Vapor pressure (water)

The vapor pressure of water is the pressure at which water boils at a given temperature, the threshold that governs cavitation. At 15 degrees Celsius it is about 1.7 kPa and at 20 degrees Celsius about 2.34 kPa, rising steeply with temperature. Cavitation begins where the local absolute pressure, lowered by flow acceleration over a propeller blade or hull, falls to the vapor pressure, captured by the cavitation number sigma = (p - p_v) / (0.5 rho V^2). Designers keep blade-section pressures above vapor pressure across the operating range to avoid erosion, thrust breakdown, and the broadband noise that cavitation produces.