Cavitation/ventilation or engine problem

[richard570]

I have a new 18' Fiesta with a 50hp Merc. I have had an intermittent problem
that the dealer blames on cavitation. Motor will be running fine at all speeds
and suddenly will lose power, appears to shift into neutral, RPM's increase. Will come back if I shift into neutral. However as soon as I throttle up, it does it again. Operates perfectly under 2000 RPM's. I am sending it back to the dealer tomorrow. Anyone have any thoughts?

 

[Jeep Man]

Re: Cavitation/ventilation or engine problem

To help you, we need a little more info. Is your motor a long shaft or short shaft? What size prop are you using? How many people on board (combined weight), and how is this weight distributed?
Pontoon boats, unlike regular hulls, do not have a defined bottom, which make setting your motor height harder to place. This combined with the fact that you have, in essence, two narrow parallel boats splashing "dirty" water in the area of the prop which causes cavitation or ventilation. Adjusting the motor downward can help. Going too far can cause drag. Some owners add a hydrofoil to the cavitation plate, which gives it more surface and keeps more water on the prop. Don't blame the dealer yet. It's a bit of trial and error to balance things out.
Another possibility, but not likely because the motor is new, is prop slippage. This is when the inner rubber section breaks loose from the blade section, causing the prop to "slip", but as I said, this is not likely to happen on a new prop unless there is a defect.

 

[richard570]

Re: Cavitation/ventilation or engine problem

Thanks for your reply.....truly appreciate the input. I believe it is a long shaft.
Problem has occured with only me onboard, 250# and at the helm. It has happened on smooth and light chop water. The strange thing is it does not happen with any seeming regularity. I used it all day last Saturday and ran perfectly. However on Sunday, continuous problems over idle speed. It went back to the dealer about 3 weeks ago when I complained of similar problems and they said they found a problem in the throttle control, sic "venacle?" and made the repair. Obviously did not fix the problem. I believe the motor is mounted low. I think the prop is 10 1/2x12, numbers might be reversed. Boat went back to the dealer today, no diagnosis yet.

 

[mayorjones]

Re: Cavitation/ventilation or engine problem

Did we ever get a diagnosis on this? Just curious.

Thanks.

 

[Silvertip]

Re: Cavitation/ventilation or engine problem

Is your motor a "Big Foot"? If not, you have a standard gearcase which does not work well on a pontoon. That is why Merc markets the Big Foot. It has a 2.33:1 gearset and can swing a 14 inch prop so there is much better push from the engine and far less chance of ventilation. Read the other posts in this forum about this problem.