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## 8134Re: Physics?

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• Nov 2, 2000
"a boat would behave the same whether ballasted with 500 lbs. of
water, or 500 lbs of lead"

Well, not true, let me see if I can give you a feel for this. I
concidered this while imagineering a catfish, which has a water-
filled keel.

Imagine you have any boat, and start filling baloons with water,
knotting the end and tying off with string. Weigh each baloon, then
dump it overboard. Tie the string to the gunnel and do another. How
many baloons will it take to sink your boat?

Well, not *quite* an infinite ammount. After all, the string and
even the baloon itself have some weight. But close.

Now start doing the same with fishing sinkers that have the same
weight as a water filled baloon. As long as the weights stay off the
bottom they will start to pull your boat down as soon as you add the
first one.

Sure, 500 pounds of water and 500 pounds of lead both weigh 500
pounds *in air*. Put em underwater and the water weighs.... zero,
nada, nothing. Just remember: all boats float cause they weigh
nothing.

It's displacement you see. A 500 pound boat displaces an volumn of
water that if you could scoop out and take out on land and weigh it
would weigh... exactly 500 pounds. If the weight of water your boat
displaced weighed any less, your boat would be sinking.

(Water inside the boat isn't displaced. It may be out of place, but
thats something different.)

The lead will have weight underwater, as it is denser then the
water. A thing is good for balast under water as long as it is denser
then water, but only by the difference in it's weight and the weight
of water it displaces.

For my 'water keel' catfish, the water filling the keel still does
a few useful things: It still has mass, hence it will help damp out
movement (it will take more time for it to heel all the way over wet
over dry), and also help carry it thru tacks. But, since underwater
water "don't weigh nothing," it will not add to the righting moment
till the boat heels to it's side and the keel comes out dry. And once
outside it gets heavy FAST, helping to stop a rollover when it's
getting disasterous.

-Ernie

--- In bolger@egroups.com, "Bill Jochems" <wjochems@s...> wrote:
> Estimado grupo Bolger
> I have a question about something which is related, I think, to
David
> Ryan's question about 100 lbs. centered vs. 50 lbs. on each side.
Bolger
> has written that water ballast is of no effect until it has been
lifted
> above the water line. I don't understand this. It seems to me
that weight
> is weight, be it water, concrete or lead; and that a boat would
behave the
> same whether ballasted with 500 lbs. of water, or 500 lbs of lead,
if the
> centers of gravity of each were the same.
> Bill Jochems
> -----Original Message-----
> From: David Ryan <david@c...>
> To: bolger@egroups.com <bolger@egroups.com>
> Date: Monday, October 30, 2000 10:17 AM
> Subject: Re: [bolger] Physics?
>
>
> >Hooray! Dr. Wah would be proud, even if he never did understand
what
> >I was doing in his class.
> >
> >Moving on.
> >
> >In a flat bottomed boat, is there a difference between putting 100#
> >dead in the center of the center, or putting 50# on each chine?
> >
> >YIBB,
> >
> >David
> >
> >
> >
> >>That's about right. Let the weight of your 900# beam act in the
middle;
> the
> >>lever arm is from fulcrum is then (15-3)=12', so the moment is
> 12*900=10800
> >>ft-lb. To balance the beam, you need to apply 10800/3 = 3600# on
the
> short
> >>end (i.e. 3*3600=10800 => equilibrium).
> >>
> >>(To figure it much, much more simply, convert your units to SI
metric,
> >>divide by 1000 and then once again by .001 ;-)
> >>
> >>Gregg Carlson
> >>
> >>At 09:58 AM 10/30/2000 -0500, you wrote:
> >>>FBBB --
> >>>
> >>>It's been 12 years since I cracked a physical text book (my
degree is
> >>>in fine art, go figure.)
> >>>
> >>>Anyone care to check me on this:
> >>>
> >>>A 30 foot beam that weighs 30 pounds per foot is balanced on a
> >>>fulcrum 3 feet from one end (and 27 feet from the other.)
> >>>
> >>>By my (suspect) calculations, there is a torque of 10935 foot-
pounds
> >>>on one side, and 135 foot-pounds on the other.
> >>>
> >>>Further (dubious) arithmetic says that if I apply 3600 pounds of
> >>>force directly on the end of the short side, the beam will be
> >>>balanced.
> >>>
> >>>Anyone wanna check my math?
> >>>
> >>>YIBB,
> >>>
> >>>David
> >>>
> >>>
> >>>CRUMBLING EMPIRE PRODUCTIONS
> >>>134 W.26th St. 12th Floor
> >>>New York, NY 10001
> >>>(212) 243-1636
> >>>
> >>>
> >>>Bolger rules!!!
> >>>- no cursing
> >>>- stay on topic
> >>>- use punctuation
> >>>- add your comments at the TOP and SIGN your posts
> >>>- add some content: send "thanks!" and "ditto!" posts off-list.
> >>>
> >>>
> >>
> >>
> >>
> >>Bolger rules!!!
> >>- no cursing
> >>- stay on topic
> >>- use punctuation
> >>- add your comments at the TOP and SIGN your posts
> >>- add some content: send "thanks!" and "ditto!" posts off-list.
> >
> >
> >CRUMBLING EMPIRE PRODUCTIONS
> >134 W.26th St. 12th Floor
> >New York, NY 10001
> >(212) 243-1636
> >
> >
> >Bolger rules!!!
> >- no cursing
> >- stay on topic
> >- use punctuation
> >- add your comments at the TOP and SIGN your posts
> >- add some content: send "thanks!" and "ditto!" posts off-list.
> >
> >
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