Re: Plywood 12 1/2 sloop-CEMENT KEELS
- --- In email@example.com, "Bruce Hallman" <bruce@...> wrote:
> > Until the water ballast comes out of the water you can
> > static behaviour of the boat from a different shape, that isthe the
> > boat shape minus that part holding water.behavior.
> True, for static behavior, but we don't care much about static
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
> heels over.Dynamic behaviour has to do with moments of inertia and
> In that case, the water ballast swings out from the center ofwhether
> buoyancy, creating a righting moment force.
> This occurs based on the location of the 'center of weight',
> 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.
- 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.
--- In firstname.lastname@example.org, 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.
> Gene T.
> "We may have all come on different ships, but we are in the same
boat now" -- Rev. Martin Luther King