Small boat-like device stability ( not for people )

Discussion in 'Boat Design' started by cs2870, Apr 1, 2011.

  1. cs2870
    Joined: Sep 2010
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    cs2870 New Member

    Hello forum members,

    I have a question about stability. This is a boat forum, and this device is not for carrying people, but I don't know where else on the net I could find a more appropriate answer to this question.

    I have built a device with a dual round aluminum pontoon design. This device is only 4 feet long, and 3 feet wide, with 10 inch diameter round pontoons. So the distance between the pontoons is about 16 inches ( 36 - 10 - 10 )

    I was considering replacing the pontoons with an enclosed, watertight box, so in essence, it would be one pontoon, 4ft long, 3ft wide.

    I was considering building a prototype just to compare stability. I know an enclosed box would have much more buoyancy because of the amount of surface area and thus the more water displaced.

    What I am wondering is , would the pontoon design be more stable when weight is shifted to one side? In my mind, the pontoons act sort of like "outriggers", like you would see on a canoe or kayak, but I could be wrong.

    Any help is appreciated. Have a great day
  2. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Stability is dependent on the displacement (weight) of the device, the vertical center of gravity, and how these barrels are floating. If the barrels (pontoons) are floating high (below centerline or max beam) stability will increase with heel, but when immersed beyond max beam (center of tube) stability will begin to drop.........
  3. cs2870
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    cs2870 New Member

    Understood about the round pontoons.
    Let me pose the question differently. Say the pontoons are 10" square pontoons, and pose the same question of a flat bottom single large enclosed box, versus two square pontoons. The load in question will be a low center of gravity, basically the load sits directly on top of the pontoons. A the load shifts to one side and the device begins to heel, which is going to be more stable? I could pm you a link, don't really want to put pictures of the device in public.
  4. aranda1984
    Joined: Jan 2010
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    Location: Vancouver, B.C.

    aranda1984 aranda1984


    Tad gave you the right advice.

    Depending on how high the center of gravity is, you need to consider a turning moment calculation.

    Why not make a cone type float and put the weight (or ballast) below the water level. That way it will never flip over.

    Meaning; lower the center of gravity below water line.

  5. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    From another thread.......

    The fundamental stability formula is...... BM = I/V

    B is the center of bouyancy
    M is the Metacenter (about which B is shown to rotate)
    I is Inertia of waterplane
    V is volume (displacement)

    We know G (center of gravity) is somewhere above B.

    We know stability is proportional to GM......

    We can increase stability by lowering G, or by raising M.......

    Going to square pontoons or a larger box will increase I, thus increasing stability.....
    1 person likes this.
  6. JEstes
    Joined: Apr 2011
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    JEstes JEstes

    Considering the givens in this problem, I must agree with Tad and Stephen. The flare in the cylinder provides reserve bouncy to handle a load, but a shifting load would cause the cylonder to tilt. The smaller the load the smaller the heel. The same goes for the box, except slab sided box will not have any reserve bouncy and would therefore be more tender. In the event you require no heel, you need to counter balance any shift in weight or restrict ther load to a very light weight item. For example, ridihg a bicycle on a ocean going barge will have little or no impact on heel.
    Regards: JEstes
  7. cs2870
    Joined: Sep 2010
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    cs2870 New Member

    I thank you all for your input. I know its hard to talk about something without seeing it.

    I dont require zero heel. I know that it will heel some, which is expected. The user will probably be putting up to 35-55% of the net buoyancy of the device on to it, but the user will be expected to load it evenly. But if all the load were to shift to one side, it shouldn't capsize, though it would be unusable until the load is corrected ( once again this is NOT for people ).

    I am thinking a single barge like box enclosure would handle the load shift better. With dual pontoons if the load shifted all to one side, the pontoon on the opposite would basically be rendered useless. At least that's my experience with prototypes I have made so far. I haven't built a solid barge type yet.

    I know we have all levels of expertise on here from yacht builders to complete beginners ( like me ) so thank you in advance for your patience.
  8. portacruise
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    portacruise Senior Member

    This is much smaller, but self righting due to the float. Does it help?



  9. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    I think the pontoon system will offer more stability so long as the amount of weight on the lower pontoon doesn't exceed the optimum buoyancy point of the lower pontoon (i.e. the middle). This means your pontoons will need to be larger diameter than would be required if the load was expected to be centered all the time.

    Alternately, you could use a reverse teardrop shaped pontoon like I've illustrated below...


    In the balanced condition it seems like this would lower the resistance and in the unbalanced condition it offers more buoyancy than the round design.


    With the pontoon design the weight of the upper pontoon in the unbalanced condition is all at the outside of the "lever" and so exerts maximum righting force.

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