'It would help to know if Joe was talking about the hull interior or a ballast tank."
Actually both Bill. The original question is geared towards material scantlings for the forces involved and not so much the behavior of the air. The buoyancy and control issues are separate nightmares to be dealt with.
Joe
From: "Joseph Perkel" <joeperkel@hotmail.com>
Reply-To: personal_submersibles@psubs.org
To: personal_submersibles@psubs.org
Subject: [PSUBS-MAILIST] Re: Forces of Nature
Date: Wed, 09 Nov 2005 16:20:05 +0000
************************************************************************ ************************************************************************ ************************************************************************ The personal submersibles mailing list complies with the US Federal CAN-SPAM Act of 2003. Your email address appears in our database because either you, or someone you know, requested you receive messages from our organization. If you want to be removed from this mailing list simply click on the link below or send a blank email message to: removeme-personal_submersibles@psubs.org Removal of your email address from this mailing list occurs by an automated process and should be complete within five minutes of our server receiving your request. PSUBS.ORG PO Box 311 Weare, NH 03281 603-529-1100 ************************************************************************ ************************************************************************ ************************************************************************Thank you for the discussion guys. As an amateur designer I am trying to visualize all forces at work here. it would be a shame to have the thing "fold up' because some twisting moment or compressive force was overlooked. Existing designs and plans give me further hints.
Thanks again
Joe
From: "Akins" <lakins1@tampabay.rr.com>
Reply-To: personal_submersibles@psubs.org
To: <personal_submersibles@psubs.org>
Subject: Re: [PSUBS-MAILIST] Forces of Nature
Date: Wed, 9 Nov 2005 02:44:46 -0500
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 becausehs 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 fullof 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 equalizeeverything 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 aboutthe hull interior or a ballast tank.I wonder at what point his hull/tank air compressability would cause him to lose buoyancy due tothe 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 withbuoyancy like you said.Bill.----- Original Message -----From: Rick and MarciaSent: Wednesday, November 09, 2005 12:57 AMSubject: Re: [PSUBS-MAILIST] Forces of NatureHi, 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=P2V2At ~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.
RickVancouver