Cavitation

A ship's propeller spinning fast enough can, under the right conditions, cause the surrounding water to boil at room temperature, not because of heat, but because of a sudden drop in pressure. This phenomenon, called cavitation, occurs when a propeller blade moves through water fast enough that the pressure on its trailing surface drops below the water's vapor pressure at that temperature. When that happens, the water doesn't need added heat to turn to vapor; the drop in pressure alone is enough to cause tiny pockets of water vapor, called cavities, to form almost instantaneously. These vapor cavities are inherently unstable. As the blade continues moving and the surrounding pressure rises again, the cavities collapse violently, and that collapse is where cavitation does its damage. Each collapsing cavity generates a tiny, localized shockwave and a jet of water moving at extremely high speed, concentrated into an area smaller than a pinhead. A single collapse causes negligible harm, but a propeller can generate millions of these collapsing cavities per second under sustained operation, and the cumulative effect, striking the same small area of metal repeatedly over time, gradually erodes even hardened steel, leaving a pitted, roughened surface. Cavitation also produces a distinctive crackling or hissing sound underwater, caused by the rapid succession of tiny implosions, which is often how engineers first detect that it's occurring before any physical damage becomes visible. Modern propeller designs attempt to minimize cavitation by carefully shaping the blade to avoid the sharp pressure drops that trigger it in the first place.