Jon boat water ballast

Discussion in 'Stability' started by curttampa, Apr 6, 2020.

  1. curttampa
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    curttampa Junior Member

    This may be way out there...but after having read an article about a small 12’ sailboat “Scamp” that uses a chambered water ballast system the usefulness of something similar on my small Jon boat interested me. My goal is to reduce the tippy nature of the Jon boat when standing on the foredeck to cast. The boat is 12’ long with a 42” bottom. It has a flat bottom no V.

    My crazy(?) idea is to build a box from 2x4” paneled with plywood top and bottom that would sit inside the boat center amidships. Epoxy waterproofed and baffles if needed. Internal bilge pump to drain accessible with a round sealed inspection port. To fill it a second external bait well pump. Depending on the overall dimensions (42x42”) this could add 250-300 pounds of ballast over the lowest point of the boat. The boat is used in mill pond flat waters. The ideal of a fillable/drainable ballast tank makes sense to me but I have zero knowledge boat design and stability. Thoughts, ridicule, improvements and other ideas are all welcome. :)
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    So when you stand at the bow, and I assume no one else aboard, the bow drops, and the boat also becomes tippy ? I guess if you are standing at a height well above the waterline, you are not a jockey who likes fishing, and you are not always distributing you weight centrally, and especially when casting, it will get a bit tippy. It is after all, a fairly small and narrow boat. I don't think the ballast idea would be worth it, it is a complication in what is otherwise a very simple boat. You might try adding a buoyancy band just above the waterline, that takes up when the boat heels, and especially when you stand right forward, you could experiment with some cheap block polystyrene foam, say 4- 6" thick. Just glue it on temporarily, and if it does the job, you can think of something more permanent.
     
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  3. curttampa
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    curttampa Junior Member

    Great reply. Thank you. I am still fascinated by the water ballast idea.
     
  4. gonzo
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    gonzo Senior Member

    There are a couple of ways of using water ballast. To fill the tank, make it so the top is not higher than the waterline when the tank is full. To fill it, have a hole on the top with a short pipe coming up. There will be a hole at the transom with an accessible plug. To fill, take of both plugs and the tank will fill. Put on the plugs until you need to empty it. To empty, take the plugs off and motor until the tank is empty. Put on both plugs.
     
  5. kapnD
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    kapnD Senior Member

    You’re asking too much of the design, what you need is a wider boat!
    You could try Lowering the casting platform to seat level, and stay nearer the center. I see many fishermen actually hanging ten in their zeal to get maximum cast distance, but obviously pushing the stability parameters of their craft.
    I doubt that sloshing some water about will do much for you, but some bricks across the transom might help keep more of the boat in the water when you’re on the bow.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    That's right a wider boat, but a buoyancy band effectively makes that. The low draught of such a boat makes ballast tankage difficult, and not going to be as effective as extra volume further from centreline.
     
  7. jehardiman
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    jehardiman Senior Member

    Remember; water ballast only works when it is above the waterline, otherwise all you did was change the shape of the boat. Lightweight and stable, there is a reason pontoon like hull forms have become ubiquitous for fishing on lakes.
     
  8. rwatson
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    rwatson Senior Member

    Huh?
    THAT idea doesn't "hold water". Water ballast in the keel works, as discussed in this article
    "Boat A - The stability numbers here again show a clear picture. At 90° of heel, she has more than double the Righting Moment of any of the 3 other boats. "
    [​IMG]
    The effects of water-ballast on sailboat stability – Bedard Yacht Design https://www.bedardyachtdesign.com/articles/the-effects-of-water-ballast-on-sailboat-stability/
     
  9. jehardiman
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    jehardiman Senior Member

    FWIW, the figure you show is poorly informed at best, deliberately misleading at worst. If boat C's KG is a DWL, then boat B's cannot be the same height above DWL as the hull was lifted by the keel buoyancy unless the keel structure has null weight. The best thing to do would be to take the weight of the keel structure on boat B and add it as ballast at the keel of boat C (maybe they did but didn't point that out).
    Anyway, if we examine the figure above, assuming they are based in some fact, it is immediately obvious that for initial stability boat C is greater than all the others. And will continue to be until the water tank is lifted above the waterline (as the case is with boat D) if the upper hull shape is identical. This is because KG is above KB for all cases and boat C has a BM at least 38% greater than boat A and 6% greater than boat D. Additionally even though boat B has the same displacement, it's shape means that its waterplane inertia is less as well as KB being less giving a lower GM. I'd need the exact hull shape to work the numbers, but boat D may have more stability than boat C, but only BECAUSE water ballast is above the waterline. Otherwise my money says BM and therefore initial stability is still lower than boat C.
    We have been through this several times, and the answer remains the same because the physics of the matter does not change. Water ballast has no effect on stability until it is raised above the water line, so for a given displacement it is better to properly shape the hull than it is to waste internal volume on water ballast. In the specific case of ultimate stability, water ballast does play any part only because it has been lifted above the waterline. If you want a worked example then I can do it.
     
