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In some submarines, divers can exit and enter through a moon pool, an opening in the bottom of the submarine.

We can clearly see ocean's water flowing in the moon pool, but it won't come inside.

Why so? What is stopping the water here from coming inside?

a drawing of a submarine with a moon pool


marked as duplicate by Jon Custer, David Hammen, Yashas, John Rennie, heather Jul 29 '17 at 14:35

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    $\begingroup$ You can do this with a cup & a bucket in your house too. $\endgroup$ – Kyle Kanos Jul 28 '17 at 11:04
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    $\begingroup$ I am not sure I have ever seen this in a submarine though. If you are more then 10 meters below the surface the water will start to come inside by compressing the air inside...Sounds a little bit risky. Where have you seen this design? $\endgroup$ – valerio Jul 28 '17 at 11:50
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    $\begingroup$ There is the equivalent of an air lock called an escape trunk. en.wikipedia.org/wiki/Escape_trunk $\endgroup$ – Farcher Jul 28 '17 at 12:26
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    $\begingroup$ @valerio92: Submarines are a bit risky. And yes, the water will pressurize the air, whether that's at 1, 10 or 100 meters depth. So? $\endgroup$ – MSalters Jul 28 '17 at 15:08
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    $\begingroup$ @valerio92, the OP is asking about a moon pool (also known as a "wet porch" or a "wet room") It's a feature that has sometimes been built-in to one-of-a-kind research and exploration subs. Divers sit in the "wet" room, breathing air that is at the same pressure as the water outside, and they can enter and leave the sub through an open hole in the floor. If it's built for saturation diving, then they can close the hatch and return to the surface while maintaining the same pressure. $\endgroup$ – Solomon Slow Jul 28 '17 at 17:29

The air is stopping the water from coming inside.

For the water to enter a cavity already filled with another fluid, it has to either displace or compress this fluid. The shape of the container prevents the air from escaping and the water can't rush in if the air inside is at the same pressure as the water outside, because then the net force on the interface is zero.

  • $\begingroup$ If pressure is equalized, then there is no limit to depth, and they would need decompression chambers going up, and they would have to store all the compressed air to create the air pressure going down. Are you assume interface pressure is zero? $\endgroup$ – Sunnyskyguy EE75 Jul 29 '17 at 2:42
  • $\begingroup$ @TonyStewart As far as I understend the sub is a big decompression chamber as the air pressure drops slowly while it emerges. $\endgroup$ – Maciej Piechotka Jul 29 '17 at 4:18
  • $\begingroup$ @TonyStewart, another possibility has been mentioned in Paparazzi's answer: the chamber where the moon pool is might be the only high pressurized part of the submarine. $\endgroup$ – stafusa Jul 29 '17 at 8:02

Air pressure is what holds back the water. If the air pressure is higher than the water pressure the water cannot enter. Typically it will just be a pressurized chamber - not the whole sub.

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    $\begingroup$ No harm in expanding that to a more detailed answer, so far imo, it's just a comment. There is a bigger audience than just the OP, and I was told this myself recently, (and it's a perfectly valid point to make), for one of my own answers :) thus this comment . $\endgroup$ – user163104 Jul 28 '17 at 11:43
  • $\begingroup$ -1 for lack of detail. Happy to upvote if you add more detail. $\endgroup$ – Chappo Jul 28 '17 at 12:45
  • $\begingroup$ en.wikipedia.org/wiki/Airlock $\endgroup$ – Hot Licks Jul 28 '17 at 21:52

As someone else has said, the air pressure is what prevents the water from entering the submarine. But let's do some calculations.

We will assume that the robustness of the hull is not a problem, i.e. the hull of the submarine can withstand infinite pressure$^1$.

The pressure inside is initially $P_{atm}=1$ atm. The pressure outside (water pressure) can be computed from the hydrostatic equation:

$$P_{out}(z) = P_{atm} + \rho g z$$

Let's assume that the air inside can be approximated by an ideal gas; we will then have

$$P_{in} = nRT/V$$

whre $V$ is the volume of air inside the moon pool chamber. We will also assume that the internal temperature $T$ is kept constant by a very efficient air conditioning system.

With the submarine design you show, mechanical equilibrium requires that


from which we obtain

$$V(z) = \frac{nRT}{P_{atm}+\rho g z}$$

The initial volume is


from which

$$V(z) = V^* \cdot \left(1+\frac{\rho g z}{P_{atm}}\right)^{-1}$$

This equations tells us how the volume of air inside the chamber decreases with depth.

For water, we have $\rho=10^3$ kg/m$^3$ and this value can be considered independent from $z$ since water is almost incompressible. Atmospheric pressure is $P_{atm}=10^5$ Pa. We round up $g$ to $10$ m/s$^2$. Therefore we obtain

$$V(z) \simeq V^* \cdot \left(1+\frac{z}{10 \text m}\right)^{-1}$$

At $10$ meters deep, $V \simeq V^*/2$.

At $20$ meters deep, $V \simeq V^*/3$...

You can see that very soon the room will be completely filled with water. In order to prevent this, it must be pressurized, and this requires the use of an airlock. But even like this, the pressure in the chamber cannot be increased too much, otherwise those who enter will risk oxygen intoxication.

$1.$ This is not such a bad approximation as it seems. Modern nuclear submarines can go as deep as $730$ m before the hull collapses, withstanding a pressure of $74$ atmospheres. A submarine with a hole in it will be filled in water well before the hull collapses (see above discussion).

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    $\begingroup$ A sub has facilities to increase air pressure. $\endgroup$ – paparazzo Jul 28 '17 at 17:49
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    $\begingroup$ @Paparazzi Sure, but there is still a limit since the human body cannot withstand arbitrary pressures. $\endgroup$ – valerio Jul 28 '17 at 17:52
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    $\begingroup$ Then they could not stand the water pressure either so that is mute? $\endgroup$ – paparazzo Jul 28 '17 at 17:54
  • $\begingroup$ @Paparazzi I analyzed the situation of a submarine with an hole in it. Then, we can discuss as much as we want about submarines in real life. $\endgroup$ – valerio Jul 28 '17 at 18:00
  • $\begingroup$ Real live human body is in scope but pressurize the sub is out of scope? $\endgroup$ – paparazzo Jul 28 '17 at 18:02

Water'd fly out of your cup if there wasn't atmospheric air pressure on it. That's actually how straws work; we create a gentle vacuum with our mouths and the water flow upwards!

enter image description here

Under the ocean, the water pressure's a lot higher, so submarines have airlocks. Once you go into the airlock:

  1. The doors on both sides are sealed.

  2. The airlock chamber starts changing pressure to match the pressure outside of the door that you want to go through.

  3. Once the correct pressure is reached, you can go through the door.

Physically, the water's obeying Newton's first law of motion:

In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

The force from pressure is the main concern because gravity doesn't have much of an effect when the port hole is facing downward.

enter image description here

If the port hole were vertical, then, yeah, water'd come flooding in because the net force would be higher at the bottom, while air'd flow out of the top. And that'd be due to gravity breaking the balance of forces, such that Newton's first law would no longer apply.


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