7/8 X 12 degree prop for it. I think another degree and a half is enough, but want other opinions from yall. It literally has to be wound all the way out right now with a normal load to hit the limiter. Me/wife/gas/gear etc etc. Any and all suggestions appreciated!

[boscoe99]

Pitch is normally expressed in inches. Not degrees.

A general rule of thumb is that for each inch of pitch change the WOT RPM will change approximately 150 ~ 200 RPM.

You really need to have a tachometer installed to know precisely what the WOT RPM is.

If you are convinced that the motor is hitting the rev limiter, and if your present propeller is the standard F25 propeller that comes with the motor (9 7/8 diameter X 10.5" pitch), then a propeller with slightly more pitch (9 7/8 X 11 1/4) might be a solution. Or as you mention, a 9 7/8 X 12.

Hopefully, the hole shot will not suffer too much.

[Jason2tpa]

I agree with everything Boscoe wrote. A little more to add that might help...I don't believe you would notice a significant performance difference if increasing pitch by 2". With small lightweight boats as yours, boat load (weight) and position of load in the boat will have the greatest effect on performance.

Four strokes do not have the low end hole shot capabilities like a 2 stroke, so take that into consideration as you increase pitch. However, if hole shot time does increase, small lightweight boats respond very well to a hydrofoil, including better hole shot, maintaining plane, and plane stability.

[rodbolt17]

F25??? hole shot????
yall gotta be kiddin me.

but I do agree on the running with a tach to prop it correctly.

[boscoe99]

Hole shot = time to plane.

With just me and my two stroke Mercury 25 on my 14' skiff the hole shot was superb.

With me, my wife, my brother, my brothers wife, the hole shot was terrible.

With a Yamaha F25 on, and just me in, my 14' skiff, the hole shot was marginal.

Now with a Yamaha F40 on, and just me in, my 14' skiff, it was an exciting E ticket ride. Boat would pop onto plane. Great hole shot. At about 40 MPH it would get real skittish. Chine walk baby. I had visions of me losing control and being another statistic on the lake. But damn, it was fun!

Is anyone suggesting that the time to plane will not suffer if and when a propeller with a larger diameter or greater pitch is used?