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Re: Plywood 12 1/2 sloop-CEMENT KEELS

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  • graeme19121984
    ... calculate the ... the the ... behavior. Yes we do. At a given angle of heel, that s a snap shot of whether you are heeled too much, past the point of no
    Message 1 of 56 , Sep 28 6:05 AM
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      --- In bolger@yahoogroups.com, "Bruce Hallman" <bruce@...> wrote:
      >
      > > Until the water ballast comes out of the water you can
      calculate the
      > > static behaviour of the boat from a different shape, that is
      the the
      > > boat shape minus that part holding water.
      >
      >
      > True, for static behavior, but we don't care much about static
      behavior.

      Yes we do. At a given angle of heel, that's a snap shot of whether
      you are heeled too much, past the point of no return, and are in the
      process of capsising, or otherwise.



      >
      > Rather, we care about the dynamic behavior: The effect when the
      hull
      > heels over.
      >

      Dynamic behaviour has to do with moments of inertia and
      accelerations.


      > In that case, the water ballast swings out from the center of
      > buoyancy, creating a righting moment force.
      >
      > This occurs based on the location of the 'center of weight',
      whether
      > or not that location is above or below the waterline.
      >

      While underwater, with respect to the centre of the Earth, the
      centre of weight of water ballast through which the force of gravity
      acts downwards is exactly vertically aligned with the centre of
      bouyancy of the water it is displacing through which the bouyant
      force acts in the exatly opposite upwards direction.

      As the forces are of the exact same magnitude, and are acting
      directly opposed, there is no effect - ie no resulting movement of
      one body of water or the other, no action.

      What does have an effect in swinging out is the CoG of the boat
      relative to the CoB of the boat. These centres of exactly opposing
      forces become more misaligned as heeling progresses, increasing the
      righting arm (RA), or the horizontal distance between their
      respective verticle axes of action. At some point if heeling goes
      far enough the RA will start to diminish as the CoG is rotated over
      the CoB. Past this point the RA becomes negative, and the boat is on
      the way to capsise :(

      When the water-ballast is lifted into air, then the centre of weight
      of that water is aligned verticaly with the centre of the volume of
      air it has displaced. As water is denser than air there is a net
      downwards force, so we can say that the water then is serving to
      right the boat.

      Cheers
      Graeme
    • 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 10:14 PM
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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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