Prop for 20' Pontoon

[ken_23434]

I am trying to figure out the correct prop for my 20' pontoon.

Mercury 4 stroke 60 hp outboard (NOT big foot)

Current prop: 11.5" diameter, 9 pitch 4 blade Michigan Wheel aluminum
Current WOT RPM: 4400
Speed at above: 16 n-mph

Previous prop: 12.5, 8 pitch 3 bladed Mercury aluminum prop (not positive of model line)
WOT RPM: could not get to WOT ... would reach 6,000 rpm around 3/4 throttle
Speed at 3/4 throttle, 6,000 rpm: 16 n-mph

I was totally shocked to find out that this new prop dropped the RPM's by so much.

I read that the 4 blade design is desirable for pontoons, so that is the reason I was going w/ a 3 bladed prop.

I have tried a few different prop calculators before getting the current prop, but since they all want WOT rpm and speed, I could not get a good recommendation. The rev limiter (along w/ me not wanting to find out) prevents it from going to the actual WOT RPM w/ the old prop.

I tried to guess and put in a value of 6,400 or 6,600 rpm to get an idea of what prop, but I am not sure how good the recommendation would be.

Another issue is the calculators assume that I am running a 3 bladed prop.

The next size smaller from Michigan Wheel in their 4 blade design is 11 1/8" diameter w/ an 8 pitch.

I assume that a 9" or 10" pitch 3 blade would be correct, but wanted to stay w/ the 4 blade style.

 

[SparkieBoat]

Re: Prop for 20' Pontoon

it sounds like your old prop was ventilating bad, you are doing the same speed at 4400 with 1 pitch smaller prop as you were at 6K and counting with your old 3 blade, it sounds like either your motor is too high or you have a lot of air coming off of the box in front of your prop caused by something churning the water bad. It would be good if you could post a few pics of your motor trimmed down and the box in front of it to see how the motor is mounted. the smaller dia 4 blade is getting a better bite and not slipping, that is why the big change in RPM, 4400 is too low, I think those motors like around 5500

 

[ken_23434]

Re: Prop for 20' Pontoon

View attachment 106436

The first couple times I took the boat out, I had water coming over the engine cover at about 50% throttle. The engine was mounted all the way down. After some research and some advice from this site, I raised the motor 2 holes.

I am pretty sure that I was having over-revving issues before raising the motor. I will have to look back and see if I posted any thing about rev's at the time I was asking about the water coming over the engine.

 

[SparkieBoat]

Re: Prop for 20' Pontoon

well you defiantly had some serious slippage going on...again some pics would be helpful. do you have vent fins on your motor?? they can help stop prop venting, you should try dropping your motor back down and try your 3 blade prop again. ....so at %50 throttle you back major stern dipping??? I assume that is why the motor was down in the water. you need to shed some weight out of the back and maybe shift some weight to the front. I had a 28 small toon boat once, it had water logged plywood seats, I ended up pulling all the seats out, they were getting old and the vinyl was cracking anyway, I got a big plastic yard box from home depot, put it in the front for life jackets and such, and used camping chairs and coolers for people to sit on, got rid of about 800 LBS. that four stroke is heavy, but most of the newer large toons can handle it better, but they all still have a lot of drag and just never seem to do good...If I ever get another toon it will be a tri toon, they stay on top of the water and can handle the weight load.

 

[Silvertip]

Re: Prop for 20' Pontoon

First, you really have the wrong engine for the pontoon. A Big Foot was made for that purpose as it has different gearing in the lower unit so it can swing a larger diameter prop (14 inch). But - since you have the small gearcase unit with a lower ratio, you need to get the engine down into the water (back where it was). The next thing you need to do is stop worrying about diameter. You didn't mention the year of this outboard but small gearcase engines have a 1.83:1 gears or 2.33:1 if it was a Big Foot. The small gearcase runs a 10-inch prop, not 11 or 12 so someone has already installed the wrong prop. A 40/50/60 HP Big Foot motor will almost always run a 14 x 9 prop. So a 12 inch prop in any pitch is very likely wrong as well. The prop is very likely ventilating so getting the engine down and trimming properly will take care of that to a degree. The small prop and wrong engine are the real issue. You might also check the boat itself to see if something is loose in the engine pod area. That water is turbulent anyway so any protrusion will make things worse.

 

[ken_23434]

Re: Prop for 20' Pontoon

The boat and motor are both 2011's. Just bought it about a month ago. This is the motor that came on it from the factory.

 

[SparkieBoat]

Re: Prop for 20' Pontoon

factory set up does not always = right.

 

[steelespike]

Re: Prop for 20' Pontoon

While the big foot may be ideal for the toon I don't think you could call the regular gearcase the wrong motor.It will require careful setup and prop selection.
I don't understand the rush to lower the motor back down.It does appear the MW 9" 4 blade has plenty of grip. The 11 1/2 " diameter is about right for a 4 blade.with a 12 or 12 1/2 about the size with a 3 blade.
From the descriptioon of the performace with the dealer setup (water over the motor,
reving way over 6,000).
I would wonder if they bothered to be sure the throttle was set up to open all the way.
Your missing a whole bunch of rpm.If your not having any venting/cavitation problems you may be able to raise the motor more.Probably won't gain much but could help.
If the throttle check and raising the motor don't get you set then a 8"pitch 4 blade may do it. While pontoons don't allow much trim try to carefully find the best setting.
Do your tests lightly loaded and try to keep your normal load carefully balanced.
I would think that toon should make 20-21 mph.

