Prop size for pontoon boat

Tompen

Seaman Apprentice
Joined
Jun 3, 2013
Messages
33
I have a 1989 24ft pontoon boat.
1996 Evinrude 115hp
13.25" x 15" prop size
WOT is 4200 RPM @ 19.8 mph (measured with GPS)

I finally checked my speed with a GPS and I'm going slower than I thought. With the numbers above, I calculated a 33% slip. The motor and prop came off a 19 ft ocean fishing boat and so the prop is not made for a pontoon boat. But still, I would expect to get more out of my setup (my mechanic predicted mid to high 20s top speed). If its slipping that much, I would expect my rpms to be higher and thus a little more speed. I just bought this motor a few months ago and the compression was very good, and it seems to runs perfectly.

A couple things to consider --
My boat's been in the water for 6 months and so I probably have dirty/algae-y pontoons.
I had 3 adults on my boat and probably another 200 lbs of 'extra' weight (reinforced roof for sunbathing) - so call it 4 adults.

The existing prop is original for the motor. I'm thinking if I was to get a 14x13 pontoon prop, I'll get more grab (increased diameter, decreased pitch, more cupping), BUT... it seems to me that while I'll probably decrease my slip, I'll STILL be around 4200-4300 RPMs @ WOT -- which would leave me well below my target of 5000 RPM.

This would imply that I should drop pitch even more and go with a 14x11 pontoon prop -- but this simply doesn't sound right to me. An 11 pitch prop for a 115 just seems very small, from everything I've read/seen.

Not sure where to go from here. Is my thought process flawed anywhere? Any thoughts or suggestions?

A key question I have when thinking about this is this - does slip WASTE energy, generally speaking?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop size for pontoon boat

As an 89 I wonder if your tubes may be a little undersized.
Some things to consider. Motor height, if exceptionally low it could be dragging.If you don't have any ventilation(cavitatiuon)
problems you may be able to raise the motor. May not gain a lot but a little may help.
Weight distribution is important If it squats too much you may be dragging framing through the water.
Motor trim usually is just to get the least ventilation(cavitation).
At 4200 your slip would be higher. From my observations a 15" prop is a bit too much.
I'd like to see your results with a light load and clean tubes.
Assuming the setup is right and the motor is sound.I would suggest an 11" prop. If it proves too light you can still use it
by watching the rpm,while you evaluate the results and not have to rush to a decision.If a 13 is too much you shouldn't use it for an extended period and your performance will still be lacking.
The 11 will get your rpm up and improve the slip.
 

Tompen

Seaman Apprentice
Joined
Jun 3, 2013
Messages
33
Re: Prop size for pontoon boat

As an 89 I wonder if your tubes may be a little undersized.
Some things to consider. Motor height, if exceptionally low it could be dragging.If you don't have any ventilation(cavitatiuon)
problems you may be able to raise the motor. May not gain a lot but a little may help.
Weight distribution is important If it squats too much you may be dragging framing through the water.
Motor trim usually is just to get the least ventilation(cavitation).
At 4200 your slip would be higher. From my observations a 15" prop is a bit too much.
I'd like to see your results with a light load and clean tubes.
Assuming the setup is right and the motor is sound.I would suggest an 11" prop. If it proves too light you can still use it
by watching the rpm,while you evaluate the results and not have to rush to a decision.If a 13 is too much you shouldn't use it for an extended period and your performance will still be lacking.
The 11 will get your rpm up and improve the slip.

Thx for the response. I will consider going with an 11 pitch.

You said " At 4200 your slip would be higher."
I'm a little unclear what you mean. Does a motor have higher slip with a WOT of 4200 rpm, compared to that of a higher WOT rpm, generally speaking?
Or perhaps you're saying that with my specific situation, if I achieve a higher WOT rpm, i'll be faster and better planed (less drag), and therefore that will decrease slip?


Also, do you know the answer to my question about slip vs. efficiency? I know the conventional wisdom is that slip is bad. But I guess I'm wondering why. Is it a loss in efficiency? Consider:
1. Say you have a boat w/ a 100hp motor with a low-slip prop (good cupping, large diameter, etc.) with a WOT of 5000 rpm.
2. Now say you have the same boat and same 100 hp motor, but with a high-slip prop (no cupping, small diameter, etc.) and with a higher pitch to compensate for the slip so that it results in the same WOT of 5000 rpms.
In both situations, the motor is feeling the same load and creating the same power output at WOT. Is option 1. (low-slip) going to be faster? If so, where is that lost power/energy going with the high-slip scenario?

Sorry for the detailed question. I'm just trying to understand whats going on.

Thanks.
 

