Cavitation/Aerateing Problem?

[stevetedder]

I think I have a aeration or cavitation problem with a 24' sundancer 90 hp johnson. The problem makes me think of a car suddenly hitting a patch of ice, the rpms go up and the forward momentum goes down. At lower throttle it does fine it only seems to do it at around 3/4 and up. Im pretty sure i has a 13-13 alum. prop. It was recently reshaped by a local marine shop.(It did this before they reshaped it also)The motor is dropped all the way down to the lowest setting on the transom. Ive been playing with the trim with limited results though. Does this sound like cavitation? Any suggestions on fixing it? Any help is appreciated. (sorry the pic is so dark but just got in from lake and wanted to post this while I was thinking of it If need be I can retake it tom.)
Update: added another pic I already had not a great angle but more light.....

 

[EGlideRider]

Re: Cavitation/Aerateing Problem?

It could be cavitation, but it is more likely to be a ventilation problem.

I assume you have the long shaft motor.

Also examine your loading distribution to make sure you don't have too much forward weight.

 

[Silvertip]

Re: Cavitation/Aerateing Problem?

Probably just "blow-out" where the prop simply blows a hole in the water and loses bite. There are props available that are specifically designed for pontoons. If you look carefully at them you will see very large rounded blades that look almost the same on the leading edge as they do on the trailing edge. These props have very large blade area and are intended for pontoons and work boats. Any reputable prop shop can help with selection.

Read this article but pay attention to the last paragraph.
http://hubpages.com/hub/Picking-the-Perfect-Pontoon-Boat-Prop

 

[steelespike]

Re: Cavitation/Aerateing Problem?

What rpm does the motor run at at wot?(wide open throttle)
Cavitation and ventilation are 2 different things.Cavitation doesn't directly affect performance.Ventilation is just that the prop becomes air bound usually requiring a throttle reduction to solve.As suggested a "pontoon"prop will probably solve your problem.Most good 4 blade props will work well.We have seen good results with the Solas Amita. It works well and is predictable.Though pontoon applications have so much turbulance results can vary.

 

[stevetedder]

Re: Cavitation/Aerateing Problem?

Im not sure if it is a long or short shaft motor(my ignorance)
I have a good bit more weight on the back than on the front(32 gal fuel tank on one side and 3 batteries on the other). Im not sure what the WOT is as it is not equipped with a rpm gauge. The guy who reshaped the prop also reshapes them for a local marina and had hundreds of them in his garage, he knew it was for a pontoon boat and I really feel like he would have said something if the prop wasnt correct. But if it is ventilation or blow out would a 4 blade prop help with that?

 

[Silvertip]

Re: Cavitation/Aerateing Problem?

Not necessarily. You first need to make sure the engine you have has a long shaft. You measure that by measuring from the top of the engine pod (the point where the engine sits) to the anti-ventilation plate just above the prop. That dimension should be 20 inches if you have a long shaft, or 15 inches if it is a short shaft. Changing props will not fix the problem if you have a short shaft engine. If it is a long shaft, then ensure the engine is mounted full down on the pod. If those two things are correct then a "pontoon-specific" prop is in order. Those can be three or four blade.

 

[ibewolfer]

Re: Cavitation/Aerateing Problem?

kinda normal for pontoons....usually happens when turning or in rough water....caused by turbalance from the toons usually...or waves picking the back up the boat up enough to cause blowout etc.....notice most of the newer toons and extensions moving the motor futher back away from the toons and usually lower in the water aswell...don't see how a prop change will help much

 

[mayorjones]

Re: Cavitation/Aerateing Problem?

Don't mean to freak you out - I had a nightmare of a problem with my 2008 Sun Tracker when I bought it last year, and ended up changing motors to a Bigfoot. It had a 50 hp 2-Stroke Mercury, and the prop was just too small and geared wrong to push a boat that size. There are lots of things you can try before doing the drastic step as I did. If your motor is adjustable, you'll want to check and see if it can be moved any lower. Also, if you're dealing with cavitation, I'd just plan to keep it trimmed all of the way down and not mess with it. I tried a different prop and it didn't work for me, but there are some out there - I ordered one off of this website, tried it and it didn't work and they took it back as long as it was still in new condition, so you might want to try that.

You'll get all kinds of different ideas, and I'm certainly not pretending to be an expert. Mine was to the point that I simply couldn't use it, but it sounds like yours is not such a problem. There can also be such a thing as a spun prop, that will act up if you're putting added stress on it, such as out of the hole or against strong current.

