Cavitation
Cavitation is the formation of vapour-filled bubbles in water where the local pressure drops below the vapour pressure — typically on the low-pressure side of a fast-moving propeller blade — and their violent collapse as they move into higher-pressure regions. The collapsing bubbles produce noise, vibration and, over time, pitting erosion of the blade metal.
Some cavitation is unavoidable on hard-working propellers, but excessive cavitation robs the propeller of thrust and efficiency, adds vibration felt through the hull, and damages the blades and rudder. It is controlled through propeller design, blade condition and avoiding overloading, and blade erosion is checked at dry-docking.
Cavitation quietly wastes propulsion efficiency, shakes the after end of the ship, and erodes the propeller and rudder — all of which cost fuel, comfort and maintenance money. Keeping it within limits through design and blade condition is a real performance and maintenance concern.
How does cavitation damage a propeller?
The vapour bubbles collapse against the blade surface with tiny but intense pressure spikes; repeated over millions of cycles this pits and erodes the metal, especially near the blade tips and trailing edges.