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Brian121804

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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, trolling 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?
 
I'd keep it as is.. high in the RPM range during light load seems ideal to me.. and your right in the middle of the range during normal load.

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Too low ... you're borderline lugging. I find the best performance is to be no more than 200 RPMs off the rated/stated maximum with the 'usual load' carried. Who cares when running alone with light load and fuel load> ... just throttle back.

You add more gear ... and boats only get HEAVIER as the season and years go on ... and you will be lugging that motor. That can kill a motor at worst or at least take years off their useful life, at best.
 
DaleH is absolutely right, and this is especially important on a 4 stroke motor!

9 7/8" x 9" is too "small"...way too small. I'd suggest a 9 7/8" x 10 1/2"-which is the stock prop for the older 4 stroke 25's. Usually found dirt cheap. Might even be able to trade someone out of one as the older 25's on more modern hulls should use a 11 1/4" pitch (which is what you have obviously)-and typically pick up a few mph over their stock 10 1/2".

If you don't have PT&T, and plan to add it via CMC or Panter, or whatever, you may find yourself liking the 11 1/4"-as moving the engine back and up can help tremedously-so much so that many folks find themselves going to a 12" or sometimes even a 13" on the better duck boat hulls; and running low 30mph range.

There is a "rule of thumb" out there that for every inch of pitch your RPM would change by 200. IMO that rule of thumb only applies to motors that are about 90hp and up; these little engines are completely different. I think a 10 1/2" will get you real close to where you need to be.
 
Higher RPMs usually means the engine is working less hard (less load) and lower RPMs means the engine is working harder (more load). Usually if you can operate at the upper range when lightly loaded that is ideal. You may see some improvement in speed but don't expect much maybe a MPH or 2. The biggest gain is fuel economy and the engine will be happier too.
 
Thanks for the replies.

Based on the info provided here & a couple other trusted sources,
I'll pitch down to 9-7/8 x 8, which should get me right at the high limit.

I'll follow up with the new performance numbers once I make the swap.
 
kdgrills said:
Thanks for the replies.

Based on the info provided here & a couple other trusted sources,
I'll pitch down to 9-7/8 x 8, which should get me right at the high limit.

I'll follow up with the new performance numbers once I make the swap.

Let us know how it goes if you don't mind. An 8" pitch is for pontoons, bet you'll be against the limiter in no time. I still say 10 1/2" is going to be the best bet, but what do I know? Never done this stuff before....
 
turbotodd said:
Let us know how it goes if you don't mind. An 8" pitch is for pontoons, bet you'll be against the limiter in no time. I still say 10 1/2" is going to be the best bet, but what do I know? Never done this stuff before....

Yes, I will follow up.

I'm actually still waiting to hear from my dealer, Hopefully I'll be able to trade in on something more appropriate if it doesn't work out.

What is your experience? This is my 1st (& likely last) new, nice, motor. Just trying to get it right, I have never done this before.
 
Try to keep the diameter close but reduce pitch. I think giving up diameter will reduce thrust, less prop footprint. 4 strokes have more time between power pulses than ring dings. Think about drag race cars, some cars will hook up better with a narrower but taller tire vs a short fatter tire. You still want to pull water with as close to factory diameter as possible.
 
Following up...

I found a 9 pitch oem for a very good price & able to return.
Was finally able to get out yesterday to give it a whirl.
With my usual load I was on the rev limiter (6200 rpm)
at not quite full throttle, mph is down by 2-3.

Looks like turbotodd was spot on in his recommendation of 10-1/2 pitch.
Got one ordered, hopefully I'll be able to get out next weekend.
 
ProduceMan said:
Try to keep the diameter close but reduce pitch. I think giving up diameter will reduce thrust, less prop footprint. 4 strokes have more time between power pulses than ring dings. Think about drag race cars, some cars will hook up better with a narrower but taller tire vs a short fatter tire. You still want to pull water with as close to factory diameter as possible.

Not necessarily.

Drag cars have the luxury of changing the final drive ratio....as well as the tire type. The tires can be replaced with ones that "grow" a little more, which is sorta how top fuel guys tune....they only have one gear ratio available per rules, so they use tires that grow quite a bit to achieve the high MPH given the same RPM (they have mandated rev limiters too-which is a sore subject). I know I've done some tire testing with radial slicks and found that the finish line MPH is a shade faster but with quite a bit more RPM...because the tires don't grow as much-plus radials also have a little less drag than bias ply stuff does. And...we automatic guys (well I'm a stick shifter at heart but I can't seem to afford that stuff) have a torque converter which also can be altered to work better with the engine and the car it's propelling. All this...and our outboards don't have the luxury of changing the gear ratio in the foot, there is no torque converter to mess with, and the prop isn't variable like a growing tire. So we're stuck with what we can get readily available which is 99% of the time the prop.
 
Got out a few times this weekend with the 9-7/8 x 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 was 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.
 
The two props could have differing amounts of cupping. There is more to the 'right prop recipe' than just pitch. Diameter, pitch, cup, rake, overall shape of blades, and # of blades all play a part. Myself, I would stick with the 10.5" pitch. As a previous poster stated, boats only get heavier.
 

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