RE: [PSUBS-MAILIST] Pressure hull materials

["Alec Smyth" <Asmyth@changepoint.com>]

Actually someone brought up the concept of an inflatable ambient sub just recently, and we were visualizing folding it up and storing in the trunk of a car after the dive. Cool! The problem I see with this concept is that although the hull might be light, this does not mean the sub as a whole can be light too -- it still needs overall to be the same density as seawater if you want it to sink. One can argue you could then put the weight saved from the hull into more useful things like batteries. But that's going to be one heck of a lot of weight to connect structurally to the flexible material. One way or another you're flexible material will end up supporting it, regardless of whether you use some sort of exostructure. Maybe this would work if attempted on a very small scale, to keep the hull volume down, such as a ride-on scooter for divers. 
 
I hate to counter innovative ideas... keep them coming!
 
cheers,
 
Alec
 
 
 

	-----Original Message----- 
	From: Dale Raby [mailto:publisher@thegreenbayweb.com] 
	Sent: Sun 12/22/2002 9:59 AM 
	To: personal_submersibles@psubs.org 
	Cc: 
	Subject: Re: [PSUBS-MAILIST] Pressure hull materials
	
	

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	Good Morning:
	
	I am new to the list and not very well versed in the subject, though I have
	to say that I find the topic of your discussions fascinating.  On the subject
	of pressure hull materials I have a few thoughts.  Perhaps someone with more
	knowledge/ability/money can make use of it.  If nothing else... perhaps it
	will provide you with some amusement.
	
	The basic function of a hull, as I understand the concept, is to provide a
	"bubble" of air for the occupants of the submarine to breath.  Now, if you
	want to maintain an atmospheric pressure similar to that found at sea level,
	you need a very rigid structure because the water pressure at depth will tend
	to compress the shell.  If one is not concerned with this, the hull's
	rigidity is not at all a concern.  Bubbles can form in great depths with
	nothing but the surface tension of the water to hold their shape.
	
	Now, suppose someone were to form a thick clear flexible plastic "bubble"
	with a conventional rigid hatch and seal for entry/egress.  One could
	concievably use an old water bed mattress.  As the "bubble" descends, the
	non-rigid hull would compress.  One could equalize this pressure by
	compensating with bottled air.  You would have to have a pressure valve
	somewhere to keep the bubble from bursting from internal pressure.
	Equalization could be maintained by simply opening the valve whenever it is
	noticed that the hull is collapsing until air bleeds out through the pressure
	valve.
	
	The craft would have to have a light framework to affix things to... life
	support components, electrical devices for light, etc.  It would be not
	unlike a semi-rigid dirigible... and the bottom of the craft would have to
	have some ballast to maintain equilibrium as well as to allow it to sink
	below the surface.  Variable ballast tanks could also be formed from the same
	stuff... though they would function more as "anti-ballast" tanks, since it
	would be the absence of air that would cause the device to sink.
	
	One obvious drawback would be the necessity of decompression... and I suspect
	that there would be a very definite absolute depth limit... maybe a hundred
	feet... but in small lakes and rivers, that would be sufficient.  You'd have
	to watch your time, just like a SCUBA diver, and come up slowly, allowing the
	overpressure to bleed out.
	
	For a model, a zip-lock plastic bag would suffice.  Put a CO2 cartridge
	inside it with a valve assembly for equalization experiments.
	
	One other question....  Bushnell's Turtle as well as the CSA Hunley utilized
	simple muscle power for propulsion.  They didn't have electrics or internal
	combustion engines.  Has anyone postulated a modern Hunley that would use the
	same type of propulsion?
	
	OK, go ahead and laugh at me.  I'm sure I've said more than a few amusing
	things... but if my flexible hull concept is flawed, I would like to know why
	it wouldn't work.
	
	- --
	Dale A. Raby
	Editor/Publisher
	The Green Bay Web
	http://www.thegreenbayweb.com
	Nothing is ever so bad that it couldn't be worse, and if it could be worse,
	then maybe it's not so bad!
	
	
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