Pontoon boat engine is ventilating / cavitating help!!!

[JV74]

Hello all. I spent last summer restoring a 24' Playbuoy Pontoon with a 40hp Mercury. Finally got it in the water and I have learned a few things. One.....never buy a real old pontoon and sink a bunch of money into it because the toons are too small. Unfortunately I didnt factor the toon size into my resto and now that its in the water I realize my toon isnt nearly as buoyant as the toons with larger circumference. Lesson learned. Now to my question. As I throttle up my 40hp it revs up and ventilates so I have to throttle back down to idle speed or just above to cruise. My issue gets a little complicated but if I can peck away at one issue at a time that would be great.
My issues
Throttle control......idle advance lever does not work. When I raise the lever it will not allow the motor to even turn over. When I lower it the engine spins like a top.
Also I think I might have a throttle position sensor issue but it is minor. The boat shifts in and out of gear pretty well.

I just mentioned the above because who knows maybe my engine issue is due to the throttle.

I have a buddy with a pontoon boat and a small engine like mine and he has the same ventilation issue so I figure this is kind of common in the pontoon boat world. I have had a couple of semi successful runs with only 2 people on board where i was able to throttle up at least 50 % before ventilating. Recently I had 4 people on board and the ventilating was terrible....we could barely get going before it would just rev up. The weird thing is that it seemed to get worse as the day went on which makes no sense at all to me.

I am assuming that my motor just isnt getting clean water at its current height setting. I was looking at the engine while running slow and by outboard standards it is running deep in the water. My toon has that engine well like most toons and I believe the engine well is pushing the water down right in front of the engine and causing bubbles or turbulence which is causing my engine to ventilate.

If this is a common issue in the pontoon world....what is the fix? Lower or raise the engine? Modify how the water flows past the engine mounting well? From what I have seen in the forum some pontoons are set up with the engine to run very deep....so i assume this is to avoid issues like mine.

I know my post is scrambled and all over the place but its a start......any suggestions would be appreciated. I have already decided to buy a new throttle set up to eliminate that portion of the problem.

[wwind3]

1) If you can-post a pic that shows where the cavitation plate is with respect to the bottom of the toons. Too low and lower unit will break up the water flow around the prop & cause cavitation. If motor is too high or you have a short shaft motor---possibly same effect.,
2) Dont rule out that prop hub is spun out. From a standing start "floorboard it" and see if rpm's overrev and you dont go anywhere. Hub is spun out. Props on pontoon boats have to work pretty hard..

[JV74]

Now see thats why I posted my long winded ramble cause I knew it was possible to get some kind of reply that would get me moving in the right or at least forward direction.

When I purchased the boat for $2500 last year.....the previous owner had recently spent over 4k doing engine maint...riggging....and other improvements on the boat so I figured I couldnt lose. The problem is that my ocd wont let a beater boat be a beater boat so I restored the darn thing and now I am in deep.

Back on subject.......there is in fact unusual prop play...what I mean by that is I can wiggle the prop forward and backward a little bit more then normal for my taste. I Installed a new water pump kit on this engine and re installed the prop so I know it is on correctly.....but it is quite possible that the hub has been spun out and it has just enough grab to get er movin but thats it. But while I am typing those words I know my buddies boat does the same thing and what are the odds that he has a spun hub too? I did not take the pics you were asking for when we were out last and now the boat is covered in snow. I am in Virginia so I wont have her back in the water until late March. I wonder if there is a way I can check the prob for a spun hub? Its just a 40hp so a new prop wouldnt cost much either way and would help eliminate that possibility. I was gunna try a diff prop anyway to see if it would help. From what I read about cavitation and ventilation I believe my engine is ventilating not cavitating. It was riding deep in the water I can tell you that for sure. Again my toons are not very buoyant and there was a rolling wave coming off of the back of the engine well mount thingamabob ...this wave had the engine down deep not to mention its not like we could get up on plane without a hull....

So currently I have a throttle problem
Possibly a prop issue
and possibly and engine height issue.....

I just have to figure it out one at a time until I am cruisin correctly........thanks for any suggestions or thoughts u may have.

[wwind3]

..sounds like you are on top of it. I've spun 2 hubs on my toon. One would accelerate up to about 3000 rpms and then let go. I could take off normally without jumping on it--but when I hit around 3K it would spin out--weird...

[bansil]

me too spun my 1st one a factory 3 blade it was old, I got I think a season out of it (USED).

WENT WITH THE PERFORMANCE 4 blade made for my engine, nos in original box. 1 side looked brand new, other side of box was almost white from uv after sitting on a shelf for 20 years

Maybe take prop off clean inside good and maybe heavy spray paint in the crack between hub and prop
It might show spinning by cracking the paint?

[JV74]

I wish it was Spring.....I would drive down to the boat tomorrow and check this theory out.....thanks for the suggestion. If this doesnt work I will try diff engine heights. If that doesnt work I will punt

[Marc K]

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Maybe take prop off clean inside good and maybe heavy spray paint in the crack between hub and prop
It might show spinning by cracking the paint?

