external water ballst

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

  1. sand groper
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    sand groper Junior Member

    Possibly dumbo question here but I'm used to it ....

    An internal water ballast tank works.
    I assume that an external tank, ie. on the bottom of a boxy hull, would also
    work in that when full the mass and therefore the 'weight' of it would be
    dragged around along with the rest of the hull and still be subject to gravity.

    Now if this is true, then would such a tank still work if it were to be filled
    and drained by small holes at launch and retrieval of a trailer-boat ?

    The assumption here is that an enclosed mass of water would still be
    largely 'captive' even though some water would move in and out of the tank
    because of the pressure of the water flowing along the hull.
     
    Last edited: Jan 26, 2010
  2. alan white
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    alan white Senior Member

    Sure. It's not necessary to actively fill and drain. Water has weight even if it's in motion.
    The only requirement would be that water stays in long enough to return the boat to an upright position should the boat capsize. Therefore, short of using a valve, the entry/exit hole or holes should be small enough to slow the process down.
     
  3. sand groper
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    sand groper Junior Member

    Thanks for that Alan.
    I've not had much to do with power boats but have a vague idea that
    some have 'wet wells' for live fish, and some aluminium dinghies have
    hollow keels that fill and drain.
     
  4. gonzo
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    gonzo Senior Member

    The chinese had bow tanks in the junks to damp pitching. That is thousands of years ago.
     
  5. masrapido
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    masrapido Junior forever

    As a deck officer to me every water compartment is internal if it is on board. Extenal would apply only if you place it outside of the boat, say hanging over the board.

    In sand groper's example, the tank is internal to me, although it is in a removable canister (did I understand that properly?). It is still inside the boat, at the bottom. And it works in the appropriate manner. The weight is working within the boats structure and it is essentially a part of it and I would treat it as such.

    The only problem with such a wight would be if the canister/tank is not secured and can fly around the space. Then you would have a few problems with it.
     
  6. sand groper
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    sand groper Junior Member

    Hi Masrapido. What I had in mind is an 'extra skin' on the underside of a hull that fills and drains slowly via suitable holes, not a tank enclosed within the hull itself. Imagine a double-bottomed vessel such as a tanker with water between the shells.
     
  7. masrapido
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    masrapido Junior forever

    Ok,thanks. Now I understand the picture., And Alan's response makes perfect sense. I wonder if the drag would not be detrimental though.
     
  8. sand groper
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    sand groper Junior Member

    The 'drag' would come from moving the mass of the water ballast system plus the drag of the additional skin or blister or tank or whatever, plus any drag associated with flow through the system (unless it was valved), and water movement within the ballast would have some small effect too perhaps.
    Obviously the hazards are corrosion, bio-growth, and so on.
    I just want the impossible - a trailer-able, roomy, wide, shallow hull that can stand up to being powered up to windward (even if only to 60ยบ or thereabouts) without falling over. Maybe that's why motors are so popular.
     
  9. gonzo
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    gonzo Senior Member

    You are describing a damping tank more than a ballast tank.
     
  10. jehardiman
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    jehardiman Senior Member

    Gonzo is correct here.

    While the mass of the water in the "tank" (actually a free flood) acts as a damper, it cannot be consider "ballast" unless a) it is totally contained AND b) is above the waterline. Otherwise all that happens is you have reduced the "ever buoyant" hull to some strange shape and maybe lowered the CG slightly due to the weight of the shell plating "exterior" and below the ever buoyant hull. But it is not "ballasting" the vessel in any way. If you want some worked examples I can show you some.
     
  11. souljour2000
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    souljour2000 Senior Member

    interesting idea Sandgroper...but with a double-walled " hull-tank" ..how would you deal with fouling in marine conditions...?
     
  12. sand groper
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    sand groper Junior Member

    I'm thinking of a trailer-boat where fouling isn't so much of a problem, though corrosion might be.
    Not sure about jehardiman's idea(s) however. My cerebral cortex isn't sufficiently developed enough to comprehend a naval architect's series of equations in worked examples, so are there any other views on his argument ?
     
  13. souljour2000
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    souljour2000 Senior Member

    sorry.. obviously missed your earlier post on that subject..I guess if you could design it so you could spray it down really well at an angle with say..a pressure washer...it maybe could work "some"how..
     
  14. sand groper
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    sand groper Junior Member

    umm ... http://www.barcrusher.com.au/technology.htm
    I found this after a tip from a work colleague -

    "STABILITY AT REST
    QUICKFLOW tm WATER BALLAST TECHNOLOGY"

    "Boat design is about trying to incorporate all the very best design features to ensure the hull gives great performance for its intended application. As boat manufacturers and designers we have listened carefully to our customers and ensured that our design technology is applied to provide the smoothest possible ride whilst achieving optimum stability at rest.

    In the past, boat manufacturers have had to compromise by building flatter hulls to achieve the required stability at rest. As we all know, a flatter hull shape means a hard, pounding ride. (You may have ridden in another aluminium boat on a windy day and found the ride to be almost unbearable).

    The Bar Crusher design uses a deep-V hull for superior ride and an innovative water ballast system for stability at rest. Enter QuickFlowTM water ballast technology...

    Running along the full length of the keel, there is a cavity in the bottom of the hull open at the transom. When the boat stops, this fills, becomes ballast, lowers the chines into the water and provides tremendous stability. The moment the boat moves forward, the water ballast is jettisoned from the hull allowing the boat to fly up onto the plane. Interestingly, there is no lag when accelerating, just a smooth and fast transition onto the plane.

    The reality is that boat manufacturers use very different design technology and accordingly perform very differently on the water."
     
    Last edited: Jan 28, 2010

  15. masrapido
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    masrapido Junior forever

    Did you mean below the waterline? Ballast can be above, but that is quite unusual for a leisure boat. I do not know one with the ballast above the waterline.
     
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