The Rooster

Ensign
Joined
Apr 28, 2011
Messages
936
Re: Prop size for pontoon boat

115 hp. Johnnyrudes of that era were rated to 6000 rpm's max. at WOT. Weather they were saltwater series or Intruder / Faststrike series, you want to be close to that lightly loaded. Need to confirm a few things first. Are the tubes holding excess water, ie: a leak. Are you dragging too much lower unit through the water. Tubes need to be clean. Check to make sure you haven't spun the bushing in the prop hub. A 15" pitch prop is about right for a 115 w/ your setup. Definetly should not require an 11" pitch. Good luck !!!
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop size for pontoon boat

I checked out 6 tests of Evinrude 115s on pontoons of about 24 ft.None used a 15 2 used an 11" and 4 used a 13
rpm on all tests ranging from 5500 to 6000.
Considering the present performance and assuming the setup is good and the motor sound I feel the 11" is a better educated gamble.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop size for pontoon boat

Tompen: generally as prop speed decreases slip goes up.Then if you consider your prop is very likely too much pitch it further aggravates slip. I don't think that a "high slip" higher pitch prop can make up for slip with pitch,as stated above I believe a pitch that is too high will agrivate slip.
Of course nothing about prop results is carved in stone.There is a post here that a has a prop that "appears" to have slightly lower slip as rpm decreases. The slip numbers seem to me to be unusually low as well.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Prop size for pontoon boat

To understand slip, tie your boat to the dock and rev the engine. You are going nowhere but the engine is running at some specific rpm. Slip numbers would be very high. Just because you succeed in getting the engine to rev higher does not mean boat speed goes up. It means the engine is not being "lugged" as it is now. True -- a severely over propped boat can pick up a fair amount of speed with just a slight adjustment of pitch because suddenly the engine can get "over the hump" so to speak (getting onto its torque curve). For best overall performance pontoons need props with lower pitch but with big, more rounded "elephant ear" blades. This provides more push and better control when in reverse.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Prop size for pontoon boat

steelespike said " Tompen: generally as prop speed decreases slip goes up.Then if you consider your prop is very likely too much pitch it further aggravates slip. I don't think that a "high slip" higher pitch prop can make up for slip with pitch,as stated above I believe a pitch that is too high will agrivate slip. "

Just as spike says, if you exchange the 15" pitch you have now with a 17" pitch, in the exact same make and model of prop, 95% of the time your RPM will go down and prop slip will increase in most cases. But whether slip increases or not or even speed increases, your motor now is even under more stress because of the lower RPM.

Tompen, " A couple things to consider -- My boat's been in the water for 6 months and so I probably have dirty/algae-y pontoons. I had 3 adults on my boat and probably another 200 lbs of 'extra' weight (reinforced roof for sunbathing) - so call it 4 adults. "

The 1st thing you need to be positive of is that the pontoons are CLEAN and not producing major excess drag that is lugging your motor. Unless this is verified there is absolutely no reason to even discuss changing props, because you have no reliable reference point to judge how much it hurts or helps.

The second thing that needs to be addressed is motor height, as it can decrease speed easily by a few MPH on a pontoon, and it is not nearly as easy to judge the right height on a pontoon as it is in normal boats.

Tompen, " Also, do you know the answer to my question about slip vs. efficiency? I know the conventional wisdom is that slip is bad. But I guess I'm wondering why. Is it a loss in efficiency? Consider:
1. Say you have a boat w/ a 100hp motor with a low-slip prop (good cupping, large diameter, etc.) with a WOT of 5000 rpm.
2. Now say you have the same boat and same 100 hp motor, but with a high-slip prop (no cupping, small diameter, etc.) and with a higher pitch to compensate for the slip so that it results in the same WOT of 5000 rpms.
In both situations, the motor is feeling the same load and creating the same power output at WOT. Is option 1. (low-slip) going to be faster? If so, where is that lost power/energy going with the high-slip scenario?
"

Slip is required for any propeller to operate, it has to do with the angle of attack of the blades to the water, so slip is not a bad thing. In fact in tug boats the prop slip is much much higher than our recreational boats. Slip has nothing to do with the efficiency of any prop.

This conversation could get extremely deep, over many peoples heads and BORING, as I have found on many occasions to be the case, so I will make it as short as I can.

99% of the time the lower pitched lower slip prop you describe in number 1 will have a much better hole shot and better lower cruising speed thus a much better ride in rough formed seas.

Normally the situation you described above is not the case, and I have NEVER heard anyone say they have too much hole shot or that they can plane at too low a speed, but the exact opposite of that is true in Sooooo many cases here. Because the more slip there is in any particular prop at maximum RPM the higher the slip is always at all lower RPM.

And YES you definitely do NOT want to get a higher pitched prop.

H
 
Last edited:

Tompen

Seaman Apprentice
Joined
Jun 3, 2013
Messages
33
Re: Prop size for pontoon boat

Thank you all for your responses. I learned a lot and I'm now able to make a more informed decision.

I'm going to try a 14x11" prop. As steelespike pointed out, there's been several tests with this low of a pitch on similar 115hp pontoons that appear to be properly sized (surprised me). I'm also going to clean my tubes and hopefully skin the bottom this winter. With these 3 improvements, I'm confident I'll pull my speed up to the mid to upper 20s, and I should still be under my WOT of 5500rpm.

Thanks again.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Prop size for pontoon boat

I would hold off on the prop until you test after cleaning and skinning.
No doubt still need a change but a test of the modifications may predict a slightly different requirement.
 
Top