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Re: water ballast

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  • Nels
    ... In the article where I read about this the water ballest tanks were divided into two areas - one in each bilge. The single crew could not right the boat
    Message 1 of 47 , Feb 1, 2004
      --- In bolger@yahoogroups.com, "Gene T." <goldranger02-boats@y...>
      wrote:
      >
      > This confuses me! Water trapped up at the highest
      > point of the hull, where the water ballast tanks are
      > in this inverted boat doesn't seem like an impediment
      > to rolling back over. The "obvious" logic escapes me!
      >
      > Gene T.

      In the article where I read about this the water ballest tanks were
      divided into two areas - one in each bilge. The single crew could not
      right the boat until he got into water shallow enough where he could
      lift the weight of the near tank high enough out of the water to flip
      it over. While in the water he could not get enough leverage to do so
      no matter what he tried. The most effective way was bearing down on
      the near leeboard - using it as a lever. If the tanks had been empty
      he could have done it easily is what he claimed. The biggest drawback
      in this particular design however, was deemed to be the large cockpit
      that was not self-draining once it swamped. The water was coming in
      faster than going out - so the weight of the water in the cockpit
      plus the water in the ballest tanks and the force of the wind on the
      furled sail, caused the boat to roll over. The guy was sleeping in
      the cuddy and when he opened the hatch, it filled with water as well.
      (The boat was designed as a "coastal cruiser" and got hit with a
      thunderstorm while lying offshore to get away from the bugs and
      rocks.)

      At any rate, that convinced me to follow what Bolger recommends when
      it comes to seaworthiness - and that is - a boat that can recover
      from a knockdown on it's own.

      I live where the water is cold most of the year, and we get some
      pretty nasty thunderstorms.

      Cheers, Nels
    • 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
        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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