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Sonotube for submarine body

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klugesmith

Tue Sept 13 2011, 07:58PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

engineer 1: 10 meters deep is what? double atmosperic or 30 psi? i think.
engineer 2: Ok, some numbers. ~3 atmospheres pressure, 50psi ish, plus 'safety margin', X2? X3?...X1.5?

Not so fast.
Water pressure at depth of 10 meters is, uh, 10^4 kgf/m^2 * 9.8 N/kgf = 98 kPa.
That's a bit less than 1 atmosphere. Maybe 1 full atmosphere in sea water.
The pressure hull could be stress and leak tested dry, by evacuating it.
I can attest that a polycarbonate bell jar with OD of about 8 inches and thickness of about 1/8 inch does not collapse under vacuum, but the indentation of its once-flat top can be scary at first.

The strength of the enclosure would not be an issue if its internal pressure is maintained at about the same as the external pressure. Could use a compressed air hose in the tether, or an on-board reservoir of compressed gas, with a simple and safe overpressure bleed valve and/or a more sophisticated regulator.

Having the enclosure pressurized could facilitate a buoyancy control system using water as ballast.

Ash Small

Tue Sept 13 2011, 09:31PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Klugesmith wrote ...

engineer 1: 10 meters deep is what? double atmosperic or 30 psi? i think.
engineer 2: Ok, some numbers. ~3 atmospheres pressure, 50psi ish, plus 'safety margin', X2? X3?...X1.5?

Not so fast.
Water pressure at depth of 10 meters is, uh, 10^4 kgf/m^2 * 9.8 N/kgf = 98 kPa.
That's a bit less than 1 atmosphere. Maybe 1 full atmosphere in sea water.

You are correct. I shouldn't post after I've been at the Merlot.

I'ts a long time since I did any diving. I had the number 15 in my head associated with one atmosphere.

It's ~15psi, and it increases by another atmosphere every 30 feet, as a rough guide, from the point of view of scuba divers, anyway.

Thanks for correcting me. (I've had a very stressful week)

Patrick

Wed Sept 14 2011, 01:28AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

I had the number 15 in my head associated with one atmosphere.

It's ~15psi, and it increases by another atmosphere every 30 feet, as a rough guide, from the point of view of scuba divers, anyway.

dont feel bad Ash, I was thinking the same thing. so its 2 Atm - 1 Atm = 1 atm increase of external pressure.

lightlinked

Thu Sept 15 2011, 09:44AM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
Location:
Posts: 115

if you build a jig that can spin the tube, such as a large metal lathe covered with painters plastic to protect it from epoxy, you can wrap it with fiberglass tape or since 30" fits within a yard width of cloth, a 1 yard roll or bolt or fiberglass. Mix your epoxy, test a small bit first to make sure it is curing right. then mix your large about of epoxy Apply your epoxy, you can use a bondo squeegee or a chip brush. If you use a lathe it should have plenty of torque, so you can take plastic wrap, used for packing stuff, you probably will have some around the shop area since it is so common for boxing junk up. hold the shrink wrap roll so you can control how tight the wrap is. wrap the tube and the plastic will do the same thing as a vac bag. 10" x 30" will probably fit in a kiln which you can borrow from the art dept or you can build a foam box with lightbulbs. of course oven curing is optional but i would recommend it if your epoxy recommends it. BTW the epoxy might not soak through the entire tube which would be a problem if the cardboard absorbs water. you could begin with a thin pvc tube instead of sonotube or build a form and slip off the fiberglass at the end.

If you are wondering about strength look at examples of fiberglass pressure vessels like for well pressure regulators and air tanks. also remember that you will be pushing in as a compression vessel and not out like a compressed tank, which a cylinder will be stronger at. if you over think it and make it too complicated you will never actually build it. plus at 10 meters you have 30 psi, from us plastics you can get 10" scheduled 80 pvc for $23.65 a foot. that is rated for 230psi pushing out, in compression it should be even stronger. $60 for pvc is going to be cheaper than a sonotube, fiberglass and epoxy. you could even precharge the vessel with maybe something like 15 psi (about +1 atm) with a bike pump or compressor by putting a Schroeder valve on the thingy

cduma

Fri Sept 16 2011, 11:16AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

It will need ballast. I am a scuba diver and if I make my self neutrally boyant at the surface there is no way that I could swim down 30' and stay there. You will need some sort of compressed air to dump a ballast tank

Ash Small

Fri Sept 16 2011, 12:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

cduma wrote ...

It will need ballast. I am a scuba diver and if I make my self neutrally boyant at the surface there is no way that I could swim down 30' and stay there. You will need some sort of compressed air to dump a ballast tank

I did mention this above. I assume you can get those small CO2 cartridges locally. They are about 3" long or so.

This is the type of system that the Navy use. (except they use re-chargable tanks, which is another option).

(EDIT: In fact, it would be best to follow conventional practice as much as possible, single propeller, conventional rudders, etc, maybe bow thrusters, if extra manouverability is required. (although contra-rotating twin props are definitely worth considering))

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