Water Ballast for small boats

Discussion in 'Stability' started by Toot, Sep 24, 2006.

  1. SuperPiper
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    SuperPiper Men With Little Boats . .

    Claus' argument made sense at first. Here is the mental analogy I created to understand Michael's reasoning:

    A perfectly cylindrical jar floating on its side would roll and not stop in any particular orientation. But, tape a condom full of water to the inside and the jar would always stop same-side-up even though the condom was still BELOW the waterline.

    And, American beer is like sex in a sailboat: f&#$@ing near water.
     
  2. Claus Riepe
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    Claus Riepe Junior Member

    Michael, Superpiper:
    Nothing wrong with what you say. And analogies do help to grasp the mechanism.

    I quite like that 'condom in a jar' analogy, which I feel would be the appropriate model for keel hulls.
    For shallow but beamy dinghy-like boats, I think a 'condom in an open shoebox' is a better analogy. Now, the choice would be to put one big water filled condom onto the longitudinal centerline, or two half-filled ones on either sides, would the two different arrangements stabilize the box differently? Or will both arrangements have exactly the same righting effect?

    Looking at this model, I feel the righting effect would be quite the same.
    -I fear I just disproved my own theory. Hat off to you!

    Anyway, the key thing remains we all agree that waterballast works in small boats.

    Claus
     
  3. BOATMIK
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    BOATMIK Deeply flawed human being

    Hats off to you Claus, you disproved your own theory in record time!!! :)

    Stability comes from the relationship of two centres.

    The centre of gravity and the centre of buoyancy.

    The centre of gravity doesn't move much on conventional yachts.

    The centre of buoyancy moves around considerably depending on the heel of the boat and other factors that make the hull change its disposition to the water.

    If you take ballast and split it port and starboard without changing its height you haven't changed the centre of gravity so the stability will be the same.

    The thing that will change is the period of roll of the boat. It might rock from side to side a little bit slower with ballast arranged on each side rather than at the same height in the centre.

    Michael Storer
     
    Last edited: Jan 2, 2007
  4. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Isn't it tempting to make a water tank able to move from side to side?

    :)
     
  5. BOATMIK
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    BOATMIK Deeply flawed human being

    Ha Raggi,

    I think you have a big idea here.

    But I think it would be awkward and difficult to move.

    But we can use the natural properties of water to resolve the problem!

    IF it was frozen it could simply be slid almost frictionlessly from side to side.

    The refrigeration machinery might require running the motor continuously which would have been against the spirit of sailing once, but that seems to be OK in these days of canting keel yachts.

    :)

    Michael
     
  6. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Salt water?
    :)

    I think I will give it a try, two sliding water tanks, stored under the sitting bench on each side of the cabin when they are in "neutral" position, dragged cloely together up to the windward side when beating...
     
  7. BOATMIK
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    BOATMIK Deeply flawed human being

    Hi Raggi,

    Well if you don't use the engine for freezing the blocks (though I think it would save building/buying two tanks) maybe you can use it for moving them?

    :-0

    Michael
     
  8. LP
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    LP Flying Boatman

    Use a scupper on the leaward side below waterline to use the hydrodynamic pressure created by forward movement to fill the windward ballast tank. Then have a simple kick mechanism that opens a drain in the leaward tank while closing off the drain in the windward side. With a little ingenuity(sic), a person could maybe develope a siphon drain system that makes the entire system passive.
     
  9. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    yes, nice :)
    How much of the dynamic pressure can be used?
    Do you have any numbers from self bailers?

    Dynamic pressure (from speed) = static pressure (from height potential):
    1/2 x Rho x V^2 = Rho g h
    V squared = 2 x height x g (g~9.81, we use 10)
    or H = (V^2) / (2

    Say your speed is 1m/s after a tack (2 knots?),
    then the dynamic pressure in the scupper equals (1/20)m or 50mm (two inches).
    At 2m/s you can lift the water 200mm (8 inches).

