Explain Cavitation

[whozleft]

I bought my first boat this year. It's a 2000 22' Bennington with a 2000 50hp Johnson motor.

Yesterday my wife and I were out on the boat and it was doing fine for several hours. Then all of a sudden it started cavitating a lot. What causes this? When we bought the boat the marina put a new prop on for us because the salesman tha took us out for the first time said he noticed it was cavitating then. At the time I didn't know what he was talking about. After theyr eplaced the prop I asked him about cavitation and he gave me a little explanation. Then he said the cavitation ring? or something in the old prop was loose or something. Maybe I misunderstood him.

Bottom line is, why would the boat run great all afternoon and then all of a sudden start having problems?

I'd appreciate some help in understanding what occurs and what I can do to help prevent this from happening. By the way, we were both sitting in the same position all day, me driving and my wife on the side near the back of the toon.

Thanks a bunch.

 

[Yepblaze]

Re: Explain Cavitation

As you run along a bit of seaweed or other aquatic debris gets caught up in front of the motor part that hangs down into the water and disrupts the smooth flow of water into the propeller causing a turbulence that gets amplified into a froth that prevents, or inhibits the forward thrust.

There are a number of other reasons, explanations, or conditions commonly called cavitation, but this might explain what you are experiencing.

Usually a moment of backing up with a turn, or lifting the motor will dislodge the debris allowing you to go on as before.

Even if cavitation does not become apparent, debris wrapped around the drive housing can lead to catastrophic failure as it can many times block the cooling water intake causing engine failure.

 

[Silvertip]

Re: Explain Cavitation

You more than likely are not experiencing "cavitation" but rather "ventilation". Ventilation simply means the prop has lost it's "bite" on the water. Weeds are a common cause of this as was explained. Having one or more passengers move from the rear of the 'toon to the front causes the bow to settle deeper in the water and the stern to rise. It can rise far enough that the prop is not deep enough in the water. The motor may not be installed low enough on the engine pan although standard practice is to mount them as low it can go so I doubt that's the issue. Lastly, most prop manufacturers offer pontoon-specific props. These props have very large and very uniformly shaped blades when compared to standard props. This large blade area provides the push and bite needed for pontoon service. It also provides much more thrust in reverse than a standard prop.

 

[rickdb1boat]

Re: Explain Cavitation

Also, as you use fuel, the weight that the full tank provides lessens. The rear of the boat rises and causes the prop to ventilate. Happens to my in-laws Crest pontoon everytime they get below a half tank..

 

[whozleft]

Re: Explain Cavitation

Thanks for the input. You have all given me some things to check.

 

[Silvertip]

Re: Explain Cavitation

Here is a very brief description of "ventilation".

http://boatpropellers.iboats.com/pr...ation/?**********=148177262&*******=406716992

And here is a very brief description of "cavitation".

http://boatpropellers.iboats.com/pr...ation/?**********=148177262&*******=406716992

And to the bottom and just to the left of the above description are titles you can click to get descriptions of cupping, rake, diamter and pitch. As you investigate the issues with your boat you will need to understand these terms and be able to communicate with this forum or propeller folks.

 

[chrisg]

Re: Explain Cavitation

Silvertip: the links are messed up (they do not display correctly)

But the explanation of what caused cavitation may explain some of my issues. My toon sits in the water all summer, and I noticed recently the motor was acting funny. When I pull it out of the water, the lower unit looks like it has a grass skirt on it.