Is this cavitation ? and what to do?

[jimbojonesmc]

Hello all
1986 24 ft Lowe Sunbird pontoon. 1986 90 HP Johnson c4 outboard that came on pontoon when new.
Runs great no missing no hesitation. Throttle past 3/4 and start hearing a sucking gurgling sound from motor and slight
lunging of pontoon. Have someone drive and i look at motor at the time this is occurring and i can see the cavatation plate and even below it on motor without problem. Motor is bolted as low as it can go on transom from factory and has the factory prop on it.
Am i describing cavatation and if so what can i do or what can i check.

 

[jrs_diesel]

Re: Is this cavitation ? and what to do?

What you're describing sounds a lot more like ventilation, not cavitation. Air is making it past your anti-ventilation (A/V) plate and into the prop. When this happens, the prop loses "grip" on the water and suffers a "blow out" meaning it can no longer move water like it's supposed to.

About the only fix that comes to mind is to find a way to get that motor lower in the water or turn it into a long shaft engine to get that prop deeper into the water.

 

[jrs_diesel]

Re: Is this cavitation ? and what to do?

Oops, double post.

 

[boobie]

Re: Is this cavitation ? and what to do?

Get some people and have them all go to the back of the boat once and see what happens when you run it.

 

[racerone]

Re: Is this cavitation ? and what to do?

Motor is mounted too high.

 

[boobie]

Re: Is this cavitation ? and what to do?

Well doing what I suggested would be a good way to prove it.

 

[midcarolina]

Re: Is this cavitation ? and what to do?

Just get a jackplate and do the reverse of most folks and lower the motor to the proper height.....

About 25 years ago I made one for a boat I had out of 4x4 aluminum angle.........worked great

 

[emdsapmgr]

Re: Is this cavitation ? and what to do?

Seems odd that this pontoon has worked well all these years as is, but now it's got a problem. Try another propeller.

 

[Grub54891]

Re: Is this cavitation ? and what to do?

^^ yup,maby the prop is slipping its hub.my 1980 something finally did the same thing,had a new hub pressed in,good to go!

 

[71Windsor]

Re: Is this cavitation ? and what to do?

If the prop was just spinning on the hub it would certainly explain the lunging described but I dont think you would get the gurgling sound described sounds more like ventilation. Did you purchase the pontoon new or did you purchase it used with the previous owners word that the engine was mated with it since new?