Best of luck.
Dwight

 

[stevetedder]

Re: Cavitation/Aerateing Problem?

It is the long shaft and placed all the way down in the bottom hole set. After playing with it a while I have leanred that initially upon takeoff I have to raise the trim to avoid a "shuddering " feeling, and if I continue to accelerate If I bring the trim back down it seems to do much better. Seems a little odd to me but that is what is working. I have also pulled the plus in the pontoons and drained almost 20 gallons of water, that was probably not helping matters. Good news is the problem is at least manageable now, thanks to all who responded.

 

[Boss Hawg]

Re: Cavitation/Aerateing Problem?

I have also pulled the plus in the pontoons and drained almost 20 gallons of water, that was probably not helping matters

Might want to address that problem soon -

 

[stevetedder]

Re: Cavitation/Aerateing Problem?

I have taken it out since then after all day on the water it only collected a gallon or two, cant see any obvious cracks or anything though,fortunately they are foam filled. That did solve my tracking problem though.

 

[Jeep Man]

Re: Cavitation/Aerateing Problem?

I have taken it out since then after all day on the water it only collected a gallon or two, cant see any obvious cracks or anything though,fortunately they are foam filled. That did solve my tracking problem though.

I wouldn't be quick to say "fortunately" when talking about foam. The foam will become water logged over time and then you have a monster problem.

 

[stevetedder]

Re: Cavitation/Aerateing Problem?

Seems pretty heavy in the back already. Im leaving the plug out all the time now when its not in use. been dripping for 2 days now.

 

[stevetedder]

Re: Cavitation/Aerateing Problem?

still dripping.....

 

[lncoop]

Re: Cavitation/Aerateing Problem?

Sounds like you have waterlogged foam. That definitely adds weight, which, as you said, doesn't help matters. Just leave the plugs out until there's no more dripping, then take it to a weld shop. Not sure what the procedure is for finding leaks in foam filled toons, but hopefully they'll be able to pressure test them and repair the leaks.

 

[Jeep Man]

Re: Cavitation/Aerateing Problem?

It is the long shaft and placed all the way down in the bottom hole set. After playing with it a while I have leanred that initially upon takeoff I have to raise the trim to avoid a "shuddering " feeling, and if I continue to accelerate If I bring the trim back down it seems to do much better. Seems a little odd to me but that is what is working. I have also pulled the plus in the pontoons and drained almost 20 gallons of water, that was probably not helping matters. Good news is the problem is at least manageable now, thanks to all who responded.

Have you always had this prop problem, or is it a recent problem? Possibly you have a spun prop.

 

[stevetedder]

Re: Cavitation/Aerateing Problem?

Ive had the prob. since I had the boat. I Had thought that about the spun prop also,but I made a mark running from the bolt in the center out into the prop. A couple of trips later it still lines up. That wouldnt be the case with a spun prop or would it ?

 

[stevetedder]

Re: Cavitation/Aerateing Problem?

Sounds like you have waterlogged foam. That definitely adds weight, which, as you said, doesn't help matters. Just leave the plugs out until there's no more dripping, then take it to a weld shop. Not sure what the procedure is for finding leaks in foam filled toons, but hopefully they'll be able to pressure test them and repair the leaks.

I have read that around 2-3 psi in the tube and some soap water is more than enough to find a leak. I did see a pic on iboats somewhere that someone put 10-20?? psi and it looked like it had been peeled open. I dont have a shraeder valve or anything installed on it anywhere but Im thinking I can just put the plug back in while it is still cool in the morning and by afternoon when its around 100 degrees the air inside the toon will expand and air should be getting pushed out of the bad spot wherever it happens to be. Just dont know if there will be enough air pressure for that to work. Worth a shot anyway.

 

[pjt123]

Re: Cavitation/Aerateing Problem?

Hi. am having the same problem with my pontoon. Did you ever solve your cavatation/ventilation problem? so would like to find a solution

 

[pjt123]

Re: Cavitation/Aerateing Problem?

What rpm does the motor run at at wot?(wide open throttle)
Cavitation and ventilation are 2 different things.Cavitation doesn't directly affect performance.Ventilation is just that the prop becomes air bound usually requiring a throttle reduction to solve.As suggested a "pontoon"prop will probably solve your problem.Most good 4 blade props will work well.We have seen good results with the Solas Amita. It works well and is predictable.Though pontoon applications have so much turbulance results can vary.

where would I find a longer shaft for a Force 50hp motor? I have3 a short shaft and it isn't working