Pontoon Ventilation Problem

Zigblazer

Cadet
Joined
Apr 27, 2014
Messages
17
I have a 1987 24' Pontoon, with a 1983 115hp evinrude with a couple problems.

I bought this pontoon last year and completely rebuilt it. All new floor, seating, console, steering cable, etc. Re-welded all the railings, made it like new again. I got the 115hp evirude for it and am using it.

The problem, I believe is ventilation. Once I get going up around 17-20 mph the prop acts like it is slipping. RPMs go up about 500 and we slow down a little. The prop was new last year (15p) and I tried marking the hub to check for slippage, and the marks stayed aligned. At 17mph the motor is sitting around 3000rpm. With more weight on board we can get up to 20mph, but with just 2 people the problem starts at 17mph. The motor is mounted as low as it can go and I tried raising it, but that only made the problem worse. If I remember right it has 23? pontoons. I tried to make a bracket to lower the motor further, but the steering cable got in the way, it is almost sitting on the mount now. I just ordered a hydrofoil and 4 blade 13p prop to try. They will be here Wednesday, and I will take it out to test Thursday. If anyone has any ideas to help let me know.

I've had a very busy spring and only got to get it out on the water for the first time last Saturday, which just reminded me of these problems, while I had been working on the trailer problems I had. I have family coming Wednesday to visit and we are hoping to take a ride down the North shore to Duluth to watch the fireworks on the 4[SUP]th[/SUP] from the pontoon.
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
Some pics might allow somebody to spot something wrong?

Assuming the prop shaft is parallel with the pontoons (not tilted), about the only thing you can do to control ventilation/cavitation is drop the motor. It might be possible to add a shim between the engine pod and cross members?

Some additional control, to a lesser extent maybe,might be to run a cupped prop if the one you have now is not.
 

1983 ercoa 21'

Chief Petty Officer
Joined
Jan 12, 2014
Messages
632
If ventilation is truly your problem a jack plate will allow you to lower your motor.
things to check
1 make sure your boat is loaded properly you don't want it nose heavy.
2 check the pin that acts as a stop when lowering the motor make sure it's not keeping the motor trimmed up to much.
the move to to 13p prop is a good one that motor needs to be running 5500 - 5800 at wide open throttle.
 

Zigblazer

Cadet
Joined
Apr 27, 2014
Messages
17
I will try to get some pictures soon. I may look further into a jack plate. I have plenty of welding and fabrication experience, and made my own, but forgot about the steering cable so it only allowed me to lower it an inch, then I figured if I lowered it any more I wouldn't be able to tilt the motor up much without hitting the original transom.

I don't believe the front is too heavy. In fact I think it is several hundred pounds lighter now than when I bought it. All the new furniture is lightweight plastic. The front sits fairly high while in the water.

I'm trying the hydrofoil because it was suggested to me by several people only to help the ventilation/cavitation problem. I am hoping it won't make things worse by not allowing me to trim down without lifting the motor more out of the water. Hopefully it will make a positive difference.

I don't know if the Michigan Vortex 4 Blade Aluminum prop I am getting is cupped or not. It was the one specifically recommended to help with the ventilation.

I won't get out until Thursday at the earliest to see if anything helps, but will hopefully get the hydrofoil and prop on tomorrow.
 

MaPaHa

Petty Officer 1st Class
Joined
Jul 6, 2012
Messages
239
I messed around with a jack plate trying to lower my motor at one point and they are not made for that. When you move the motor back and try to configure to lower it, they do what you say and won't tilt up without hitting. Also, as you mentioned the cables are a problem. I got mine to work but I was the only one that could drive the boat for fear of wiping out the front of the motor.
I went with a long shaft conversion kit from "Bay" that made it a 25" shaft. Not too hard to install but somewhere between reading the directions and figuring it out. It comes for several models and use what you need sort of thing.
Now you can use the jack plate for what it was made for and set the motor height where you need it. I think it cost $750.00 or so.
I'm pretty good at re-engineering stuff and I can tell you that a jack plate is not made to set back and lower a motor. From day one, if I was going to do only one or the other, I should have bought the extension kit and saved the TIME and money on the jack plate, but with both it works better.
I also run a 4-Blade prop but one that was recommend for pontoons. There are too many styles out there to just say 4-Blade in general. Mine is a Power-Tech Off-Shore prop factory drilled with exhaust ports and it does quite well and holds it's bite in the water in the turns.
My boat is a home built 22.5 feet tri-toon with all lifting strakes, 150 long shaft 1994 Johnson motor (now) , jack plate and PT 4-Blade prop. It runs right at 40 MPH, turns on a dime and will be on plane so fast it's unbelievable. Cruises real nice around 4,000 RPM in the mid 30's.
 
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