Explosions underwater

What happens when a bomb goes off underwater? An explosion in air will force air outwards, causing a pressure wave away from the bomb which will spread and disperse. Since, as I understand it, water cannot be compressed, such a pressure wave should be impossible.

My guess is that the bomb forces water outwards, causing a ripple wave and creating a vacuum bubble around the bomb, which, moments later, collapses as the water is sucked back into the vacuum. Is that somewhere near correct?

That’s how they show it in the movies when there’s an underwater explosion.

This isn’t strictly true; water does not compress easily, but it certainly can be compressed, so can solids (that’s how sound is able to travel through liquids/solids.

Hmmm… didn’t think about that. So when Cecil said that liquids are noncompressible, he was fibbing it? Or simplifying?

Simplifying, I should think; everything is compressible - what would happen if the liquid was poured into a neutron star? - it’s just that in everyday terms, the compressibility of liquids is too small a factor to worry about, but the forces generated in explosions are quite significantly higher than ‘everyday’ ones.

You may just be missing the point of what Cecil said. In an easily compressible material, you can get compression waves, which have smooth pressure transitions. If enough force is applied to the material, you can get shock waves, which have sharper pressure transitions.

In liquids, which are much less compressible than gasses, it’s easier to produce a shock wave. That was what Cecil was talking about. The bullet produces a shock wave with rapid pressure transitions that disrupt organs.

To be strictly correct, maybe Cecil should have said “relatively incompressible”, but that would have clouded the point that was being made.

Anyway, back to the OP.

What exactly do you think an explosion is?

You take a small volume of some explosive compound, detonate (ignite) it, and it rapidly undergoes a chemical reaction that turns it into a large volume of hot gas.

That’s what you’re seeing in films of underwater explosions. The expanding gas displaces water, and causes a shock wave through the water.

After a time, the underwater ball of gas begins to cool and lose pressure. The water rushes back to where it was.

I would have thunk the gas’ sudden urge to travel to the surface to be more of an issue than it compacting due to falling temperature.

No, at first it’s big and very hot. Then it rapidly cools and shrinks. Then you’re left with gas at (roughly) water temperature which rises to the surface.

Of course, you’d see different effects if the explosion was close to the surface and the initial gas bubble breached the surface.

From my classes in college on how to sink ships, a deep explosion below a ship will have the described overpressure “bubble” initially, then collapse again, all the while it is rising. After the collapse, it rebounds to another overpressure bubble (lower pressure, but at a lesser depth and therefore lesser water pressure), and continues this cycle for a short time. If the “bubble” comes up under the keel of the ship, the keel has no water there to support the ship, and it can break.

Any Navy-types out there to corroborate this?

Your version is better than mine, UncleBill. (IANA"NT").

As others have pointed out, water is compressible, but the effect is almost always negligible. However, some of the confusion here seems to be in what incompressible means. Incompressible simply means that the volume does not change when pressure changes. You can still have pressure gradients, but they do not affect the density of the fluid.

Affirmative, UncleBill. The MK48 ad cap torpedo is designed to do this, as most heavyweight torpedos used by the world’s navies.

Here’s a few links…

Test firing

BF, ex-boat-sailor

I remember, back in my road-building days, an old-timer saying that “gunpowder blows up, but dynamite blows down.” It took a long time for me to figure out what he was trying to say…

Gunpowder explodes more slowly than dynamite. Gunpowder, on a rock face, will just shoot outward, pushing the air out of the way, and hardly scratching the rock. But dynamite explodes so fast that it compresses the air around it into a kind of rigid shield; the force of the explosion is unable to escape that way, and causes much more damage to the rock. It’s a little like a shaped charge, with the air itself serving as the shaping shield.


Back when the US was doing nuke tests, they detonated a couple of nukes underwater. The first time they did this, the blast was stronger than they predicted and they cancelled the next test. (It wasn’t until the 60s, IIRC, that they finally did the second test.) You can see clips of the tests in the movie: Trinity and Beyond: The Atomic Bomb Movie.