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

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.