Prop Question?

[IslandManMitch]

I have a '86 40 hp Evinrude on a stripped (just railing and stand up console) 20' Bass Buggy pontoon running a 11 3/4 x 17 aluminum prop. It runs 5500 rpm at wot around 20 mph light loaded. I have a 12 x 14 Power Tech S.S. spare prop. I know the overall speed will be slower and the engine will over rev at wot but how much difference is my question?

 

[Johny25]

Re: Prop Question?

Well your going to be running 600 rpm higher at least and maybe even more considering it is stainless. Stainless tends to run more RPMs than the same pitch aluminum. So my guess is with the info you provided you would be roughly 6100-6200 rpm with the switch and you will most likely lose 3-4mph possibly more?

I wouldn't recommend running a prop that will over rev your engine though

 

[IslandManMitch]

Re: Prop Question?

Thanks Johny, I didn't know if SS props would be cupped more getting better-less bite or more-less slip and make a difference. If I have to use it for a spare I'll watch the RPMs.

 

[steelespike]

Re: Prop Question?

A 17 is way too much prop. Your speed is ok but I have a feeling the rpm isn't accurate.
You need to verify the tach Perhaps its set wrong. Is it a factory tach?
Some times if you operate the switch a few times it will clean the contacts.
The usual prop size is around a 13" pitch.
It is possible the high pitch is causing a lot of slip or is ventilating.
I have a feeling your rpm is really around 4000 or 4500.
If your motor is operating at those rpms over time it will cause damage.

 

[IslandManMitch]

Re: Prop Question?

Steelspike, it is the factory tach that came with the motor when bought new, but it is over 25 years old and may not be accurate. From reading the pontoon forum I thought the pitch was high but never questioned the tach and let it go at that. It has been running the 17 pitch for over 25 years but rarely over 3500 rpm indicated so maybe that is why it has lasted? How do you check tach accuracy/RPM short of buying a new tach?

 

[Howard Sterndrive]

Re: Prop Question?

I suspect your prop is not a 17 then. It might say 17 on it, but maybe been rebuilt to something else.
I have a 1990 40Hp with a 17 and at 5500, I'm doing right around 36 mph.

 

[IslandManMitch]

Re: Prop Question?

I suspect your prop is not a 17 then. It might say 17 on it, but maybe been rebuilt to something else.
I have a 1990 40Hp with a 17 and at 5500, I'm doing right around 36 mph.

On a pontoon boat?

 

[Howard Sterndrive]

Re: Prop Question?

just doing some looking online - looks like 1986 and 1990 40HP have different lower unit gearing so not a useful comparison

 

[steelespike]

Re: Prop Question?

You could perhaps borrow a shop tach or the Tinytach is reasonable easy to in stall and will work on any motor.
I would hope 3500 was not wot. And you just cruised at lower rpms generally.

 

[steelespike]

Re: Prop Question?

I suspect your prop is not a 17 then. It might say 17 on it, but maybe been rebuilt to something else.
I have a 1990 40Hp with a 17 and at 5500, I'm doing right around 36 mph.

He's pushing a pontoon.

 

[steelespike]

Re: Prop Question?

just doing some looking online - looks like 1986 and 1990 40HP have different lower unit gearing so not a useful comparison

My info indicates the 86 and 90 are the same 2.42 ratio.

 

[pootnic]

Re: Prop Question?

Down below...forgot quote

 

[pootnic]

Re: Prop Question?

Well your going to be running 600 rpm higher at least and maybe even more considering it is stainless. Stainless tends to run more RPMs than the same pitch aluminum. So my guess is with the info you provided you would be roughly 6100-6200 rpm with the switch and you will most likely lose 3-4mph possibly more?

I wouldn't recommend running a prop that will over rev your engine though

Really;I thought it was the other way around?(SS vs. aluminum)
If I had a 13 pitch prop and wanted a SS,I thought I'd look at a 12 pitch?
Sorry if I'm butting in,just wanting to hear his views.
Like the rest,a good tach may help you and seeing that your motor is 25 years old and running,I don't think you hurt it to much.

 

[steelespike]

Re: Prop Question?

Changing prop material isn't as cut and dried as that. Going to stainless results would depend on how well the al prop is doing .
And all prop vary in their design criteria a ss could load the motor more than al but at the same time the design should turn easier
through the water and may get the boat up on the water better improving speed and rpm.
To further complicate things there are al props that deliver close to ss performance.
Certain well established ss props have performance results that can be predicted and have a certain reputation.
Just as certain al props have recognized performance and reputation.

 

[Howard Sterndrive]

Re: Prop Question?

He's pushing a pontoon.

so pontoons make propellers slip 50% more?
it doesn't matter what the boat. If he has a good 17P prop and the motor is really at 5500 rpm, whatever it's pushing is doing over 30 mph.
If he tied his motor to the dock, that dock is going 30mph plus...

I think everyone pretty much agrees that either the tach reading or the prop is not what is appears

or he has way more than 40hp(the hood says 40? or the welchplug) , or some goofy lower unit gearing..

 

[Johny25]

Re: Prop Question?

Really;I thought it was the other way around?(SS vs. aluminum)
If I had a 13 pitch prop and wanted a SS,I thought I'd look at a 12 pitch?
Sorry if I'm butting in,just wanting to hear his views.
Like the rest,a good tach may help you and seeing that your motor is 25 years old and running,I don't think you hurt it to much.

As steelespike eluded too, it is not that cut and dry. Stainless can be formed with thinner blades and better cupping due to it's strength advantage over aluminum. I know that they make some higher end aluminum's that do perform like stainless props though. But this all depends on manufacturer, blade design and quality of craftsmanship. From my experience SS runs about 50-100 rpm higher than the same aluminum pitch and I believe it is because of thinner blades and better overall design that SS typically has.

Not sure anyone has ever mentioned this but I also believe the hardened smooth finish/surface of the stainless also helps it slide/cut through the water. Aluminum blades are painted and tend to have a rougher surface. Especially when the paint starts coming off. If the blade was only spinning at 50 rpm this may not even factor into the equation, but at 5000-6000 rpm I would think it can make a measurable difference. Just my opinion though

 

[IslandManMitch]

Re: Prop Question?

My buddy put a shop tach on my motor and my tach is reading correctly. I was looking on the pontoon forum about a similar situation. "20' Starcraft with a 91' 48hp spl Evinrude, 11 3/4 x 17 prop, WOT 5400-5500 rpm @ 17.5 - 18 mph". Mine is running a little faster top end but his numbers are very close to mine. From what I understand 17 pitch is too much for a pontoon boat running a 40-50 hp. So does this mean I am getting "slippage" as Steelspike suggested? Yes Steelspike the 3,500 rpm is cruising rpm. 5,500 is WOT. Nothing is mismarked or changed out. My motor is one owner '86 40hp Evinrude with 11 3/4 x 17 OMC supplied factory prop that has never been taken off the motor since new. If it is excessive slippage what do you do to correct it? Is it a motor mounting height or prop issue? Or, it's been working like this for 25 years leave it alone?

 

[IslandManMitch]

Re: Prop Question?

Took the pontoon out today with the 12x14 prop on it. WOT 5,600 rpm. Did not have my gps so not sure of the speed but it seemed to be the same as the 11 3/4 x 17 prop. The only difference I could tell was it came out of the hole faster and climbed to top end faster. I will keep it on the boat and put the aluminum in the box for the spare. I guess it is not exact science.