Cavitation

Cavitation is the formation of vapor cavities where local pressure on a propeller blade drops below the water’s vapor pressure, then their violent collapse downstream. It erodes blade metal, raises radiated noise and hull pressure pulses, and at the extreme breaks down thrust. The governing parameter is the cavitation number $\sigma = (p_0 - p_v) / (\tfrac12 \rho V^2)$. Designers stay inside the section’s cavitation bucket by limiting blade loading and area (high expanded area ratio).