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Re: water ballast

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  • oneillparker
    Free surface effect? I don t follow. Nor did I say anything about water ballast sloshing around in the bilge, because you re right, sloshing water does
    Message 1 of 47 , Feb 1, 2004
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      Free surface effect? I don't follow. Nor did I say anything about
      water ballast 'sloshing around in the bilge,' because you're right,
      sloshing water does nothing for righting moment - but neither does
      water ballast in a bilge filled with sloshing water. picture those
      plastic jugs of water 'ballast' tied with string to a frame member in
      the bilge of a skiff half filled with water. The jugs meader around
      at neutral bounancy (disregarding the jug itself) contributing zero
      to stability until the boat is heeled enough to bring a jug out of
      the water.

      But in a dry bilge where the water in the jugs no longer has to
      displace it's own volume in water before it acts to weigh down the
      hull the jug is continually pressing against the bottom of the hull,
      weighing the boat down, adding to stability.

      I think your statement that it depends on how you "draw the envelope"
      is dead on. In a given boat, whether ballast in put in the boat or
      tacked on outside the boat has the potential to make a huge
      difference depending on the density of the ballast, and can actually
      contribute nothing to stability (water) or even give negative
      stabilty (foam) because tacking on ballast outside a given hull in
      effect adds to the overall volume of the hull. Placeing the same
      ballast inside the hull makes the boat float deeper, but doesn't
      change the shape of the boat - no matter what the displacement of the
      ballast itself - thus potentially adding to stability depending where
      it is placed.

      On a boat STILL ON PAPER, the distinctions between outside and inside
      water ballast can be less clear depending on the priorities of the
      designer, because one way or the other room has to be found for the
      ballast. If the boat has to be given more volume in order to add
      ballast without compromising other priorities (like headroom) then
      the distinction between 'inside' and 'outside' ballast - as far as
      the designer is concerned - becomes fuzzy - because if to make room
      for the ballast the designer has to deepen the keel or harden the
      bilge - thus changing the overall volume - the designer is, in
      effect, tacking on the ballast 'outside' despite the fact that the
      builder will actually be placing the ballast 'inside.'

      John O'Neill





      --- In bolger@yahoogroups.com, "John Bell" <smallboatdesigner@m...>
      wrote:
      > Free surface effect. Water sloshing around in the bilge does
      nothing for
      > righting moment. Bad example. ....

      >
      > ....Basically it all depends on how you draw the envelope.
      >
      >
      >
      > ----- Original Message -----
      > From: "oneillparker" <jboatguy@c...>
      > | If you put the water ballast back in the boat, and then flood the
      > | boat enough to cover the ballast, measure, then take the jugs out
      and
      > | measure again, the amount of force required to tip the boat a
      given
      > | amount will be equal.
    • oneillparker
      Free surface effect? I don t follow. Nor did I say anything about water ballast sloshing around in the bilge, because you re right, sloshing water does
      Message 47 of 47 , Feb 1, 2004
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        Free surface effect? I don't follow. Nor did I say anything about
        water ballast 'sloshing around in the bilge,' because you're right,
        sloshing water does nothing for righting moment - but neither does
        water ballast in a bilge filled with sloshing water. picture those
        plastic jugs of water 'ballast' tied with string to a frame member in
        the bilge of a skiff half filled with water. The jugs meader around
        at neutral bounancy (disregarding the jug itself) contributing zero
        to stability until the boat is heeled enough to bring a jug out of
        the water.

        But in a dry bilge where the water in the jugs no longer has to
        displace it's own volume in water before it acts to weigh down the
        hull the jug is continually pressing against the bottom of the hull,
        weighing the boat down, adding to stability.

        I think your statement that it depends on how you "draw the envelope"
        is dead on. In a given boat, whether ballast in put in the boat or
        tacked on outside the boat has the potential to make a huge
        difference depending on the density of the ballast, and can actually
        contribute nothing to stability (water) or even give negative
        stabilty (foam) because tacking on ballast outside a given hull in
        effect adds to the overall volume of the hull. Placeing the same
        ballast inside the hull makes the boat float deeper, but doesn't
        change the shape of the boat - no matter what the displacement of the
        ballast itself - thus potentially adding to stability depending where
        it is placed.

        On a boat STILL ON PAPER, the distinctions between outside and inside
        water ballast can be less clear depending on the priorities of the
        designer, because one way or the other room has to be found for the
        ballast. If the boat has to be given more volume in order to add
        ballast without compromising other priorities (like headroom) then
        the distinction between 'inside' and 'outside' ballast - as far as
        the designer is concerned - becomes fuzzy - because if to make room
        for the ballast the designer has to deepen the keel or harden the
        bilge - thus changing the overall volume - the designer is, in
        effect, tacking on the ballast 'outside' despite the fact that the
        builder will actually be placing the ballast 'inside.'

        John O'Neill





        --- In bolger@yahoogroups.com, "John Bell" <smallboatdesigner@m...>
        wrote:
        > Free surface effect. Water sloshing around in the bilge does
        nothing for
        > righting moment. Bad example. ....

        >
        > ....Basically it all depends on how you draw the envelope.
        >
        >
        >
        > ----- Original Message -----
        > From: "oneillparker" <jboatguy@c...>
        > | If you put the water ballast back in the boat, and then flood the
        > | boat enough to cover the ballast, measure, then take the jugs out
        and
        > | measure again, the amount of force required to tip the boat a
        given
        > | amount will be equal.
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