Stringer drive in a pontoon

[4doorcat]

Hello, I have a 1984 24ft sun tracker party barge. It was equipped with a 85hp mercury outboard that drank lots of fuel so in an attempt to help I removed the outboard and placed a 140hp omc stringer drive in the boat. Now here's the problem when I start to take off the boat runs up to about 10mph and 2500 rpms. Then it cavitates so badly that it will go no faster. I have been looking around this site and found some good info on drive placement on fiberglass boats, however nothing on pontoons. Can anyone here help? The boat has a 13 3/4 x13p prop on it that looks very similar to the one on the 85hp.

[PAR]

What year OMC is it? It's not a well liked drive setup, fraught with issues, but there are still plenty of them around. The first thing that comes to mind is the distributor advance weights are frozen, stuck or rusted in place, but frankly it could be a bunch of different things.

The best advise you could receive is to buy a repair manual for your engine/drive assembly.



This one covers all the Stringers and is about 30 bucks. It'll pay for itself the first time you don't have to take the boat to a professional.

[gonzo]

How did you set the engine? If it is in the outboard well, it is too high and will suck air. Ventilation is more likely than cavitation.

[4doorcat]

Thank you PAR I have one those manuals. It is a 1979 drive. It dosen't seem to be a motor issue. The outboard well was removed and a new well for the drive built. It may be a ventilation issue rather than cavitation. Is there a forumla for figureing out the proper height?

[Chase_B]

just curious,..how far does the prop sit in the water just floating? and then ..how far does it sit in thewater once you start moving?,.. could the wake from the pontoons be disturbing the flow to the prop as you start moving?

if your new well sets below the top of the prop,..it would cause cavitation

[4doorcat]

The new well is about 3 inches above the cavitation plate. Just sitting the plate is about 2 inches below. When you start to move the cavitation plate is at water level.

[Chase_B]

I know this must be frustrating ,..the train of thought being that you just replaced one drive with another it should work,.. on my pontoon I have the long shaft motor and the cavitation sits below the water level until I start moving ,..once the boat starts moving ,..it planes off to where the cavitation plate is right at waters level,..so it sounds like your height is right,..what was the string drive on before you installed it on the pontoon? what I'm thinking is it might be a pitch issue ,.. on mono hull boats the water going under the hull actually gets compressed before it hits the prop,..so it doesn't get a chance really to create a suction voticy that reaches the waters surface,..like when you pull the plug on your bath tub,.. if the pitch is too much for your application ,.creating a vorticy in front of the prop "like pulling the plug out of your bath tub" and aereating the water before it gets to the prop,..which creates cavitation,.. the answer ,..would be to make the new Well,..as a third pontoon config,.so it actually stopped even with the bottom of the other 2 pontoons,..or ..change the pitch to match the outboard you had on previously?
I probably should have asked if the pitch between the 2 props .outboard and stringer drive were the same first ,..but ..just trying to save some typing

[4doorcat]

The drive was on a pontoon also. When I removed it I made some detailed drawings of the well it was in on the other and made mine to match. The prop is a 13p that is the same pitch that was on the outboard. I did make a longer front mount to lower the prop some but have not had a chance to test it

[Chase_B]

Ok then the only other thing it might be that I can think of would be an exhaust leak that is creating the cavitation?

let us know what you find out ok?

[Chase_B]

try this Gary's outboard Motor Repair 573-762-2777,.. he is in or near Flat Missouri,.. he is very knowledgable in all marine drives ,..hope he can steer you right,..not sure how far Festus MO is from flat ,.. but over the phone he can diagnos allot of things

Chase

[tunnels]

if its a party boat try getting more people in the back of the boat rather than the front and see if that helps with the airation /cavitaion problem !! movable ballast !!