Design Ideas for Fabricating 3rd Pontoon

Discussion in 'Boat Design' started by SlowCruise, Dec 14, 2011.

  1. SlowCruise
    Joined: Dec 2011
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    Location: Orangevale, CA

    SlowCruise Junior Member

    Hi everyone. Just found this forum and it is just what I was looking for! Awesome.

    I am going to fabricate a center pod for my pontoon boat. Basically a 3rd pontoon, but shorter and meant to act like the "Water glide" hydrofoil you may have seen.

    My brother in law is a kick-butt sheet metal man with all the equipment I will need to make just about anything. So I want to make this thing out of aluminum myself. I just need some guidance on the design. I have come up with the basic concept (see pics). when you look at the pics, keep in mind the nose is simply going to be flat triangles welded together. No Radius work, just to keep it simple (I only have a day or two of his time to work with) Im just not knowledgable on the engineering and design requirements on it.

    I'd like to know the angles for the nose, etc to maximize lift and hopefully help get my boat planing a bit (I have a 60hp 4 stroke, so I know the planing idea may be shaky, but I mostly want some extra floatation and at least give the planing a shot)

    Id also like to hear your thoughts on positioning of this pod. I know I want it 7 feet up from the motor so the prop will still work. I have had some issues with "submarining" a bit in rough water, so Id also like it to extend toward the front. But the waterglides normally go at or near the center of gravity, so I was going to slide it back that way a bit. any advice would be appreciated!

    Also I thought I'd build this thing 18" deep. (My outside pontoons are 23") The thought is not to drag the boat down too much with too much metal in the water, but I am really unsure on this. I may need to get it 23" deep to get the full upward push from the strakes, shape and nose on this thing?

    I plan to use lifting strakes on the outside pontoons (But only on the inside of them) and also on my new center pod.

    Any thoughts on the design for this?

    I'll definitely keep you posted as this thing gets built, too over the next month or so.

    Thanks in advance!
     

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    Last edited: Dec 14, 2011
  2. messabout
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    messabout Senior Member

    This a sticky wicket. More details will be required. For openers you might do better by re-thinking the new attachment. In the side view you have a tapered section that is 34 inches long. You have it connected to the remainder of the hull with a sharp corner. The two surfaces should be connected with a smooth curve. No hard lateral corners allowed.

    You also show a fairly steep vee bottom. That is fine if you run in rough water. The vee bottom that you have drawn will not contribute as much lift as a flatter vee or even a flat bottom. Flat bottom will aid planing more than the vee bottom. Unfortunately the flat bottom will tend to pound and also it would require more internal bracing. Sorry about that. If it is a boat you always have to trade one thing for another.

    Are your main pontoons round or some other shape? Please specify.
     
  3. SlowCruise
    Joined: Dec 2011
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    SlowCruise Junior Member

    Thanks messabout...
    The design of the hull is still very much open for suggestion.
    I could easily go with a flatter V on the bottom. As for the curves, if I did a slight rounding of that bottom front, could I keep this general plan? How much rounding do you think I'd need?
    And on the original pontoons, they are perfectly round.
     
  4. messabout
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    messabout Senior Member

    Neither air nor water like to change directions abruptly. The sharp corner that I criticized is a place where the water will become vioently disturbed thus sucking up a lot of your ower. The smoothest curve you can make will serve you best, To translate that remark just think in terms of the least change of impact angle that you can tolerate. Making the front section longer with a smooth transition to the bottom surface will be a better deal. That implies a gentle curve not an abrupt change of direction. You can still maintain a bit of vee that goes all the way to the pointy front end if you like..

    Round pontoons are about the worst cross section you could contrive for a planeing hull. You are stuck with what you have so do not despair. Just do not expect great performance with the power that you have. Adding a middle pontoon might help but maybe not as much as you would like. Weight is a big deal. If the boat was light enough then 60HP would be adequate even with round pontoons. But pontoon boats are party boats and they tend to get heavier and heavier the more creature comforts that you add. (including the middle ski like thing) Please forgive me for sounding like the voice of doom. It is not me that is being negative it is the laws of physics.
     
  5. SlowCruise
    Joined: Dec 2011
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    SlowCruise Junior Member

    Awesome. Give it to me straight!
    I mostly want the floatation, but I am hoping for the best on the speed upgrade.
    I just found this video showing what happens with the water under a 2 pontoon boat at various speeds. Check it out here if you like...
    http://www.youtube.com/watch?v=cK67dnWeXUk
    What I see is A LOT of water being channeled down the center of the boat, and it seems to me if I had a pontoon or whatever with some angle to it, that the boat would almost have to kind of rise up on that wave of water. Any thoughts on that?
     
  6. gonzo
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    gonzo Senior Member

    The sharp corner in the bow will create a lot of drag. If you want to plane, less is better. You are adding weight but keeping the same HP. If it is not planing now it will be even less likely after you add more weight and drag.
     
  7. SlowCruise
    Joined: Dec 2011
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    SlowCruise Junior Member

    OK Gonzo. I'm glad you checked in. I have enjoyed some of your other posts.

    I just thought it wasn't planing now primarily because the pontoons aren't meant to plane. I figured adding some flat areas like the strakes and the bottom of my new pod could get those to work a little better, even with the added weight of the pod.

    Did you happen to check out the video link I put in above? It seems like there might be diffferent dynamics going on here with pontoons versus other more efficient catamarans, etc. There is so much water being pushed inward and down the center of these when they are running, it seems like I could get it to ride up and "surf" that water a bit if I added something that is of the right shape and put it right down the center/front. Do you think there might be anything to that?
     

  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Adding weight, drag and centerline turbulence to a possably under powered already, less then efficient set of hull shapes seems self defeating, unless compensated with additional power as well as a third pontoon.

    In short, adding all this will not make you go faster, it will make you go slower, even if the third pontoon adds a fair bit of dynamic lift. Getting a pontoon boat to move requires power, lots of it in comparison to most other hull shapes. The third pontoon will detract from your power to weight ratio, it will add drag and provide the prop with a lousy flow of water to chew on. None of these things are particularly beneficial, unless you can toss some more power at it, to over come the additional burdens.
     
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