Cavitation/trim problem

[jfellows]

I recently bought an old Weeres 20-foot sportsman pontoon, with an adjustable transom. It has a 1981 35 HP Johnson short-shaft motor and runs well, but I can't power up the boat without serious ventilation and cavitation. I can ease the power up and get up to speed,but at any time it may begin cavitating. It feels like the prop is coming out of the water. I had a marina replace the prop -- they put on a 9X9 pitch. I've tried raising and lowering the motor transom; it seems to work best at the highest setting ( the ventilation plate is well under water, but the water flow is turbulent). The prop did not help. It occurred to me the problem might be trim, but I don't know what trim angle is appropriate. ANy ideas?

 

[EGlideRider]

Re: Cavitation/trim problem

I'm certainly not an expert on this subject but I doubt if you are cavitating, probably only ventalating. The normal solution to ventalation is to lower the motor as much as possible and and trim the motor such that the prop is as close to the boat as possible.

Prop size can affect cavitation but has little to do with ventelation.

Please post back here if you correct the problem.

 

[Jeep Man]

Re: Cavitation/trim problem

I had the same problem on my older toon. It had a short shaft and would only run without ventilating if it was trimmed all the way down, which affected performance. I did not have an adjustable transom. My only solution was a long shaft motor. To answer your question, your motor should be close to vertical when up to speed.