Where do the underwater bubbles come from when you shoot a bullet into water, or from a spinning propeller?

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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?

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9 Answers

Anonymous 0 Comments

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.

Anonymous 0 Comments

Not sure about submarines but most sterndrive and outboards (that I have seen/used) have thru-hub exhaust which would account for the bubbles you would see behind the prop.

Anonymous 0 Comments

Ryan wasn’t all that accurate there, at least not for the soldiers deeper underwater. Fast-moving rifle bullets, especially at that very shallow angle, don’t have much penetration in water. Here’s Mythbusters doing it with a [.50 BMG](https://youtu.be/yvSTuLIjRm8), didn’t go far.

Otherwise, fast-moving things in water (or water moving fast through things) can cause cavitation. The bullet leaves a low pressure area around it, and that pressure drops below the vapor pressure of the water, which means the water basically boils and you’re looking at steam, not air.

This has been used on purpose to make torpedoes faster. They’re not travelling through high-friction water, they’re travelling through a low-friction tunnel of steam generated by cavitation. This has also ruined a dam, as they did an emergency flood of the tunnels, water started cavitating at imperfections in the tunnels, and eventually started carving out livingroom-sized rocks.

Anonymous 0 Comments

>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.

That scene is not realistic because the water stops the bullet quite quickly. The bullet trace and blood in that scene is CGI that is not realistic.

The bullets will create areas with no water behind them like that but not as long as distance. The bubble behind the will collapse a lot faster than in the move

You can look at https://www.youtube.com/watch?v=cp5gdUHFGIQ and https://www.youtube.com/watch?v=OubvTOHWTms for real guns fired underwater filmed with a high-speed camera. https://www.youtube.com/watch?v=R3rRKtwjrNc with gund fired into the water.

One thing to consider with those videos is that the gun is in or close to the water so the gases released from the propellant of the gun are also involved in what you see. In the move, the defender on Omaha Beach is not in the water with the attacker but onshore so only the bullet will be relevant

You can see that scene and others in the movie being discussed https://youtu.be/AWOm1wxOjmQ?t=124 with someone involved in the movie. The bullet in the water is CGI there for the emotional impact. They knew that is not how bullets work in water but did it anyway. It is a great way to show the danger and the incoming bullets to the audience because you do not see them the same way in the air.

I would also assume that the number of bubbles you see from submarine propellers in movies is fake. The propeller can cause cavitation like that. The problem is that when the bubble collapse there is a lot of noise generated. So submarines where being undetected is one of the main ways you survive will be the vessels where you take the most care to avoid cavitation.

So they have moved something that occurs to a lot higher degree on surface vessels to submarines. Bubbles in the water show that something happen so good in a movie bit terrible for a real submarine.

Anonymous 0 Comments

For one, films are not necessarily accurate. With a bullet being shot into the water though I do have an explanation. A fast moving object pushes air out of the way and leaves a low pressure zone behind it. Air wants to move to these low pressure zones. When it is shot into water you can actually see this low pressure zone of air behind the bullet before it fills back up with water.

With a propeller that is already submerged I only have an educated guess. There are air bubbles in water. The propeller pushes the water out of the way and since air moves quicker than water, the bubbles of air fill that low pressure zone before water can.

Anonymous 0 Comments

The bubbles contain steam or water vapor. It is caused by *cavitation* which is when the local pressure in the water is so low that the water basically boils. With a propeller you are creating high and low pressure areas behind and in front of it. This pressure difference provides the thrust which *propels* the boat. If the low pressure falls below the vapor pressure for the water temperature, it boils and forms a bubble. This bubble will sustain until it moves into a higher pressure area where it will collapse. If it collapses near the propeller, the shock-wave can actually melt a small bit of metal and over time this can result in holes.

With bullets there is a low pressure area right behind it which causes similar bubbles.

Anonymous 0 Comments

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Anonymous 0 Comments

If an object enters the water, some air is dragged behind it ad the object breaks the surface of the water. There’s also a thing called cavitation that affects underwater propellers (and probably bullets as well): a fast moving object can “rip” the water apart, creating a bubble (cavity, hence the name) filled not with air but with vapor from surrounding water. These bubbles collapse quickly but they are still visible.

However, don’t look into Hollywood scenes too much, most of the bubbles are just added for visuals. Actual propellers are made to reduce cavitation since it can damage the blades. Maybe some submarines have some sort of exhaust near the propeller but I won’t make any guesses here.

Anonymous 0 Comments

This is known as cavitation. As a bullet travels through water, or a blade spins quickly through water, they create pockets of low pressure behind them. If the pressure is low enough then the water turns from liquid to gas, and this is the source of the bubbles. The bubbles are composed of water vapor.