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Re: water ballast

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  • oneillparker
    Nels, consider, in an inverted craft with water ballast tanks awash while it is certainly true that the ballast is contributing little or nothing towards
    Message 1 of 47 , Feb 1, 2004
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      Nels, consider, in an inverted craft with water ballast tanks awash
      while it is certainly true that the ballast is contributing little or
      nothing towards righting the craft, it is also certain that it is
      contributing nothing towards actually sinking the craft either.

      If, however, the same weight of water ballast was replaced with lead,
      and considering that the craft is barely afloat in the first place,
      the craft would now sink. The denser lead would displace less water,
      and down she would go.

      If a designer didn't want that scenario to happen then the designer
      would have to find space for flotation inside the hull anyway - and
      lots of it - to offset that lead. Whereas the water-ballasted design
      is already not sinking, and only needs enough flotation high up to
      make the boat unstable when inverted.

      So, using lead as ballast because it takes up less space is a great
      argument if you don't mind sinking. But personally, afloat in the big
      wide water with all that can go wrong I'd rather have a boat that
      will stay with me when disaster strikes, no matter how crippled she
      may be, than a SINKER that will give up the ghost, abandon me, and
      disappear under the waves....

      Which reminds me of a report I saw recently, in Latitude 38, of a
      pinky schooner that did just that recently in the South Seas. Filled
      with lead ballast down she went. No storm. No reef. No collision. No
      floating containers in mid-ocean. Just a light fog (as I recall).

      No, this sinker caught her big keel in a fisherman's trawling net and
      got pulled sideways enough to flood. It took two or three minutes,
      and she was gone. Her skipper, sailing solo, had just enough time to
      take to the inflatable....

      John O'Neill

      --- In bolger@yahoogroups.com, "Nels" <arvent@h...> wrote:

      > There is another disadvantage of a boat with water ballest tanks.
      > it becomes inverted and fills with water, it can be almost
      > for the occupants to return it upright as they would have to be
      > strong enough to lift the weight of the water filled ballest tank
      > above the water and over 180 degrees to get the boat back on it's
      > feet. Not easy to do with 500 pounds of water while you are
      > in the water. Your only option is likely to drain the tank which
      > be impossible when outside the boat and in the water. It happened
      > one of Bolger's designs apparently.
      > "A couple years ago, one of my water-ballested designs met with an
      > accident that completely flooded her. The wood structure had
      > bouyancy, so she didn't sink, but she floated bottom up with the
      > outside ballest tanks awash. Some foam high up in the hull would
      > righted her and saved some inconvenience,even danger in cold
      > BWAOM page 245
      > So in this case I would rather be in a boat without water ballest
      > tanks. Also without the tanks, it would allow more options to load
      > down with a more effective ballest than water. i.e. lead, rocks,
      > guys filled with beer.
      > 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
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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...>
        > 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
        > | measure again, the amount of force required to tip the boat a
        > | amount will be equal.
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