24 ft Pontoon w/115hp Johnson

[Aqua Chalet]

Gentlemen
I have a 24 ft Aqua Chalet pontoon boat with a 1984 115 hp Johnson outboard. I think it has a 16 pitch prop. I just rebuilt the powerhead and carburators but am still having issues with the performance.
When accelerating from mid throttle to higher throttle, the boat will gradually move faster and then the RPMs will suddenly go higher and seem to lose thrust.
Can't seem to get faster than 10 to 15 mph. Only guessing because have not gotten around to replacing the gauges.
Reducing the throttle does not seem to help. Though I see water churn, it is only when I shift back to neutral and then reengage the throttle does the boat reaccelerate.
I thought it might be cavitation so I installed the motor on an adjustible transom mount and lowered it 3 inches. It made the motor quiter but performance got worse. The motor was 5 inches further back from the transom and pushed the back of the pontoon down an inch deeper in the water and the front up a bit.

The weight distribution on the pontoon isn't balanced yet

Is it the Prop?
This is an through exhaust prop. Is it possible it could be ventilation?
Could it be the lower unit?
Any suggestions?

 

[The Rooster]

Re: 24 ft Pontoon w/115hp Johnson

Almost sounds like a spun bushing in the hub of the prop !!!

 

[lncoop]

Re: 24 ft Pontoon w/115hp Johnson

Almost sounds like a spun bushing in the hub of the prop !!!

Distinct possibility.

 

[steelespike]

Re: 24 ft Pontoon w/115hp Johnson

remove the prop and draw a waterproof line across the hub.then test run and look for seperation of the mark.
I think a 16 is too much prop I think a 13 is closer to what you need. To do a proper test you need a working tach and a gps or speedometer.

 

[EGlideRider]

Re: 24 ft Pontoon w/115hp Johnson

Yep. Probably the hub. And the torque of that big engine with a 16 pitch prop may be compounding the problem. I agree with steelespike that you need a 13. A 16 is much too big.

 

[Aqua Chalet]

Re: 24 ft Pontoon w/115hp Johnson

Thanks Gentlemen
I will give it a try with a new prop at 13 pitch.