In the same way SCUBA divers do? And if not, why not?
I assume they don’t, since the subs are pressurized. (unlike a wet suit)
Quite the contrary, subs are not pressurized, which is why they tend to implode if they descend too deeply (external pressure high, internal pressure = ~atmospheric pressure at ocean surface). If they were pressurized, then yes, the submariners would have excess nitrogen dissolving into the bloodstreams, and would need to undergo a decompression process to safely ascend.
Are subs de-pressurized? They shouldn’t pressurize much but they aren’t built like bathyspheres and should compress somewhat as they descend.
They also don’t descend to the kinds of depths that bathyspheres are designed for.
It’ll take a submariner who’s seen an onboard barometer to definitively answer this question. But given that the pressure hull must be constructed so as to avoid a buckling failure, it will be considerably more rigid than a comparably sized compressed-gas storage tank; in short, I don’t believe it will compress much at all, and therefore won’t raise the internal pressure much. Imagine a sub with a 30-foot (360-inch) inside width. Suppose for the sake of discussion that it gets squeezed by an improbable 1.25 inches, so the new width is 358.75 inches, or 99.653% of what it was. If the height and length of the sub are similarly reduced at depth, then the new internal volume is .99653[sup]3[/sup] of what it was at sea level, or 99%. This is the equivalent of taking an elevator from the 20th floor to ground floor; you probably won’t even need to clear your ears.
Let’s think about this.
In normal operation, the interior spaces of a submarine is sealed. It has an interior atmospheric pressure of (say) 1013 hPa (1 atm) on the surface and unbuttoned, and before submerging it sealed up that volume of atmosphere.
Now it’s submerged. How would the pressure change? If you say “the sea pressure is much higher at that depth”, how would that pressure change be transmitted through the sealed pressure vessel? It can’t be higher-pressure air, since the sub is surrounded by water. (Assuming you aren’t venting the buoyancy-control pressurized air tanks into the space. Which you wouldn’t do, because the hull is supposed to rigidly resist the surrounding pressure, and doesn’t need the reinforcement of higher air pressure inside.) Water? Not if you even intend to surface again. And if the hull is letting the pressure through, it’s because it has failed and deformed. This is called crush depth, and it’s fatal.
So. The sealed hull is enough to protect a standard-atmosphere interior from the high-pressure water outside it, and therefore the interior air pressure is pretty close to 1 atm.
Submarine’s hull’s do compress when submerged. Here is a site from the US Navy Museum.
For fun, submariners sometimes tie tight strings to different portions of the hull. As they dive/ascend, they can pluck the string and hear the change in pitch as the string becomes looser/tighter.
However, it is a small amount and not likely to lead to any substantial pressure increase.
Look at item 6 on page 5 of this PDF.
Interesting, if not surprising. Presumably the assumed compression of delta 1 inch for 198 inches of radius is based on real-world data. From the previous cite, it’s enough that subs have to built with floating decks to prevent deck buckling. In any case, the small change in volume produces negligible change in air pressure.
As an aside, the same thing in reverse happens in airliners, where pressurization imposes expansion stresses on the fuselage. The first passenger jet airliner, the de Havilland Comet, suffered a catastrophic series of structural failures and metal fatigue issues before these stresses were well understood, leading to much improved designs when the first Boeing 707 and DC-8 were introduced.
Also, I don’t know if they bother, but the submarine’s life support system could * detect when the pressure inside is above 1 atmosphere and compress the excess gas, storing it in a tank or pumping it overboard. This is kind of how on an airliner, the cabin is sealed and at higher altitudes it can theoretically remain pressurized, but if too much air leaks out, they can add more compressed air (from the aircraft’s engines) to keep it at the same pressure.
*though they deliberately let the cabin pressure drop below sea level pressure in an airliner to save money on how reinforced they have to make the hull.
It’s not random “leakage”. Air is intentionally released through an outlet valve and replenished from compressor bleed air in order to provide ventilation. Fun fact: back in the day, brown goop could be observed staining the outside fuselage area in the vicinity of the outlet port, and the valve itself would keep getting gummed up. The reason? Tobacco smoke, back when smoking was allowed!
LEELA
Depth at 45 hundred feet, 48 hundred, 50 hundred! 5000 feet!
FARNSWORTH
Dear Lord, that’s over 150 atmospheres of pressure.
FRY
How many atmospheres can this ship withstand?
FARNSWORTH
Well it’s a spaceship, so I’d say anywhere between zero and one.
The idea of floating decks was surprising. I would have thought that decks could have been useful as structural bracing to help reinforce the sub’s walls.
My guess (and someone may come along to correct me) is that floating decks also allow you to dampen sound. Any sounds that happen on the decks are not directly transmitted to the hull.
More or less. See this old post for more if curious: http://boards.straightdope.com/sdmb/showthread.php?p=7694926#post7694926
I suspect there’s two things going on. There are structural decks and sitting on top of them are sound damping floating decks that adapt to the shrinking and growing as the sub’s cross section changes.
Much like the solid subfloors and floating hardwood floors common in houses.
The stresses on the pressure hull are actually lower if it’s allowed to remain cylindrical as it compresses, without being forced out of round by any stiffeners that would induce local bending moments. Surface ships don’t have that concern. So, betcha those floating decks on a sub *are *the decks.
The OP answer is that interior pressure is held at or very close to sea level pressure no matter the depth, so there’s nothing to decompress from.
Submarines are pressurized to about normal 1 atmosphere, but if they are underwater for lengthy periods, the pressure inside builds up. And that can’t be corrected when you are trying to ‘run silent’. So that when they surface, they have to be careful about opening the hatch – it could be enough to actually blow a crewman out of the hatch. They need to just crack the hatch and let the over-pressurized air inside whistle out until it is equalized with the outside air pressure.
Based entirely on my reading of memoirs from WWII submariners – personally, I’ve never been on a submarine or even seen one.
Any idea why the pressure inside would increase over time? Something to do with crew respiration increasing ambient humidity, or exchanging O2 for CO2?
10-year sub vet here. I’m not a design engineer, though, so there may be some “boat knowledge” that is closer to an old wive’s tale than actual physics.
For example, the interior of subs are pressurized slightly above atmospheric, in order to slow the inrush of water in the event of a leak. Obviously, this is more effective at periscope depth than test depth. It’s not enough to need to decompress like a SCUBA diver, but definitely enough to “feel it” when a seal gets broken and the compartment decompresses.
I have done integrity leakage testing that required pressurizing manned compartments to a few pounds of pressure for a number of hours, and we did have a decompression period and a master dive officer as you would for diver ops.
We did this once. And only once. On the surface, the string was tied from the port to starboard hull at chest level. At depth, it touched the floor.
Can confirm.
Floating decks accomodate the changes to the hull, and keep the sub much quieter.
Large depth changes will cause some of the paneling on the mess deck or the wardroom to creak, lovingly referred to as “This Old Boat.”