Pontoon blowing out @ 4,500 rpms.

[limoman]

I have a 22 foot pontoon with a Mercury 115 EFI out board with a 4 blade pontoon prop (4th one I have tried). It takes off fine and I just tap the trim to stop the strain and it does great until about 4,500 RPM's. At this point it occasionally acts like it is coming out of the water or the prop is pushing air and the rpm's shoot up as I slow down. Forgive me if I used the wrong terminology but I know very little about boats.

It appears that the prop itself is about 4-6 inches under the water so I am looking for solutions. The water line in the photo looks like it may sit a little deeper in the water?

Maybe lower the motor or is it too deep and maybe need to be raised? I have no idea what issues either may cause but see no reason why being too low would cause any issues? HELP PLEASE...

 

[MH Hawker]

Re: Pontoon blowing out @ 4,500 rpms.

Drop it down a bit.

 

[Bamaman1]

Re: Pontoon blowing out @ 4,500 rpms.

Let me guess. You've got 3-4 people sitting in the front of the boat? My old boat would ventilate when too many people sat in the front.

Your boat will run best with the anticavitation plate parallel with the bottoms of the toons. The twin tube nosecones plow through the water, and there's really no reason to trim your boat except when in shallow water or when putting it on a trailer. Trimming the motor too high will result in ventilation on any boat.

See if running the engine parallel to the toons helps your problem. Next step would be to lower the engine on the transom, even if it had to be slightly cut out. Or, you could put a manually adjustable jack plate on the transom.

 

[limoman]

Re: Pontoon blowing out @ 4,500 rpms.

Let me guess. You've got 3-4 people sitting in the front of the boat? My old boat would ventilate when too many people sat in the front.

Your boat will run best with the anticavitation plate parallel with the bottoms of the toons. The twin tube nosecones plow through the water, and there's really no reason to trim your boat except when in shallow water or when putting it on a trailer. Trimming the motor too high will result in ventilation on any boat.

See if running the engine parallel to the toons helps your problem. Next step would be to lower the engine on the transom, even if it had to be slightly cut out. Or, you could put a manually adjustable jack plate on the transom.

I do not trim the boat, just hardly tap it for 1/2 second to ease the strain sound from the motor pushing water straight down. I never even get it close to being level with the toons. 90% of the passenger weight is in the rear with only small kid's up front.

 

[lakegeorge]

Re: Pontoon blowing out @ 4,500 rpms.

Your cavitation plate should be even with bottom of boat or 1 inch below bottom.

 

[limoman]

Re: Pontoon blowing out @ 4,500 rpms.

Drop it down a bit.

It's in the bottom hole now. I am starting to think that the Larger toons on this boat need a longer shaft on the engine? Maybe a jack plate or something so I can drop it a little more?

 

[limoman]

Re: Pontoon blowing out @ 4,500 rpms.

Your cavitation plate should be even with bottom of boat or 1 inch below bottom.

I do believe it is a good bit lower but I will confirm this weekend.

 

[nosmoke1]

Re: Pontoon blowing out @ 4,500 rpms.

I do believe it is a good bit lower but I will confirm this weekend.

Hi I'm new to this forum but if your plate just above the prop is even with the bottom of your tubs or just below a little your motor when trimmed a small amount cant raise the front of the toon so it can cause air to get to your prop not good I think you would be better to have the prop cupped that will give it more bit in the water but it also increases the pitch about 1" you can also try a fin on your bottom plat some of these fin will help hold the water in the prop helping to hold the water around the prop. The reason I referred to the bottom of your tubes is some times the motor is not set to their bottom when a boat can get to a certain speed it can lift its self to that area of the lowest point of your boat in this case your pontoon and if the prop is not set below that area it can try to pull the top of the prop out of the water that introduces air and the prop slips. If this is true you will have to drop the motor plate down below that plane .

 

[BobGinCO]

Re: Pontoon blowing out @ 4,500 rpms.

I do not trim the boat, just hardly tap it for 1/2 second to ease the strain sound from the motor pushing water straight down.

This doesn't make sense to me. "Ease the strain"? What strain? "Pushing water straight down"? I've never seen a motor push water down.

I usually run my motor trimmed all the way down, because trimming it up doesn't actually make any difference. If you're running your prop 4-6 inches under the surface, I would say you are WAY too shallow. My 115 would be shooting a rooster-tail way back if it were running that shallow. Try putting your motor all the way down, and see how it runs.

 

[Triphammer]

Re: Pontoon blowing out @ 4,500 rpms.

Cav plate even with the bottom of the tubes or the bottom of the motor pod? Mine was bottom of tubes so I built a non- adjustable (except cutting & fitting) jack plate to move motor out & up( 7 inches up to bottom of motor pod) but I think too far up. Some advice, please?

 

[limoman]

Re: Pontoon blowing out @ 4,500 rpms.

This doesn't make sense to me. "Ease the strain"? What strain? "Pushing water straight down"? I've never seen a motor push water down.

I usually run my motor trimmed all the way down, because trimming it up doesn't actually make any difference. If you're running your prop 4-6 inches under the surface, I would say you are WAY too shallow. My 115 would be shooting a rooster-tail way back if it were running that shallow. Try putting your motor all the way down, and see how it runs.

When I have the trim all the way down it feels/sounds like it is under strain. If I just tap it for 1/2 a second the vibrating/strain feel goes away and it runs way smoother but the prop is still pointing downward more than level. I have it in the bottom holes now so thinking I may need a jack plate to lower it more. It has large pontoons and a mid shaft motor. One marina told me that for those toons I should have a long shaft motor (about 4-5 inches deeper than what I have). When I dropped it 2 bolt holes to the bottom like it is now it runs way better and will go to 4,500 RPM's which before it blew out around 3,500 RPM's.