Re: [PSUBS-MAILIST] Pressure hull materials

[Dale Raby <publisher@thegreenbayweb.com>]

On Sunday 22 December 2002 16:17, you wrote:
Thanks for the welcome.

You are correct, a regulating system would be necessary.  My idea would 
involve the operator watching the hull to monitor compression and manually 
opening a valve on an air bottle to cause the hull to expand as the craft 
descends.  A safety valve, much like that found on any steam boiler, would 
have to be present on the hull... perhaps on the bottom of the craft.  This 
valve would be set somewhere under the point at which the hull would fail 
from internal pressure.  Unlike a steel hull, it would be possible without a 
valve like this for the hull membrane to burst like a baloon... a rather 
frightening prospect for the occupants of the craft.  Equilibrium would be 
reached when bubbles come out of the safety valve.  

It would indeed be rather critical to maintain the hull fully inflated.  This 
would define displacement... a very important factor in determining buoyancy. 
 Probably you'd just open the air bottle to bleed a continuous stream into 
the atmosphere and watch your descent speed to make sure that bubbles 
continued to come out of your valve.  When you reach operating depth, you 
could, in theory, close the air bottle and perform your mission with normal 
life support engaged.  In all actuality, you'd probably just let the bottle 
bleed slowly.  The CO2 you'd exhale would naturally sink to the bottom of the 
chamber and be forced out the valve at the bottom.  Thus you'd have a viable 
life support system without any CO2 scrubber... so you'd need no electricity, 
other than a flashlight for light.  I suppose you could even go so far as to 
use a candle like they did on the Hunley.

Ballast tanks would be separate from the passenger compartment... though in 
reality, the passenger compartment would function as a ballast tank in itself.

The craft I envision would be constructed of something like the rubberized 
fabric they use to make Zodiacs.  I suppose you could, in theory, make the 
hull from Saran wrap... but I think I'd want something more stubstantial than 
that.  If I drop a screwdriver, I want it to bounce, not puncture!  The hull 
would be suspended in an external "cradle" or exoskeleton that would resemble 
the framework that they used to raise the Hunley.  There would be braided 
Nylon lines through grommets attached to the hull suspending it inside the 
framework.  

It would look like something Jules Verne cooked up... and would use 
technology he would have been familiar with.  It could even, in theory, have 
a hatchway on the bottom that would allow EVA for a SCUBA equipped passenger.

I don't think you would want pressure much above what is necessary to keep 
the hull expanded.  At just above equilibrium pressure, if you happened to 
drop an opened pocket knife into your hull, you'd have a minor hull breach 
that would begin to leak air bubbles.  If you had pressure of say five 
atmospheres inside the rising sub and dropped a screwdriver causing even a 
minor defect in the hull material, you could end up with an explosive breach. 
 The safety valve in your hull would eliminate that problem.  As the sub 
rises, the expanding air inside the hull would escape via the valve.  You 
would have to allow sufficient time for decompression... again, just like a 
SCUBA equipped diver would opperating at depth.

Now, you could do all these things with a steel hull as well, but then you'd 
need a whole bunch more money, a huge crane to lift the thing, and a pretty 
heavy duty trailer to transport it.  

I too am just an amateur... perhaps not even that.  I'm not sure I'd have the 
drive to actually attempt to construct the device I'm envisioning... even if 
I did have the money.  But there is nothing wrong with amateurs.  Bushnell 
was an amateur.  So were the men who built Hunley and the Holland subs... at 
least by our standards.  Real engineers of the time said it couldn't be 
done... these "amateurs" took that as a challenge.  As an amateur, you are in 
good company indeed.


> Dale,
>
> Welcome to the list.
>
> Your idea of a plastic bag sounds ok. I think there is one problem. You
> need a regulating system that keeps your internal pressure a little higher
> than the pressure of the surrounding water. If not the hull will become
> smaller as you dive. Ballancing the sub will become verry difficult. You
> might end up sinking like a rock. A view months ago I suggested an
> inflatable (foldable) sub that has a constant internal pressure. I  think
> it has some special advantages:
>
> 1) You only have to deal with internal pressure. One could build a
> submarine balloon from non stretching materials such as used in modern
> sails. 2) If you want to build a "high speed" ambient sub you have to
> suffer from some physical discomfort and maybe even danger (going up to
> fast). An inflatable sub can have a constant (high) internal pressure. 3)
> You do not need a lot of air for pressure compensation.
>
> In an inflatable sub you pressurise the hull to a level depending on your
> maximum dive depth. After the dive you can let the air escape in a rate
> corresponding the decompression tables.
>
> Maybe i have overseen a lot of essential problems. I am just a daydreaming
> amateur.
>
> Greatings,
>
> Thijs Struijs

-- 
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!