F25 Propeller advice

[kdgrills]

I've got several hours on my re-powered rig now.
Looking for advice as to whether re-propping is worthwhile.

Motor is 2017 Yamaha F25SWHC, WOT range 5000-6000 RPM.
Boat is 1970's Mirrocraft f3614 (14 foot length, 60 inch beam)

Current performance with the factory equipped 9-7/8 x 11-1/4 propeller:
Light load: WOT is 28ish MPH @ 5800 RPM (Myself, 6 gallons fuel, battery, *****ing motor).
Usual load: WOT is 24ish MPH @ 5400 RPM (light load + passenger, cooler, tackle, etc).
Heavy load: haven't had one yet.

Overall, I'm happy with the current performance, although it seems at my usual load a few hundred
more RPM would be better (?), and especially with the occasional heavier or extra passenger.

Considering pitching down to 9-7/8 x 9, gaining 300-400 RPM (150-200 RPM per inch of pitch seems the norm).
Light load I'd be bumping the rev limiter.
Usual load I'd be comfortably in the upper RPM range.
Heavy load I'd hopefully still be in the mid RPM range.

What say you? Worthwhile, or not much to be gained, I'm over thinking it?

[boscoe99]

I think you can gain some performance by using a lower pitched propeller. The motor needs to be able to turn at or near 6000 RPM when the boat is loaded. Seems you are about 600 RPM's short.

Both the motor and you will be happier.

I would try a Yamaha propeller with an 8" pitch. If you are ever alone in the boat with no gear and you feel the rev limiter kicking in, back off of the throttle a bit. Of course with a tachometer installed you would never need to have the rev limiter engage.

[kdgrills]

Thanks for the reply.

I did see that there is an 8 pitch available, was concerned about ending up under propped.

With all the possible variables you obviously can't know for sure,
but in your experience is 150-200 RPM per inch of pitch fairly
typical with a smaller motor such as this?

I do have a tach (you kindly helped me sort that out as well), so yes,
I'd simply need to keep an eye on it while running light. Perhaps swap
the 11 pitch back on if I anticipate several days running light.

[boscoe99]

150 ~ 200 RPM change per inch of pitch is the rule of thumb. All else being equal. Change the style of propeller and all bets are off as to what the results will be.

Many will run different propellers in the summer time (hot and humid air) versus winter time (cool and dry air). It is not unheard of to change propellers depending upon the load in the boat.

My personal preference but if there are only two choices I prefer to be under propped rather than to be over propped.

[kdgrills]

Thanks again, I owe you a beer.

I'll follow up with the new performance numbers once I get it swapped.

[kdgrills]

Following up...

I initially tried a 9" pitch & was well under propped.

Now running a 10-1/2".

With a light load I hit the rev limiter (6200 rpm) before wot, top speed is 26 mph (2 mph loss versus the 11-1/4 pitch).

With my usual load (actually several lbs lighter than) I'm right at 6100 rpm at wot, occasionally hitting the rev limiter,
top speed is 26 mph (2 mph gain versus 11-1/4 pitch).

I'm gonna leave the 10-1/2" on it, unless I know I'll be running light for several trips.
I was quite surprised that less than an inch of pitch would result in a ~7-800 rpm change.

[boscoe99]

Thanks for the feedback.

Changing propellers can result in some strange results at times. It can be a lot of trial and error.

Did you change pitch only (staying with the same brand/model of propeller) or was the model of the propeller changed? If changing models results can be all over the place and at times no where near what was expected.

[kdgrills]

All three were Yamaha OEM 3 blade x 9-7/8" dia.

I've been told the smaller motors are more sensitive to pitch changes, my experience seems to back that up.

[boscoe99]

All three were Yamaha OEM 3 blade x 9-7/8" dia.

I've been told the smaller motors are more sensitive to pitch changes, my experience seems to back that up.

I suspect that you are correct. Good info to keep in mind.