Re: 2nd opinions on prop selection

[fisherman6]

As I said, I’m no expert in hydrodynamics by any means. I have 3 14′ aluminum boats powered by tiller controlled outboards and a traditional 2-toon pontoon boat with the small diameter toons. The toons are 19’11" long and the deck is 16′. I have a 1976 Johnson 55hp on it. When I bought the motor it had a 17P prop on it and with that prop it would run along at 15-16mph and top other at maybe 4000rpm. It was lugging with that prop. I changed to a 13P prop with a slightly larger diameter and it was like a whole different boat. Motor runs at about 5000 rpm at WOT amd the boat moves along at about 20mph. Most of the time this boat is running at about 50-75% throttle and about 10-12 mph. I’m on the 3rd or 4th trim pin hole out where the AV plate is approx level with the water line when the boat is in the water. This seems.to be the ideal setting for my boat. That’s with me, my wife and 7 year old daughter on board. More people and it gets slower pretty quick. The tri-toon may be a bit different. I have no personal experience with tri-toons, but I know they often come equipped with much larger motors than traditional 2 tube pontoons. For whatever that is worth.

I know there is a lot different and more turbulent water flow under a pontoon boat hull than a traditional planing hull. I was not aware that major boat builders were building tri toons with the center tube lower on the water than the two outer ones. I’m not sure why they are doing that. My guess is that with the much higher power capability of the tri toon, the lower center tube would act more like a center keel and allow for more precipe steering with a large motor pushing the boat at high speed (40+ mph). With the boat moving at a speed that is barely planing the outer tubes (18-22 mph) the lower center tube seems like it is simply adding drag to the hull. The added turbulence created by the center tube that requires the deflector to keep water out of the boat has to be slowing it down. I’m also imagining that turbulence is making the prop cavitate when it is trimmed up. These are only theories from the seat of my pants and my limited experience. I could just as easily be "all wet" so to speak.

For what it’s worth, I also have a small pontoon I built out of an old pedal boat that I use on my pond. It has 8′ long toons and a 6′ deck. With a 4hp tiller outboard on it, I’m very limited on how quickly I can turn without prop cavitation. The turbulence created by the tubes in a turn really messes with the water flow to the prop. I have to back the throttle off so it doesn’t pull air from the surface into the prop thru the eddys. None of that is present with my planing hull boats. Pontoon hulls seem to have a different set of rules. Just my 0.02.
-Ben

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