system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do ship propellers create all those underwater bubbles, even when fully submerged?
gpt
There is a lot of various gases dissolved in water (well, not *all* water, but certainly most).

When the propeller spins it creates areas of high pressure on one side, and low pressure on the other. Where the two meet you often get very low pressures - sometimes so low that the water actually gets pulled apart leaving a vacuum, called cavitation.

That's *not* what you're seeing streaming off the ends of the blades - the vacuum bubbles collapse very quickly, but sometimes gases dissolved in the water come out, and form bubbles. They may eventually dissolve again but not immediately.

The other alternative is that some outboard motors pump the exhaust through the prop hub, and you're seeing that.