Cavitation inception

Cavitation inception is the condition at which vapor cavities first form on a propeller blade or foil, occurring when the local pressure drops to the fluid vapor pressure p_v. It is set by the local cavitation number sigma = (p_0 - p_v) / (0.5 rho V^2) falling to the section’s inception value, sigma_i, fixed by the minimum pressure coefficient -C_p,min over the surface, so inception begins near sigma = -C_p,min. Higher speed, higher loading, and a steeper angle of attack all lower the local pressure and bring inception on earlier. Designers keep the operating sigma above the inception margin at the blade tip and leading edge, where suction peaks are sharpest, to avoid erosion, noise, and thrust breakdown.