  10. rwatson
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    rwatson Senior Member

    But they do the righting moments IN the example ?


    Edit:
    "While it seems counter-intuitive that placing water in the hull (which is, after all, close to the same density as the water outside the hull- fresh vs salt water) would add any stability, adding water ballast below the vertical center of gravity increases stability. The water ballast does not need to be lifted above the waterline to affect stability, as any material having greater bulk density than air will have an effect on the centre of gravity. It is the relationship between centre of gravity and centre of buoyancy that dictates the righting moment."

    ... and of course, the epitome of below the waterline water ballasted yacht

    311px-MacGregor_26X_sailboat_2593.jpg

    And I know from first hand experience the difference it made

    I also had a professional NA do righting moments for a water ballasted Motor Sailer, with the tanks below water.
    I attach relevant diagrams
     

    Attached Files:

    Last edited: Apr 11, 2020
  11. jehardiman
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    jehardiman Senior Member

    I read the article associated with the figure and found the following quote at the bottom of particular interest: "* Note that these sailboat designs are not intended to be realistic, they are provided for comparison purposes only, as well as simplified and optimized to emphasize the discussion." There are other inaccuracies in the text also, but that article actually supports the fact that water ballast only works when above the static waterline. To quote:
    I don't understand what you are doing here except attempting to obfuscate the physics for the sake of trolling. Even the article you point to agrees that water ballast is only effective when it is above the static waterline.

    Additionally:

    I do not know where this quote comes from, but after reading it closely, it is apparent that the author has a improper conception of "the ancient interface". His/her idea that "any material having greater bulk density than air will have an effect on the centre of gravity" is patiently incorrect. ALL materials have mass and therefore an effect on the center of gravity. The author seems to have confused and separated in their mind what buoyancy is and how it is applied. Yes, all items (including gases) have a mass density, and all non-rigid materials produce "buoyancy" on other objects immersed in them and will tend to separate themselves by mass density (see the "Brazil Nut Effect"). While most of the time we ignore this "air buoyancy" (except in deep submergence where it does matter), in their statement the author makes the false mental assumption that the entire vessel, including the water ballast, is in the air all of the time. This is incorrect as everything below the static waterline is in the fluid and therefore produces a buoyancy, in this case the water ballast is producing a buoyancy and center of buoyancy identically equal to its weight and center of gravity. While the author is correct in their statement that "It is the relationship between centre of gravity and centre of buoyancy that dictates the righting moment" , what they miss is that water ballast below the static waterline has the exact same effect on CG as it does on CB. By adding volume to the ever-buoyant hull and then filling it with water of the same density, you did not change the effective separation between CG and CB. (i.e. the case of boats A and B above) Similarly, adding volume in the ever-buoyant hull and then filling it with water of the same density, only changes the effective buoyant shape of the ever-buoyant hull, it does not change the effective separation between CG and CB. Also, this is not to say that stability is not effected if fluids of different mass densities are used to fill the tanks; filling internal/external tanks with air, gas, fresh water,oil, molten sulfur, etc. are different from the specific case of using water of the exact same density.

    Actually, you attached a summary, not any of the relevant documents that would allow a competent naval architect to educate you on your flawed conceptions about water ballast. I am glad that you are seeking professional help, perhaps they can explain to you the difference between initial stability and ultimate stability and why water ballast only effects one of those. FWIW, the two incomplete calculation sheets show that the vessel has a higher initial stability at 10 degrees prior to adding water ballast. Even at 20 degrees there is only a 6% increase in stability even though you increased the displacement by 25%. Most likely this increase in stability is caused by changes to the ever-buoyant hull form (i.e. the CB moved up while the Iwp increased due to the change in buoyant shape causing an increase in GM). There really isn't any significant improvement in stability until list is 30 degrees, by which time I would expect any competently designed water ballast to be lifted above the static waterline. But, as I said, there is no way to tell what is actually going on with the documentation you provided.
     
  12. rwatson
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    rwatson Senior Member

    So, tankers that add water ballast below the waterline do not benefit from increased stability ?
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I am surprised that became a matter of contention, presumably all ballast is best placed as low as possible, for the purposes of improved stability, though not necessarily for comfortable motions.
     
  14. jehardiman
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    jehardiman Senior Member

    No, tankers add water ballast to improve powering...I'm.......
    <let someone else take over because I'm not getting through to this person/(troll?) and may say something inappropriate>
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    If the purpose is to reduce this "tippy" nature, you need to define 'tippy'...as this is not clear.
    And static stability is not the same as rolling stability and roll period.

    If referring to motions, then JEH is correct. This reduces the GM and thus increases the roll period. Ergo, not so 'tippy'.
    It is also placing the ballast above the centre of rotation too...which is a positive action.
     
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