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Thread: width compared to length?

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  1. #1
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    Question width compared to length?

    Is 12 feet wide by 35 feet long tritoon (3 pontoons) in good proportion for stability and maneuvering? I've heard width and length needs to be in certain perportions. I'm wanting to buy the pontoons-maybe 30 inch diameter and build a 20 ft cabin.

  2. #2
    Super Moderator OLD HOUSEBOATER's Avatar
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    Fine the wider the better for deck space. Know how your going to transport it though.
    The fries are cold so we gave you extra.

  3. #3
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    Quote Originally Posted by OLD HOUSEBOATER View Post
    Fine the wider the better for deck space. Know how your going to transport it though.
    I would build it at my house and have a moving company to transport it to the lake I would stay on--thank you very much.

  4. #4
    Senior Member Tony B's Avatar
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    If you were building with a normal power boat hull (planning, semi-planning, etc.), the ratio would be about 3:1 which would definitely put you in the ballpark. However, you are going with pontoons, which is a totally different dynamic since each pontoon becomes a separate hull. Generally, the wider, the more stable. I think your limiting factors would be how far you can span between hulls which would be an engineering question. The easily found solution would be "what are the manufacturers using". The other limiting factor would be how wide are the slip spaces that are available to you? In your case, neither would apply if you only want a 12' beam because you are probably well under the standard beam (width) for that length.
    The only place I can see you getting into trouble would be using smaller than normal size pontoons for that length boat just because you plan to carry less weight. That could be a stability issue. Loading weight could play into this some. F'rinstance, too many people sitting all on one side. And a fly bridge, adding weight up high will be a stability consideration.
    Curious question, how come you only want 12 feet? After all, you are the builder and you can have what you want. A walk around deck will really shrink your interior.

    I built my own ultra-light airplane from scrap parts and taught myself to fly from reading books. Not knowing anything about aeronautical engineering, I studied the specs on well over a dozen manufacturers models. They all had almost exact specs on the dimensions. I cant remember that far back but I think all the wingspans were between 30 and 32 feet. All the chords (wing edges from to back of wing) were 5 feet. all the overall lengths were around 16'. Like I said, I cant remember the exact numbers, but you get the idea. There is probably a really good reason that most manufacturers specs are almost exactly the same for any given length pontoon boat. Once you know the reason, then maybe and only maybe, you can know what deviations you can do from their plans.
    The same goes for engine sizes. There are simple formulas for that. Without understanding what you are doing, you can way overspend on a motor, or even worse - under spend.
    Houseboater at Heart
    1986 Mainship 36 Dual Cabin Pointed Ended House Boat

  5. #5
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    I am thinking economically on size=fuel use, slip rent, etc. cabin will be all way across deck, no catwalk. live aboard for one and a dog or maybe two people with very little visitors. I don't have the money for BIG and too old to go in debt. Plan to be anchored out whole lot. I don't have experience with houseboats so need small to learn to get back into dock. Thank you Tony B. -I enjoyed that glen-l site! I am determined its never too late to live out your dream-even if I cant convince my husband I will find a way!

  6. #6
    Senior Member Tony B's Avatar
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    This should be a big help to you: Glen-L.com. They sell boat plans and are excellent people to deal with. .
    When you get on their home page, a few inches down from the top and on the left side, hit "Boat Plans Catalog". Then drool over the houseboat plans. It's nice to know the specs BEFORE you build.
    The plans include building your own pontoons. here is a photo that someone sent to them from one of the plans that shows the pontoons under construction. http://www.glen-l.com/designs/house/dsn-hfnc.html .

    Once you buy their plans, they are dedicated to help you succeed. You can ask loading questions and they can usually tell you how far down you will sink for each additional thousand pounds. That alone would be worth the price of the plans. Way back when, I had built 3 different boats from their plans.
    They all went exceptionally well. They even tell you what size motors to put on there and what to expect if you go larger or smaller. That alone can save you thousands.
    Anyway, have fun pawing through the houseboat section.
    If you don't buy their plans, they wont help you, however, from their online plans info, you can pretty well figure out what to expect if you made your own from your aluminum pontoons.
    best of luck and have fun.
    Houseboater at Heart
    1986 Mainship 36 Dual Cabin Pointed Ended House Boat

  7. #7
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    Check your private messages.

  8. #8
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    I don't see anything in my messages??

  9. #9
    Senior Member Endurance's Avatar
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    Tony brings up a good point about the material you use to span between your pontoons. For what it's worth, I have an 18 foot wide pontoon boat on which the manufacturer spanned between the pontoons with aluminum C channel crossmembers that are 4" high (called the web) and have legs (called the flanges) that stick out 2". The web thickness is .150" and the flange thickness is .230". The material is 6061 (also called T-6) aluminum and the manufacturer spaced the crossmembers on 12" centers. The choice of 6061 is a good one since it approximates the strength of steel with a lot less weight and a lot better corrosion resistance. Aluminum like this generally comes in 25 foot lengths, so a 12 or even a 12.5 foot wide boat would limit waste.

    I'm also with Tony that for a pontoon boat, the wider the better. That's especially true if you have twin engines. The wider you can space twin engines, the better the boat maneuvers at low speeds. You'll also see less side-to-side rocking in open water with a wider boat.

    You will probably want to give some thought to the issue of insuring your boat. Some insurers don't like homebuilt boats. Amelia (one of the frequent posters here) might be able to help with that issue.

  10. #10
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    Is it possible to get ABYC credentialed inspectors to inspect the boat in stages of building to affirm it is built to standards for insurance?


    Quote Originally Posted by Endurance View Post
    Tony brings up a good point about the material you use to span between your pontoons. For what it's worth, I have an 18 foot wide pontoon boat on which the manufacturer spanned between the pontoons with aluminum C channel crossmembers that are 4" high (called the web) and have legs (called the flanges) that stick out 2". The web thickness is .150" and the flange thickness is .230". The material is 6061 (also called T-6) aluminum and the manufacturer spaced the crossmembers on 12" centers. The choice of 6061 is a good one since it approximates the strength of steel with a lot less weight and a lot better corrosion resistance. Aluminum like this generally comes in 25 foot lengths, so a 12 or even a 12.5 foot wide boat would limit waste.

    I'm also with Tony that for a pontoon boat, the wider the better. That's especially true if you have twin engines. The wider you can space twin engines, the better the boat maneuvers at low speeds. You'll also see less side-to-side rocking in open water with a wider boat.

    You will probably want to give some thought to the issue of insuring your boat. Some insurers don't like homebuilt boats. Amelia (one of the frequent posters here) might be able to help with that issue.

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