Pontoon Boat Hulls - Why Round?

Discussion in 'Boat Design' started by MattJ, Feb 11, 2019.

  1. MattJ
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    MattJ Junior Member

    Hi, I am just curious, why are all pontoon boats made with round hulls and not a vshape style hull like a cat?

    Is it just for manufacturing costs? Would it not be faster, plane, ride better if it was a tri hull with v-shape style bottom?

    Sorry I am new to all this and I was curious as to why.
  2. Blueknarr
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    Blueknarr Senior Member

    Welcome to the forum.

    Manufacturing cost is probably the greatest reason.
    Round also provides the largest buoyancy per skin ratio. Lowering cost of skin and friction allowing smaller (cheaper) engine. Cylinders also resist comprehensive forces well; reducing need for expensive internal support.

    Given the typical length to beam ratio of individual pontoons, planing isn't really possible. Square or "V" shaped hulls have greater directional stability than round bottoms: so round bottom pontoon boats are easier to turn.

    Tradition could also be to blame.
  3. MattJ
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    MattJ Junior Member

    Why would planning not really be possible? If you were to use a Cat style hull with a flat floor (and lower side) then what would be the downside over manufacturing this rather than round pontoons?

    I mean manufacturing costs aside would it be a better riding, faster and better handling boat if it was cat style? Most pontoon boats seem to be designed for smooth American lakes rather than more for Australian waters like Sydney harbour etc where it gets a lot choppier and has to deal with a lot of extra traffic? Also something a bit beamer than the standard 8ft
  4. fallguy
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    fallguy Senior Member

    In the simplest terms, for the same shape, a round cylinder offers more displacement than any shape that will fit inside it.

    i.e. floats higher

    And when it floats higher; it will generally have a lower wetted surface area which reduces drag.

    Pontoons are planing hulls, ftmp.

    A cat hull as you describe can be a displacement hull.

    If you look at more modern pontoons; they exit narrower to reduce drag and thus run faster.

    But in general, a pontoon is designed for what?

    A bunch of people/load.

    So the lowest wsa for the maximum displacement is sought.

    Also, most pontoons run a single engine. By changing the hulls to vees, steering gets harder for a single prop, doubling engines drives cost.

    Once you want something like a cat hull for rough Austrailian waters; the entire idea of a pontoon is mute.

    They have their place and the need in some other locale is not the same.
  5. kapnD
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    kapnD Senior Member

    Check out the recent thread “canoe with square bottom hull “
  6. MattJ
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    MattJ Junior Member

    Thanks for your feedback, I understand what your saying.

    It is a "Pontoon Boat" I was wanting, something flat decked, to hold 15 or so people, was wanting twin outboards as looking for speed and better parking/steering, but also want the stability at rest hence why keeping with the pontoon theme. I want it to be a fairly fast boat to get around the waterways quickly, also wanting it to be wider than standard pontoon something around 3m to give the extra space. So trying to figure out what hull shape.
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd say the cylindrical pontoons have the advantage of easier and cheaper construction. If you are going to potter around at low speeds most of the time, and the stern ends are tapered, is a pretty good solution, however, if you want to cover larger distances at speed, a hull with a more conventional planing shape is probably better, but you will need a lot of power.
  8. JSL
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    JSL Senior Member

    if you want to go fast, a flat bottom might be better than a cylinder. Another item is buoyancy and loading: Once the draft is at the middle (50% the diameter)of a round tube hull the pounds per inch immersion rate is at its maximum and if further immersed, it decreases. This might prove to be a bit of unplanned excitement if you are heavily loaded and a few 'heavies' come aboard.... you just might experience that sinking feeling!
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Fifteen people is a lot to be carrying around, it will need to be a substantial vessel.
  10. JamesG123
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    JamesG123 Senior Member

    Or they all want to look over the same side of the boat! That gets very exciting fast.
  11. MattJ
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    MattJ Junior Member

    Yes looking to build something around 3m x 8m with twin outboards
  12. Barry
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    Barry Senior Member

    Pontoon hulls are generally round because they minimize the amount of material for rigidity of the surface material . A good topic to look up is "Thin Shells of Revolution"
    Propane tanks, pressure vessels, tanker trucks are built this way because of this. ( and spherical is even better) If you were to build with flat panels, you would need significant interior ribs and frames to get the same rigidity of the surface.

    While some have been citing minimum surface drag DUE TO SKIN FRICTION, when comparing a square flat planing surface to a round planing surface, the flat will win out with very little
    cost of drag due to the difference of the immersed surface area.

    For example
    A 2 foot diameter pontoon, immersed halfway at one foot, will provide a cross sectional area of 1.57 sq.ft and an immersed circumference of 3.14 ft. (so 1.57 cubic feet of displacement per lineal foot and 3.14 sq.ft of immersed skin per lineal foot)

    A 2 foot square pontoon, only needs to be immersed .76 feet for the same buoyancy per lineal foot and will have an immersed perimeter of 3.52 square feet per lineal foot.

    Many manufacturers are using round pontoons with added lift strakes to "square up" the running surface to increase planing ability
    Last edited: Feb 14, 2019
  13. sharpii2
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    sharpii2 Senior Member

    Most of the pontoon boats I have seen are driven at 6 to 10 kts. And this can be done with a relatively modest engine. My father had one with a 6o hp outboard. With just two people aboard, it planed nicely. With three people aboard it tended to squat. The 60 hp, IMHO, was too much engine. Typically the boat cruised the lake at 6 to 8 kts. Usually, there were four to six people on board. Its main purpose was as a sight seeing platform.

    I now see three pontoon versions with spray rails welded on. These are clearly intended to go fast. They may have two 60 hp outboards on their brackets.
    Arguably, the center pontoon improves the total buoyancy of the boat and thereby reduces the chance it stuffing a bow.
    I have never seen a two pontoon one ever stuff a bow, but I'm sure it could happen, especially at speed.

    I have never seen the large pipes, which become the hulls, manufactured. But I imagine they use a continuous roll-forming process, with the mill making the cuts which allow the bows to be formed, with every chop of the finished pipe. circular bulkheads can be added next and puddle welded into place if needed before the transom pieces are welded on. Such a simple process would be much more complicated with a different sectional shape.

    First, the skin would have to be thicker. Second, the roll mill would probably need more passes. Aluminum does not like sharp bends which a square or "V" section would require.
  14. Village_Idiot
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    Village_Idiot Senior Member

    I have piloted an aluminum flat-bottom, much like a barge but with rounded front at the waterline. This craft was ~7.9m x ~2.6m, and would plane easily with 1000 kilos cargo and a single Mercury 150hp four-stroke outboard for power.

    All of that to say that you will get better performance and load-carrying with a large monohull than two or three pontoon hulls.

  15. MattJ
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    MattJ Junior Member

    I can appreciate that but the monohull will not give me the resting stability like a cat/pontoon style.

    It sounds like the cat style would be the way to go if manufacturing costs were not a concern, which they are not.
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