Score or strike a line between the parts. If the prop moves in relation to the hub, you will see it.

[JV74]

Yep I watched some vids about it after u guys mentioned it. I will check it out....thanks

[steve1313]

I'd suspect a spun hub, too.

But the engine height is really easy to check when it's sitting on the trailer. The cavitation plate should be at just about the same height as the bottom of the toons. If it's not, that can create cavitation, etc.

[JV74]

This is my first pontoon boat and again....its a 1984 old school toon that I restored. These skinny old toons barely keep this thing floating ( exaggeration ) ...... I might be wrong here but I am wondering if the engine height is a little different in the pontoon game vs the exact science in the fiberglass v hull game. The last boat I restored was a 18ft center console fishing boat. When installing the engine I used straight edge rulers and all kinds of stuff to make sure my cav plate was exactly where it was supposed to be. In the end I installed it correctly and had no issues but the stern of the fishing boat stays quite buoyant and consistent but the stern of my 1984 toon changes with the number of people moving around etc much more then the fishing boat did. Heck I never took more then 3 people out on my fishing boat and yet the pontoon can easily hold 6. Ultimately I made a mistake by buying such an old toon with the skinny toons....it would be fine for a small lake but for any big water at all I would be in trouble or at least very wet.

My boat looks normal dont get me wrong.......but you guys are pontooners you know when u see an old school toon vs a modern one the difference in floatation is extreme. My toons measured 19" across my neighbors toons measures 25" across.....just to give you an idea. I will enclose a pic soon.

So a 40hp outboard on a 24ft old school toon there isnt a lot of high speed getting up on plane stuff going on.....at least not yet.

Again I caught myself rambling. I always seem to post right after my morning coffee when I am jacked on caffeine.

[JV74]

Lets see if I can show u guys a pic of the ole gal on her maiden voyage......she wasnt finished but had to take her for a spin to motivate me to finish the job.

Just lookin at this pic myself....man if it sits this low with nobody aboard imagine 4 200lb people in the stern cruisin

[steve1313]

Not a great pic, but my boat sits pretty low even on 25" tubes. But I've also got the horsepower and the third tube so that when it is underway it sort of planes and rides a lot higher in the water.

wplane.JPG (157.01 KiB) Viewed 9051 times

[BobL]

I have had a couple of semi successful runs with only 2 people on board where i was able to throttle up at least 50 % before ventilating. Recently I had 4 people on board and the ventilating was terrible....we could barely get going before it would just rev up. The weird thing is that it seemed to get worse as the day went on which makes no sense at all to me.

I am thinking a spun hub as well but I would also look at weight distribution and motor height. A couple of questions.

1 - where was your passenger sitting when there were 2 people on board?

2 - where were your passengers sitting when there were 4 people on board?

3 - where is your fuel tank located? Looking at the picture, it looks like it is in the back, next to the engine (which is normal). When you said things "seemed to get worse as the day went on" and if you tank is in the back, then your rear weight was getting lighter as the day went on due to fuel consumption.

Looking at your picture, your boat appears almost level in the water. Most boats have a decent rear "squat" stance at rest. If you have a lot of weight in the front then that can create a scenario similar to what you are describing. If your hub is not spun, I would throw some weight in the back and see if that helps with the problem.

[wwind3]

40 hp with 19 in tubes and a 24 ft boat? You are basically trying to push a dumpster thru the water. Gonna be tough to get much out of it. The main thing with a toon is horsepower-the more the better up to the boat rating. Mine is rated for 90.

I have a 20ft 25 in logs and a 90 Optimax... Mine performs great from idle to wide open. Can trim it out a lot when by myself and can run it at the over rev limit at 27-28 mph and 5800 rpms 16 inch pitch stainless 3-blade prop.. Also have underskinning to smooth out the ride and maybe provide a little bow lift--esp into the wind. Like I said tho'--I've spun my prop out twice...

[JV74]

Lets see if I can remember the questions......
My successful runs were with 2 people each time but both brief as we are still working the bugs out. Both times when running so called successful We had 2 people sitting in the aft quarter of the boat. Gas tank is just a 10 gallon tank and it sits port stern.

Our unsuccessful day was with 4 people all sitting aft quarter of the boat but never got past 1/3 throttle before high revs........we tried to move some bodies forward 400lbs worth and it did not help.

As for the 40hp on the barge......this was just gunna be a cruiser all along.....for $2500 bucks I figured it would get me on the water....of course I am into the boat now for more like $7k dont ask me how I sunk 4k into this resto cause I tracked every dollar and it went fast. New deck....new flooring....all new wiring with add a battery system....bought used furniture....bimini.....new console.....some lights.....a few new cross beams.....and boom...money gone! Thats boating 101.......
But I just wanna be able to slow cruise the lake on a nice day then sorta run in before dark at say 10 to 15 mph...ok maybe 20 would be nice. In the next season or two I will upgrade to a more modern rig.

I am hoping this is a spun prop or some mystery in the throttle making the engine think its between neutral and fwd or something......