pontoon w/4toons?

I am building a 28ft.long, 10ft. wide pontoon w/ 4 pontoons. I took the lower unit off of a 3.0 liter omc w/ 4 cly. gmc block added a bw/velvet drive trans. which I put on the deck and now ready to add shaft & prop. My question is= does the angle of the shaft w/prop need to be at any particular degree of angle.how deep in the water should the prop be, and what size should the prop be if the "wot" is 42 to 4500 rpm?

 

Hi Billy,

There are a few things to consider when putting together this setup. I think it can work- heck, a Thai longtail can go like stink and that setup is even less sophisticated than this- but do think it through before cutting any metal.

The fact that you've made it a 4-toon suggests that you intend to be carrying a lot of different loads of wildly varying weight. That makes it a bit difficult for you to select a suitable prop- something that will give you acceptable performance when empty will likely overload the engine when you're piled with gear. Can you offer some guess as to how much the thing weighs empty, and how much you hope to carry with it?

I'm also trying to picture exactly what you are hoping to set up for the driveline. It sounds like you have the 3.0 I4 at deck level with the transmission bolted right to it? So the old outdrive is just sitting in a corner somewhere? (You can upload photos to the forum if you have any- it would be very helpful in trying to figure out what we're dealing with.)

Shaft angle could be a tricky issue. See some excellent discussion on some of the problems with inclining the shaft at: http://www.boatdesign.net/forums/inboards/prop-shaft-systems-24636.html . I wouldn't advise going with anything totally unorthodox without doing the engineering on it, but there's some insightful information there c/o Rick.

 

pontoon W/4 toons

Marshmat,Thank you for your reply & encouragement! Don't have a clue as to weight,but this will be recreational- not commercial. May have 2 to 16 folks@150 lbs. avg.=2400lbs. Engine & trans are mounted to deck with 8ft. shaft. Spent some 2 hrs. with "prop shaft systems"(thanks again) which alerted me to the concern of alignment of prop. There could be as much as 20to30 inches from deck to prop under water(20 degrees?) As for prop size--- being a pontoon we aren't interested in "out of the hole" performance but a prop with maximum speed when trying to beat a storm back to the dock. 90% of the time will be at speeds "just fast enough to keep the flies off". Thanks again for your time& input.