    Now that's theory, does it work in practice?
     
  10. LP
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    LP Flying Boatman

    Just throwing ideas out there. Overactive imagination thing.

    Maybe my old worn out Force 5 could become the test platform. How do I get the tanks in without carving the thing up. Thinking out loud here. Tire innertubes might work. They're pliable. Just stuff them in under the rails. Might work to other means too. The windward tube would fill while the leaward would collapse. You don't have to worry about what to do with the air in a rigid tank. Two scuppers per side. One for pressure and one for suction. Pressure for windward ballast and suction for the leaward anti-ballast(drainage). As your speed increases on the tack, pressure and suction would also increase.

    How much head pressure do you get at 6 kts? . . . 7 & 8 kts?

    Actually, a test bed on an existing hull would be pretty inexpensive proposition. Too bad it's fricking winter here.
     
  11. BOATMIK
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    BOATMIK Deeply flawed human being

    The most widespread and successful method of water ballast is that used by New Zealander Jim Young to reduce the trailer weight of some trailer sailers of quite reasonable performance and ability.

    Probably a few hundred boats launched.

    The tanks fill when you put the boat in the water and empties when the boat is removed from the water.

    I believe there is a valve to keep the water in or out as required so hydrodynamic effects or gravity effects don't empty the tank at the wrong time.

    MIK
     
  12. LP
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    LP Flying Boatman

    I was looking at doing a waterballast trailer sailor and came up with a system that sounds very similar to the one you are talking about. The only addition I might have had was to incorporate a 12 volt air pump to assist in removing the water prior to trailering. Make it float higher for loading on to the trailer, but otherwise, very similar.

    http://boatdesign.net/forums/showthread.php?t=8190&highlight=trailer sailer

    Sounds like some one took the their idea to task and came up with a viable solution. Is there a website to go to?
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    Just to return to the 'condom in a jar' analogy, if you tape the condom on the outside of the jar it has no effect unless it is out of the water. That was the point I was trying to make in my "mind experiment" earlier. Much the same thing happen in a water ballasted boat that's swamped. The mind experiment was intended to show the importance of the hull shape to the success of the water ballast strategy. It will not work on every boat.

    Wish I'd thought of the condom analogy, but I've led a sheltered life ...
     
  14. SuperPiper
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    SuperPiper Men With Little Boats . .

    A.K.:

    You are from Alliston. I am from Peterborough. But, this response is being posted from a hotel room at the base of Mt Ste. Anne. Not a flake of snow in Ontario.

    We should be sailing.
     

  15. LP
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    LP Flying Boatman

    Another concept or property of water that needs to be dealt with is the fact that it is a fluid and has very low moment of inertia properties about the areas that are concentric with centers of rotation. (Roughly, the center of gravity of the vessel) As an example, take a full 2-litre bottle of pop with(no condoms though please) and spin it with the cap. Almost no resistance to motion. Put the same bottle on it's side and spin it on it's side. Now the areas (volumes) at the ends develope moments and there is a significant increase in resistance to motion. Freezing the bottle now will increase it's moment of inertia even further, though I'm not suggesting a refrigerated ballast tank.

    Should a person want to use a water ballast system, the chamber(s)s need to located at a distance from expected centers of rotation (bilge tanks) or the chamber(s) need to be sufficiently baffled that accelerations/rotational forces experiences by the vessel are tranfered to the water ballast. This will increase the MOI of the ballast and increase resistance to roll (and/or pitch) of said vessel. The point being to reduce eddy currents to a bare minimum within the ballast tank to make it respond more like a solid than a liquid.

    In the example given earlier where sailboat went over on its beam ends, I could speculate that a lack of internal baffling in the ballast tank could cause the lack of initial resistance to rolling moments. Encountering an unexpected gust could send the boat over on her ear very easily because once the water mass(ballast) is in motion, it will try to stay in motion and aggravate the condition.
     
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