Prop size for 40hp rude on 18' pontoon??

[tawd77]

I just got a 91 Suntracker Bass Buggy 18' pontoon boat. It has a 93 evinrude tracker 40hp motor on it. I took it out and the boat was really slow, which i somewhat expected from a pontoon. But today I noticed that the engine is tilted inward at the prop. So that might have quite a bit to do with it being soo slow due to the engine being adjusted to where it was pushing the nose down... the motor (manual tilt) is now adjusted on its outermost adjustment hole. Its still not 90 degrees to the boat, but its alot better than it was.... Will this be ok?

My question is, the prop is needing replaced. The prop is a 12.5x13 and the boats maximum HP is 60 - The WOT RPM was around 5250 with the motors tilt adjusted wrong. Should I replace with the same prop, or should I change the pitch or size? Thanks!

 

[Silvertip]

Re: Prop size for 40hp rude on 18' pontoon??

WOT rpm for that motor is 4500 - 5500 rpm so you are approaching the top end which is ok. However, that motor likes to be closer to 5500 RPM (5800 would be optimum) so a prop change might help. But before you do that, a dirty hull, load, and an engine in need of a good tune up can be a problem so don't buy a prop until you have looked at this. 13P is about right for that motor. You may need to adjust trim some more.

 

[tawd77]

Re: Prop size for 40hp rude on 18' pontoon??

there is a 11-3/4 X 17 on ebay. What would that do compared to the prop I already have?

 

[Silvertip]

Re: Prop size for 40hp rude on 18' pontoon??

I re-read your original post and you said "it is really slow!". What do you consider really slow? 17P is going the wrong way on pitch. More pitch means the prop takes a bigger bite on the water. Bigger bite pulls off rpm so rpm would drop roughly 800 rpm putting you way too low in the rpm band. That is referred to as lugging the engine and is extremely hard on it. If you insist on changing props, you should look for a 11 or 12P and not worry about diameter as it has less effect on performance than pitch. You will find that 12P will likely be available only in stainless steel. 11P will put rpms at about 5600 - 5700 which would be best. But again, a poorly tuned engine or one that is down on power just because of age and wear is no reason to change props. Check the cheap stuff first and then look at props.

 

[tawd77]

Re: Prop size for 40hp rude on 18' pontoon??

I didnt have a speedo but I would say it was doing around 10-12mph. But like I said, the tilt on the engine was wayyyyy back. So I just adjusted it today all the way out (which is still not 90 degrees to the boat) and I will see if that helps. I kind figured it would go a little closer to 20 mph. engine runs great. I dont think it needs a tuneup. I just figured I would ask about a better prop pitch since im going to have to buy one anyways. I can stick with the same pitch that I have, I just thought maybe it would help to get a few more RPM's...

 

[K.P.]

Re: Prop size for 40hp rude on 18' pontoon??

13 pitch is WAY too much for that small of motor pushing that large of boat. I'm surprised you're getting as high RPM as you are but that is most likely due to excessive ventilating. Even 11 pitch will most likely be too much but it'd definitely be better than the 13. Make sure you try to stay within 1/2" of the current diameter.

Changing your trim angle may help a little but you have to face the reality of what you purchased. It's not a planing hull son it's never going to be a speed demon. 20 MPH may be out of the question but I think you should be happy if you get 15 to 17.

Good advice on getting the motor tuned up and a carb rebuild. Clean and polished pontoons will lower resistance too.

 

[Silvertip]

Re: Prop size for 40hp rude on 18' pontoon??

Throwing pitch number out there are not valid suggestions unless gear ratio is taken into consideration. 40 & 50 HP Johnny/Rudes have 2.43:1 gear sets in them and can easily twist a 13P prop. They routinely run 15P and 17P on 15 - 16 foot "V" hulls. If he had a Mercury/Mariner then a lower pitch would be indicated if the motor had a 1.83:1 gearset. If it were a BigFoot motor which has a 2.43 gear set again, you are back at 13P. In short: deep gears = high pitch. My 40HP Suzy four-stroke twists an 11 pitch prop with 2.27:1 gears.

 

[K.P.]

Re: Prop size for 40hp rude on 18' pontoon??

You are correct on gear ratios being a critical factor in prop selection but you can not compare how a certain pitch performs on a planing hull boat to a pontoon that is like moving a block of cement through the water when powered with that small of motor.

If his prop is indeed 12.5 inches in diameter, then his motor does have a larger gear case. 1.85 gear ratios on that size motor typically turn 10+ inch diameter props. Big Foot type gear cases typically turn 12 and 13 inch diameter props. So I'm not "throwing" prop pitches out there.

It's more likely his pontoon is not getting good forward progress and is ventilating. He could even have a slightly spun hub. That's why I was surprised that size diameter and pitch was getting that much RPM.

 

[Silvertip]

Re: Prop size for 40hp rude on 18' pontoon??

K.P. -- read the first post again. He has a Tracker/Evinrude, not a Tracker/Mercury. 40 HP Evinrudes of that vintage run a 2.42 gear set and were never offered in a 1.85. 12-1/4 x 15 and 11/3/4x17 props are very standard for a "V" hull and the only comparison I made was that those two props are not acceptable on a pontoon. 12-1/4x13 is a common pontoon prop as is an 11P depending on boat size and weight. A 10-1/2 inch prop will not work on this engine/boat combination.

 

[tawd77]

Re: Prop size for 40hp rude on 18' pontoon??

I re-read your original post and you said "it is really slow!". What do you consider really slow? 17P is going the wrong way on pitch. More pitch means the prop takes a bigger bite on the water. Bigger bite pulls off rpm so rpm would drop roughly 800 rpm putting you way too low in the rpm band. That is referred to as lugging the engine and is extremely hard on it. If you insist on changing props, you should look for a 11 or 12P and not worry about diameter as it has less effect on performance than pitch. You will find that 12P will likely be available only in stainless steel. 11P will put rpms at about 5600 - 5700 which would be best. But again, a poorly tuned engine or one that is down on power just because of age and wear is no reason to change props. Check the cheap stuff first and then look at props.

I took your advice and ordered a 13x11 prop. I will let you all know how it does once I get a chance to get the toon out to the lake. Thanks for the help!

 

[deltaguy]

Re: Prop size for 40hp rude on 18' pontoon??

any updates?iam very curious