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Re: [bolger] Re: Recent News from PCB, Cartoon 40 - Now "Smart Water"

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  • derbyrm
    The clue is in the let the outside shape of the hull remain the same. Then it doesn t matter if the filling is lead, pudding, or water. Only its density
    Message 1 of 2 , Nov 28, 2006
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      The clue is in the "let the outside shape of the hull remain the same." Then it doesn't matter if the filling is lead, pudding, or water. Only its density matters.

      (unless you have significant free surface, then the dynamics get nasty)

      Roger
      derbyrm@...
      http://home.insightbb.com/~derbyrm

      ----- Original Message -----
      From: Gene T
      To: bolger@yahoogroups.com
      Sent: Tuesday, November 28, 2006 10:50 AM
      Subject: Re: [bolger] Re: Recent News from PCB, Cartoon 40 - Now "Smart Water"


      Let the Alchemy begin (or however that is spelled). If I hadn't taken all those physics classes this wouldn't make me suffer so!

      Sincerely,
      Gene T.
      "A house ashore is but a boat, so poorly
      built it will not float ---- "

      ----- Original Message ----
      From: adventures_in_astrophotography <jon@...>
      To: bolger@yahoogroups.com
      Sent: Tuesday, November 28, 2006 9:32:28 AM
      Subject: [bolger] Re: Recent News from PCB, Cartoon 40

      Hi Peter,

      > No.

      > Water ballast is a tricky and misunderstood thing, and I don't mean to
      > start a long, fruitless discussion, but the above sentence is not true
      > in this case, even if PCB did write something to that effect. The
      > comparison is between the hull with water ballast and the hull with
      > the "ballast tank" removed (in the sense of being taken off the
      > outside of the boat). In this case, the "tank removed" hull would have
      > the deepest part of the v-bottome removed - a completely different
      > hull shape.
      >
      > If you keep the v-shape and fill the bottom part with water ballast,
      > you lower the center of gravity (or, equivalently raise the center of
      > buoyancy) which improves stability.

      Such a discussion may not be frutiless, so I'll bite - politely I hope.

      I agree with your statement that filling the bottom part of the v-shape
      with water improves stability, but not because it acts as ballast all
      the time. It is simply neutral bouyancy, not the positive bouyancy
      that the empty space would have, and not the negative bouyancy that
      lead or another heavy ballast material would have. For stability,
      neutral bouyancy is better than positive bouyancy in that part of the
      hull. However, it won't help the boat when heeled as much as negative
      bouyancy in that location unless it gets above the waterline where it
      then acts against the heeling motion of the boat. Any empty volume
      submerged by heeling will resist the heeling force, but the water will
      do nothing to counter this force as long as it remains below the
      surface as neutral bouyancy. The v-shape of this hull concentrates the
      flooded volume near the centerline, thus requiring more heel to get
      much of it above the surface, which I claim makes it less effective as
      ballast than denser materials.

      Because of this, I maintain that my original statement is valid, but is
      perhaps better if restated: Replacing the lead/steel/concrete called
      for on the plans with neutral bouyancy in the same location will not be
      effective *as ballast* unless the boat heels enough to get some of that
      volume above the water line.

      I'm not saying it won't work at all in this boat, I just think it won't
      be as effective as it might seem at first.

      Jon Kolb
      www.kolbsadventures.com/boatbuilding_index.htm

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