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Re: [bolger] Re: Plywood 12 1/2 sloop-CEMENT KEELS

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  • Kenneth Grome
    ... So Graeme ... when you say Yes, Ron, there is. are you talking about a center of gravity difference? If so, I agree that there WOULD be a center of
    Message 1 of 56 , Sep 28, 2007
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      >> Put 100 pounds of rocks in your boat and
      >> note the water line. Take out the rocks
      >> and put in 100 pounds of water. Is there
      >> a difference?
      >
      > Yes, Ron, there is.


      So Graeme ... when you say "Yes, Ron, there is." are you talking about a
      center of gravity difference? If so, I agree that there WOULD be a
      center of gravity difference because the rocks are denser than water so
      the center of gravity will be lower with rocks than with water.

      But if you are suggesting that there will be a waterline difference I
      disagree completely ... because 100 pounds of anything is going to push
      the boat down to the same waterline regardless of whether that 100
      pounds is made of rocks or steel or water or lead or feathers.

      Sincerely,
      Ken Grome
      Bagacay Boatworks
      www.bagacayboatworks.com
    • graeme19121984
      When the boat is rolled, one side dips down and the other raises up, but unlike a see-saw, the balance point, the Centre of Bouyancy also moves. The balance
      Message 56 of 56 , Sep 30, 2007
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        When the boat is rolled, one side dips down and the other raises up,
        but unlike a see-saw, the balance point, the Centre of Bouyancy also
        moves. The balance point is not the centre of ballast mass, nor
        centre of the entire boat system mass of the level boat. When rolled
        there is more of the ballast-water (that has affected the entire
        CoG and reduced initial stability (form stability) of firm bilged
        and similar boats) now offset to one side of the Boat CoB. With
        sufficient rolling that ballast water will have a significant impact
        on increasing secondary stability as any of it is lifted above the
        waterline; having initially reduced stability of the unballasted
        firm bilged boat.

        At the other extreme to the firm bilged boat, the deeep ballasted
        keel as on the 12&1/2 will have the same action on secondary
        stability if water is used as ballast - it won't act to right the
        boat until it comes out of the water, in this case at nearly
        90degrees heel! If other material was unavailable then water-
        ballast in the keel would serve to increase the initial stability of
        the slack-bilged 12&1/2 over that of the unballasted boat, but it
        would still be quite tiddly as water-ballast in the keel couldn't
        sink it sufficiently to effectively widen the water-line beam.



        Graeme




        --- In bolger@yahoogroups.com, Gene T <goldranger02-boats@...> wrote:
        >
        > Water doesn't act like a pendulum? I think it does. The boat is
        a complete system. The water and air in a boat act as a unit. Roll
        the boat and more water is on one side of center and more air on the
        other. All below the waterline. Think of it any way you like, more
        air below the water on one side and more water on the other acts as
        a pendulum and tries to right the boat.
        >
        > And ballast doesn't change the shape of this boat we are talking
        about. First pick a boat. Ok, now you have a shape. Then put
        ballast in it. Doing the argument any other way is comparing two
        different boats, which is not what this is all about.
        >
        > Sincerely,
        > Gene T.
        > "We may have all come on different ships, but we are in the same
        boat now" -- Rev. Martin Luther King
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