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

Question

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

Answer 1

>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.