In Saving Private Ryan there are scenes where the soldiers are underwater and bullets are shown going into the water with bubbles trailing behind them.
In films with submarines, the propellers often have bubbles coming out.
Where are these bubbles, or the air inside them, coming from?
In: 5
First rule of things you see in TV and movies: don’t think what you’re watching is real, because nine times out of ten it isn’t.
Let’s start with the prop bubbles. Fast moving propellors (called *screws* on watercraft like ships and submarines) *can* generate small little bubbles. It’s called *cavitation*, which is the creation of a low-pressure bubble due to the fast moving screw through the water. The bubbles are essentially water vapor, as water boils at a *much* lower temperature at lower pressures. The bubbles collapse back into liquid water pretty quickly, though, so they don’t stick around too long. The problem with cavitation is that it can damage the screw and other surrounding equipment, and it’s noisy as hell due to being a unnatural sound in the water. As a result, military vessels such as ships and, especially, submarines have their screws designed to prevent cavitation as much as possible because replacing screws it a stupidly expensive process (they have to take the ship/boat completely out of the water to do it) and because the less noise generated in the water, the better (especially true with submarines). The sound of a cavitating screw is pretty easy for a sonar technician to pick out of the water at amazingly high ranges, and it immediately tells the sonar techincian that a man-made object under power is out there, somewhere. This is why submarines aren’t all that fast on or near the surface, too, because the screw speed is limited by the surrounding water pressure for preventing cavitation (meaning the deeper they are, the faster they can go without risk of cavitating).
Now on to bullets in the water. Bullets will have much the same cavitation effect as screw will, but only for about a foot into the water. Water slows down bullets *really* quickly, so much that if you’re more than a couple of feet under the water it doesn’t matter what the enemy is shooting at you with; you’re going to be pretty safe. Most anything bigger than a .22lr is going to fracture from the water pressure resisting it’s velocity, too, which make the fragments go everywhere and not in a straight line and with very little ability to cause harm to an unprotected human. There are plenty of videos online of guns being fired into water (and even with the gun being fired underwater) and filmed with a high-speed camera where, yes, there’s some cavitation early on, but the bullet slows down enough within the first half-foot or so where cavitation is no longer possible. More streamlined bullets (and ones spinning faster) go a little further, but after about a foot in there won’t be any more bubbles created by the effect.
Movies are notorious for misrepresenting things like this for dramatic effect. The put fake bubbles behind moving sumbarines (either by putting an air hose in a model or just using CGI) and ships because you get a better sense of the boat/ship moving. They show bullet whizzing through the water (again, usually CGI, although there are some practical effects that can use used) to add tension and drama to the scene. Hell, sometimes the effects they add just make things look cool, like the F-14s in Top Gun apparently drawing contrails behind them at low altitude when in reality the bird is dumping fuel to create the effect (looks nice, though).
And don’t get me started on the various gun myth perpetuated by Hollywood, but that’s an entirely different topic, altogether.
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