Pontoon Boat & Deck Boat Forum

We bought our pontoon used last year, had to do some motor work and some upgrades, got a slip for it at the local marina and put it in water for the first time Saturday. Get to the marina Sunday, noticed the pontoon didn't sink (that's good) so we took her out on our first trip. We (me, the wife, and my son) were going with the current (might be important bit of information), I opened her up and it was singing at 5500rpm, right where she should be. No issues with the motor, prop, nothing. It was such a great feeling to know all the hard work has paid off.

On the way back, going against the current, I had issues with ventilation just going straight. The wife and son were in the same seating as they were when we first came out. The motor was not trimmed any differently. I did lower the trim a little but didn't seem to help. the wind was picking up also which made the river slightly more choppy but not white capping or anything close. I know the river conditions were not as good as they were when we first went out.

Sorry I don't have any pics of the back of the pontoon to see where the cavitation plate is in regards with the toons.
I'm running the stock 10.63 x 12 prop, one of the blades has a little damage on it and it will be replaced sometime soon.
No hydrofoil added on (if that makes a difference)

Is it possible that the motor may just need to be lowered just slightly? I think it can be lowered another notch. Maybe I'm right on that line?

Sounds like, as you mentioned, ventilation (and not cavitation). Dropping it down a hole may help. In general, the cavitation plate on the motor should be even with the bottom of the mounting box.

You may want to wait on dropping it down a hole until you experience normal weather/water conditions then see what happens.

In the meantime, below are some good threads discussing the issue (there are more, but this is what a quick search revealed).

viewtopic.php?t=19069

viewtopic.php?t=21263

viewtopic.php?t=21468

viewtopic.php?t=11108

viewtopic.php?t=25241

Thanks Bob, I was doing some searching also and came across many of them. I'll be taking her back out this weekend to hopefully better conditions. The fact that it didn't do it on the way out is the one thing that makes me wonder about it. Once i get her back out of the water on dry land I'll have a better idea if the motor is too high. I know this is all part of a trial and error process, part of the fun of owning a boat.
I do need a new prop...this one has some slight damage and probably is the original prop to the boat.

Bill, For what it's worth:

If I trim my motor up to an "ideal" position for the current load and weight balance on my boat - it will "blow out" when getting into waves, choppy water or crossing a wake when running wide open.

Once my prop loses its bite on the water and ventilates, I don't get it back until I cut power for a second or two, or trim a long way down, then ease it back up. I have learned to know where I need to set the trim under various conditions.

I feel that my prop is about right because I get 5800-5900 RPM running wide open and that is in the specified range for my Yamaha.

Marc

so update on this....when I had the pontoon out of the water saturday i could see the cavitation plate on the motor is just slightly above the bottom of the toons...so I think I am ok with the motor height. I can't go any lower anyways. When I took the pontoon back out sunday I only had one issue with ventilation and it was the same scenario as before...going against the current in a slight chop. so I'm going to say that the motor is in the right spot and it was the conditions that was causing the ventilation and not anything motor related.

I’ve noticed that mine is extremely sensitive to where passengers are seated. Any weight in the front causes blowout. When I installed my trolling motor, I mounted it’s battery forward under a side seat. Nope, ain’t gonna work there. Had to move it to the rear near the starting battery. Even having the top up will cause the boat to lift just enough to cause occasional blowout. I can trim up only a very little before it blows. Engine is alreadymounted as low as it can go. I just had to learn it deal with it.

A 4 blade prop will solve all your issues, might be able it raise the motor once it's on. You may lose a mph top speed with it. I loved the one I had on my 50 Merc. Pontoon props are never in hard water so the extra blade makes all the difference.

So what would you recommend as far as pitch if I go from a 3 blade to a 4 blade? I would think I should go down a bit in pitch, but how would I determine the amount? Is there some formula or simply trial and error? Thanks for any input you offer.

Usually you drop a couple inches going to a 4 blade. I was running a 9" on my 50. I don't remember the diameter but they all change slightly as the pitch changes. I actually called and talked to the guys at Michigan wheel and they recommend the size. Most prop manufacturers have calculators on their website. If you are running where the rpms need to be then dropping 2 inches should put you close to where you are now.

Thanks LA. I appreciate your responses. You’ve got me thinking more about this issue and the possible 4-bladed cure. I’ll keep you in informed.

Finally the boat is out of the water so I can take some pics: And another one... So now we can see where the motor sits in correlation with the toons and transom.
Does it look like the motor is sitting too low? If I trim the motor up just a little blowout will happen. I keep the motor trimmed all the way down and it works well...so damn frustrating.

Is your prop new? Make sure the hub is not spinning out. Going against the current puts more load on the prop. I've had my prop rehubbed twice---pontoon boat puts a lot of strain on a prop.. Like I say--like pushing a dumpster thru the water.


I have a 20ft Lowe with fishing seats. I can trim mine out similar to a bassboat w/o it blowing out. 90 Optimax with 13.xx x 16 prop. Not sure of diameter. 3 blade. Takes off well and trims out nice... Top speed 27 @ 5800 rpm. Wife and I went out yesterday---cruises at 18mph at 4200 rpm... Hope this helps.

I've towed a tube with 3 or 4 people in the boat with no probs---18 mph WOT....but it's working pretty hard!

wwind3 wrote: ↑

Mon Jun 15, 2020 10:48 am

Is your prop new? Make sure the hub is not spinning out. Going against the current puts more load on the prop. I've had my prop rehubbed twice---pontoon boat puts a lot of strain on a prop.. Like I say--like pushing a dumpster thru the water.

Prop is new, that was one of the issues before, the old prop was pitched wrong and I was going like 10mph at 5500 rpm.
I would figure if the prop was spinning I would see it even with the motor trimmed down like I have it now. It's just and adjustment to the trim and I get ventilation. Also the water is really turbulent at the back and even throws water up onto the back of the pontoon...another reason thinking the motor is too low...

Your motor appears to be at a decent height from the pictures. If you're blowing out at that motor height it's going to get worse if you raise it . How is the boat loaded , fuel batteries ,etc? Pontoons are plowing through the water so crazy spray is kind of normal.

Raising the motor doesn't cost anything so if you want to satisfy your curiosity give it a shot. As I mentioned to another member above in the thread a 4 blade prop is great on a pontoon. All the turbulence causes "soft" water that a v hull doesn't ever see. I spent a bunch of money playing with props on my old boat and since a couple friends pontoons. A prop change can solve so many issue with blow out. I swear on my last boat I could trim half out of the water lol

Two batteries, one on each side of the motor, a 18 gallon gas tank on the port side of the back that normally has 3/4 full of gas, 2 people (me and the wife) and we sit in the back. So most of the weight is around the back...I might look into the 4 blade prop...that's a simple change that doesn't cost a lot.