cavitation.....HELP!!!!!

[bmdakk]

Ok I have a 1986 crest 25 foot pontoon, New to me. 50 HP evinrude ( 86 also) (e50belcde) Got the motor running great and it pushes the boat along at a pretty good clip, However, I noticed that the motor doesnt set down on the transom all the way. it is bolted to it but the lip that would rest on the top of the transom is off about 2 inches. I can see where someone who had it before me had drilled new holes to put the motor in the current position.. My problem I noticed today when I took it out is that it has real bad cavatation at WOT going straight. I figure it is where the motor is raised, but upon further inspection I can see that the motor skeg would be touching the ground if I lowered the transom plate. I am gonna replace the transom anyway as it appears to be starting to rot. Do you think by lowering the plate back down will fix the cavitation? would a hydro fin help any. Im just not sure what to do about this.. While just sitting in the water the cavatation plate is about 4-5 inches under the water level but at WOT I can see the cav. Plate. Thanks.

 

[MaPaHa]

Much of this trial and error because it's hard to judge where the motor should set on a pontoon. I would say this is more of a "ventilation" problem than cavitation because it seems the motor is running out of good water and sucking air. One thing to check is to make sure the prop is a "pontoon" prop rather than an ordinary one. They are often called big ear props and they help with ventilation especially in turns. Was the front of the boat loaded down when this happened or was it balanced? I've found that a properly selected 4 blade prop holds in the water better than many of the 3 blade props but that may be overkill to switch unless you're changing it anyway. A good prop makes a big difference.
I have used fins off and on for 30 plus years on different boat and always had good luck with them. I dont think you would go wrong there and would probably help in several ways. After that, you could drop the motor a notch at a time to see what works best but I wouldn't drop it more than you have to.

 

[ahicks]

I'm confused. Does this boat have adjustable transom height - a big lever that lets you raise and lower the engine? If so, is it set so the engine is down all the way, or somewhere else?

If you don't have that feature, then I would lower the engine on the transom.

And if there were any question of a rotted transom, I'd be replacing that sooner than later. An engine hanging from it's cables, one half of the transom, or worse, on the bottom of the lake, is not a pretty sight.....

 

[Scott Danforth]

welcome aboard

here is a bit of knowledge gained on pontoons over the years.

set up the motor for how you will run it most of the time.

if you have a bunch of people on board with coolers and stuff most of the time, the motor will be higher, and ventilate the few times you go out alone.

if you only run by yourself most of the time, the motor will be lower, and the boat will be a dog the few times you go out with people on board.

the prior owner probably had a hand full of people most of the time, so the motor was raised accordingly

you should see the anti-cav plate when on plane.

 

[bmdakk]

after some reading I think it may be a combination of a bad prop along with the motor height The prop has some nicks and one blade is got a bent spot in it. Im trying to upload pics so You can see.

 

[bmdakk]

HOPEFULLY HERE ARE SOME PICS. iN THE ONE YOU CAN SEE THE WATER LINE. when running WOT I can see the cav plate.

 

[bmdakk]

OK Lets try this.>>>>

 

[Mi duckdown]

Can't see the prop. But I will tell you something, That transom wood NEEDS to REPLACED

 

[bmdakk]

Yea Its getting done this weekend. Just need to know where to mount the motor back.

 

[HarborRat]

Rebuild the transom with the same hole locations...but lower the motor. You can see that the motor used to be mounted in the top holes, and can see the marks on the top of the bottom slots where it used to line up. I don't see where anyone drilled new holes, they just raised the motor. Start with the top holes and see how it goes.