 

[ken_23434]

Re: Prop for 20' Pontoon

While the big foot may be ideal for the toon I don't think you could call the regular gearcase the wrong motor.It will require careful setup and prop selection.
I don't understand the rush to lower the motor back down.It does appear the MW 9" 4 blade has plenty of grip. The 11 1/2 " diameter is about right for a 4 blade.with a 12 or 12 1/2 about the size with a 3 blade.
From the descriptioon of the performace with the dealer setup (water over the motor,
reving way over 6,000).
I would wonder if they bothered to be sure the throttle was set up to open all the way.
Your missing a whole bunch of rpm.If your not having any venting/cavitation problems you may be able to raise the motor more.Probably won't gain much but could help.
If the throttle check and raising the motor don't get you set then a 8"pitch 4 blade may do it. While pontoons don't allow much trim try to carefully find the best setting.
Do your tests lightly loaded and try to keep your normal load carefully balanced.
I would think that toon should make 20-21 mph.

Why would you recommend raising the motor more? I thought the general idea was to keep it as low as possible? After I raised it two holes, I no longer have the problem w/ water coming over the engine cover. Honestly, I have thought about dropping it one hole, so see if that location would work also.

When I use the Mercury Marine prop selector and use the option to search based on the boats weight, type of boat, and motor installed, it recommends a 9 pitch or 10 pitch prop (12.25 dia on the first, 11.5 on the second). So, I do not understand the comment earlier that the prop that came on the boat being too large in diameter.

Could you explain how I would identify the cavitation/ventillation?

 

[steelespike]

Re: Prop for 20' Pontoon

While raising the motor on a toon can be tricky; the less motor in the water the less drag.
Ventilation sometimes refered to incorrectly as cavitation is when the prop loses its grip
motor revs boat slows sometimes requires lowering the throttle till the prop gets grip back.
If you have no ventilation then there may be room to raise the motor more. You should find the highesty point that has no or limited venting.The 4 blade prop will resist venting
and should have lower slip numbers,toons produce generally higher slip numbers than the typical planing boat.The lower the slip numbers the better the forward motion.
Also the 4 bld. will have better control around the dock
If the prop is designed for your motor don't worry about the diameter.
Very generally as pitch goes up diameter goes down.You will find that the 4 blade will generally have less diameter than a typical 3 blade of the same pitch.

 

[SparkieBoat]

Re: Prop for 20' Pontoon

the reason i recommended lowering the motor, was to try the 3 blade 8 pitch again, and get some idea how it effects WOT RPMs when not venting. he may indeed be able to raise the motor up some more with a 4 blade prop and in the end you do want the motor as high as you can get it without venting. but raising it and lowering it is easy enough to do and only takes a few mins. and he already has a 8 pitch 3 blade. I would also recommend a anti-vent-plate, which should be on top of the water when running at WOT and trimmed up right. do they make a 6 pitch 4 blade for that motor??

 

[ken_23434]

Re: Prop for 20' Pontoon

Thanks again for the information/advice. My old boat had an I/O, so motor height and all that just wasn't anything I ever had to deal with.

The smallest 4 blade that Michigan Wheel makes is an 8 pitch.

I used a prop calculator and put in my #'s to find what I was getting for slip w/ the 4 blade. I then used that #, put 6,000 rpm (max recommended WOT for my motor) and a speed of 20 (manufacturer states 20 to 22 is expected speed for this boat/motor combo) and it returned a 8.25 pitch prop. So, the 8 pitch 4-blade might be correct. I just don't see dropping pitch from 9 to 8 changing my WOT RPM from 4400 to 5500-6000 range.

Before raising my motor, I was having issues w/ the boat over-reving w/ my 8 pitch 3-blade prop.

Monday, I plan to call up the prop shop and see if they are willing to work w/ me to swap the new 4-blade for the smaller one (it only has 1 day on the water, no marks) or to experiment w/ a few 3 blades until I can get it resolved.

I am pretty sure the prop is not ventillating. I do not experience a sudden jump in RPM w/ a drop in speed. I have also put it in a sharp turn at full speed and haven't noticed anything like what I think ventillation would be.

Next time out, I plan to drop the motor 1 hole and put the 3 blade prop back on for comparison.

 

[SparkieBoat]

Re: Prop for 20' Pontoon

report your results from motor drop and 3 blade 8 pitch.

 

[ken_23434]

Re: Prop for 20' Pontoon

I never got around to dropping the motor.

I talked w/ the prop shop and he said he could take an inch or two of pitch out of the 4 blade prop and could add a little cup to the 3 blade. (did both for free !)

I took it out yesterday w/ the modified 4 blade. WOT was right at 6,000 rpm w/ me and two kids (up from 4,200 ish). So, I am happy now. I will test out the modified 3 blade at some point and see how that one works out.