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Hi Rick.
You wrote...."At ~33 feet seawater, the dived
volume is half its original surface volume and is now half as buoyant,
the forces applied to either side are indeed
cancelled out but not the forces applied above and below the air bubble.
They ae unequal and this is where buoyancy comes from."
I had the same unvoiced initial thought for Joe,
that the hull or tank air volume would compress by half every atmosphere, but I
did not mention it because.that should not affect anything in his case
because
hs valve would be constantly adding more air
pressure to compensate for the water trying to come in and not allow his
internal air volume to be cut in half or else his hull or tank would be half
full
of water wouldn't it? So even though his air is
compressed inside the vessel (hull/tank) by half for every extra atm deep he is,
he would always compensate for this by putting more pressure inside to
equalize
everything and never be in a situation where his
hull air volume was only half and therefore only half as buoyant, unless it was
flooding a tank and not the hull. It would help to know if Joe was talking
about
the hull interior or a ballast tank.
I wonder at what point his hull/tank air
compressability would cause him to lose buoyancy due to
the air molecules being compressed and taking up
less space and weighing more per space than they did previously? Like a full
scuba tank in the water vs all the air released from the tank.
As long as he added more pressurized air into the
hull/tank to equalize the pressure against the outside water pressure, the only
other thing in play force wise is gravity which you overcome with
buoyancy like you said.
Bill.
----- Original Message -----
Sent: Wednesday, November 09, 2005 12:57
AM
Subject: Re: [PSUBS-MAILIST] Forces of
Nature
Hi, Joseph - Important questions.
My learning curve is
flattened again, too many distractions, Please help me with some basics
here.
#1 One submerges a
bubble of air (enclosed in any material) and open to ambient
pressure to 1atm . Equal forces of pressure are applied to either side
of the material. I assume that the lifting force of the air in pounds of
buoyancy is not cancelled out by the equalization of that pressure. That the
material is subjected to a stress in psi equal to that force. Then if one
were to apply an opposing force ,as in ballast, that the material is then
subjected to both forces. Is this a correct assumption?
No. Boyle's Law states that a given
volume of air will compress to half its original volume for every atmosphere
of pressure applied or P1V1=P2V2
At ~33 feet seawater, the dived volume is half
its original surface volume and is now half as buoyant. The forces
applied to either side are indeed cancelled out but not the forces applied
above and below the air bubble. They ae unequal and this is where
buoyancy comes from.
#2 One submerges a piece of marine ply to one atm. Is
the cellular structure of the wood irreversibly compressed creating a
permanent change in density and therefore buoyancy?
Yes, to some degree it does. Elasticity of wood,
or the ability to bounce back from an applied force, is limited. To
what degree this is important is debatable. If we are talking
continental shelf pressures, probably not a heck of a lot. If you fall
off the edge, eventually I could see it playing a role.
At a practical level I believe it's a moot point,
especially within the first two hundred feet. It depends on the wall
thickness, whether you've used cold moulding, the pressure hull shape,
pressure focal points, etc.
Rick
Vancouver
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