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59405Re: Water Ballasted Chebacco

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  • mcdennyw
    Jan 11, 2009
      Someone suggested that I try the water balast calculation with the
      boat as-designed, that is without rounding off the bottom. After
      finding this post from a respected NA, Tad Roberts, on the Woodenboat
      Water ballast is useful and does work, but only in very specific
      circumstances. In the typical lead mine sailboat (and your physics
      experiments above) that Bob is referring to, it would be useless.

      Mr. Bolger, Hunter, and I believe Macgregor all use water ballast to
      good effect. These are all hulls that are shallow, wide bottomed (in
      Bolger's case rectangular), and lightweight. The water ballast is
      contained in tanks that are the full width of the boat, but not very
      high vertically. The rectangular or almost rectangular midsection
      means these boats rely on form stability at low heel angles. As the
      boat heels, the water ballast rises above the waterline very quickly,
      and becomes working ballast. The barge hull form runs out of form
      stability quickly, and this is were the ballast becomes effective.
      But only at fairly high heel angles, probably 20 degrees or more.
      Most of us aren't used to sailing at those high angles any more.

      It enables a hull of very light trailering weight to also be self
      righting (if she has either floation or a watertight cabin) from a
      complete knockdown. Water ballast will also smooth out motion and
      give a very light hull some momentum. In the case of the Macgregor it
      enables a useable (and reasonably safe) sailboat, to dump her ballast
      and plane off under outboard power. Another excellent feature is that
      if your water ballasted boat is knocked down and fills, she will not
      sink. Coupled with extremely shallow draft on launching, which can be
      very handy, it makes sense.

      As always, horse's for course's.

      All the best, Tad.
      The Chebacco, as designed, is just the form Tad suggests would
      benefit from water ballast. I revised my Freeship model to put 500#
      of water ballast in the 19' flat bottom boat and...

      ta daa - it is much stiffer than my round bottom idea. At a 20
      degree angle of heel (sailing to windward in a fresh breeze, the flat
      bottom water ballasted boat has a righting moment of 1880 ft-lbs, 30%
      better than the round bottom boat and 80% more than the unballasted
      boat. Evan at 5 and 10 degrees the water ballasted boat has almost
      double the moment of the stock boat. The extra 500# causes the boat
      to sink down just 1.5" deeper.

      The ballast tank is essentially a false bottom parallel to the
      waterline and 5" deeep at its deepest point in the middle of the
      boat. Sounds like an even better idea than the original post.

      Denny Wolfe
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