Cavitation

Cavitation is like when you're blowing bubbles in the bath tub. The bubbles form when the air you blow in the water creates tiny air pockets (cavities). In a similar way, when a boat moves through the water, the water around the propeller is forced out of the way and creates cavities. When the water comes back together to fill in the empty space, it creates an area of low pressure that can cause the cavities to collapse. This sudden collapse creates a loud noise, and the tiny bits of air cause tiny bubbles to form in the water. The bubbles then implode, creating a shock wave that can damage the propeller. That's cavitation.