external water ballst

Discussion in 'Stability' started by sand groper, Jan 25, 2010.

  1. masrapido
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    masrapido Junior forever

    Consider copper nickel alloy. Zero fouling, no painting needed.
     
  2. jehardiman
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    jehardiman Senior Member

    No, this is not an idea or theory, it IS the physics of the stability equations. Water "ballast" is fraught with perils and quirks, each of which needs to be addressed in the overall whole. It is also important to remember that static and dynamic stability as well as initial and final stability are all seperate subjects that each need to be addressed

    Water "ballast" weight does not increase static stability unless it is, or is lifted by heel, above the waterline (see why below). This is why active ballast systems pump the weight to the high sides and passive "ballast" systems for sailboats work best on flat, wide hulls. The contained mass does effect dynamic stability though, both good (retards motion) and bad (continues motion once moving)

    Adding water "ballast" volume below the waterline does not increase dynamic stability unless it causes the waterplane inertia to increase in greater proportion than the increase in volume (i.e. the geometry leads to an increase in BM == Iyy/Volume). Again, tanks high and outboard on wide shallow hulls work best.

    Adding water "ballast" weight and volume below the waterline to an existing hull decreases both static and dynamic stability by dulting the effect of existing ballast (i.e. it lowers KB in faster proportion than KG is lowered) and it lowers BM by increasing volume on the same waterplane inertia.

    Free surface, from any tank system, decreases both static and dynamic stability. This is why most "ballast" systems are operated fully pressed up.

    If you want examples, or an analysis of a given case, I can do so. Until you go through all the math, some things are not obvious.
     
  3. masrapido
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    masrapido Junior forever

    jehardiman, I'd be interested to see some examples. Iit is true that sometimes one needs to go through the numbers, particularly to understand the physics of moments and their interaction.

    It is common to place ballast tanks on commercial ships high above the waterline, for example, but I never thought that the same principle would apply to leisure boats given their small size.

    This maybe outside the thread, but I think that other readers may find it interesting too if you show us some examples here.
     
  4. sand groper
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    sand groper Junior Member

    This business reminds me of the school question - "what is heavier - a tonne of feathers or a tonne of lead ?".
    Does a cubic meter of water weigh a tonne on the deck of a ship and a tonne when submerged ?
    A steel box containing a cubic meter of water and welded inside a hull weighs a tonne, and does it also weighs a tonne if welded to the underside of a hull ?
    If this water box is welded to the outside of the hull, will the hull displace an extra tonne ? (ignoring the weight of the box itself of course)
     
  5. Typhoon
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    Typhoon Senior Member

    I know of a small (13-14ft) fibreglass centre console that had a very deep V and a flooding chamber for stability at rest. Was called a Dingo if anyone every comes across one. It needed the ballast to sink the chines into the water at rest, else the boat would flop around up on the deep hull.
    It was simply a 6" diameter tube (from memory) running inside the keel of the boat, open to the transom. Coming off plane it filled quickly and getting on the plane emptied equally as fast. It worked very well.

    Regards, Andrew.
     
  6. gonzo
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    gonzo Senior Member

    A ton weighs a ton, but a ton of steel creates a lower metacentric center than a ton of water. That means that the stability is different.
     
  7. sand groper
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    sand groper Junior Member

    Would a thin 'skin' of alleged water ballast give a similar metacentric height to a not-much-thinner steel 'skin' on the underside of a hull ?
    If not, might the stability 'improvement' over an un-ballasted hull still be useful anyway ?
    To get my floating Winnebago to windward would require enough power to overcome the hull's limitation, ie. being a fat boat. No amount of power will make it close-winded or fast, but it doesn't need to be.
    It just needs to stay upright.
     
  8. gonzo
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    gonzo Senior Member

    No, the center of gravity of a material of less density will always be higher.
     
  9. sand groper
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    sand groper Junior Member

    My contemplated camper boat hull designs are wide and shallow.
    I know they'd fall over if given sufficent rig to have enough power to get to windward.
    Being a tradesman (and not a naval architect and therefore unable to apply theory) I tend to think of practical problem-solving strategies. In this case that would be to shovel sand into the hull until it stopped trying to flip.
    If that worked, then the next step would be to work out what mass of ballast is needed and then the next logical step is to replace that sand with a lump of steel for reduced ballast volume.

    If that would do the job, then this steel slab could go on the underside of the hull presumably. If this is the case, then rather than have this slab of 1/2" (or so) steel built-on and therefore having to drag this weight up and down hills when trailing, would it not be possible to attach fore-and-aft aluminium strakes along the underside of the hull and then cover them with a sheet of stainless to make the proposed water chamber ?
    Well it could be done of course, but would it work as intended ?

    I'm not convinced it would work but have a gut feeling that it should.
    Aussie blokes are born knowing how to do everything, and never need to look at the instructions. Well, not really, but you get the idea ....
     
  10. gonzo
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    gonzo Senior Member

    That is not a practical solution to the problem. The sand will have a much higher center of gravity. If you put enough sand it will make the boat top heavy and capsize it. Naval architects think of practical problem solving techniques too. The stability calculations are not so extremely difficult. Another solution, is to make a model and try adding steel to the bottom until the boat is stable enough. Then you can scale up the weight.
     
  11. sand groper
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    sand groper Junior Member

    How can a layer of sand in the bottom of a hull make it top-heavy ?
     
  12. troy2000
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    troy2000 Senior Member


    It might not be literally top-heavy. But the boat would be noticeably less stable because the weight would be higher--inside the hull, instead of below it. And since sand is much lighter than steel, an equivalent weight of sand would have a higher center of gravity than the same weight of steel laying inside would.

    Of course, on a set of sharpie plans Chapelle drew, he specified bags of sand in the bottom of the flat-bottomed hull for ballast. But sharpies are different animals completely...
     
  13. sand groper
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    sand groper Junior Member

    What do you mean by "the weight would be higher " ?

    Okay, the whole lot now weighs more, but would the CoG now also be higher ?
    Sand is not as dense as steel for sure, but a layer of sand in a hull bottom would have a centre of mass only slightly higher than the center of mass of a slab of steel would it not ?
    A little bit higher would not raise the whole boats CoG that much (as compared to a hull with a slab of steel ballast inside) and the extra weight would add stability IMHO.
    But I've been wrong plenty of times in the past and no doubt will be in the future.
    How are sharpies different as far as ballasting goes ?
    (Your quote on idiots reminds me of the Chinese saying "when you argue with a fool, then two fools argue" - and I'm not implying idiocy in anyone here.)
     

  14. sand groper
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    sand groper Junior Member

    My thinking on this subject is along these lines -

    Here is a barge. To get it to windward it must have a lot of power and therefore a lot of rig, and therefore a high sail plan CoE which will try to make it capsize.
    So to stop this it must be ballasted to the point that the rig will blow over the side in a sufficently high wind, rather than have the barge flipping over.
    Practically, this point need never be reached with a bit of judicious boat-handling, so ballast can be reduced from this hypothesised required weight.
    So the result is a vaguely weatherly, ugly but comfortable and roomy horror.
    Think Thames barge, bark 'Endeavour', botter, and so on.
    But I want to tow a 6m version of this beast at minimum weight so a water ballast system seems the way to go.
    But rather than mess around with pumps and so on, why not have external, self-filling and -draining water ballast.
    But maybe there's no free lunch after all.
    Oh for a battery as energy-dense as a bucket of petrol and then I wouldn't mind towing that weight.
     
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