Which condition helps prevent cavitation in a marine propeller?

• A. Higher seawater salinity.
• B. Adequate Net Positive Suction Head (NPSH).
• C. Increased hull draft.
• D. Lower propeller speed.

Answer: (B)

Cavitation happens when the pressure at the propeller’s suction side drops below the vapor pressure of seawater, causing tiny vapor bubbles to form and then collapse as they move into higher-pressure areas. The best way to prevent this is to ensure there is enough pressure head on the suction side so the water never reaches its vapor pressure. That direct control is described by having adequate Net Positive Suction Head (NPSH). When NPSH is sufficient, the suction pressure remains high enough across operating conditions to keep cavitation from starting, preserving blade integrity and propulsive efficiency.

Other factors can influence cavitation but are not as reliable or direct a preventive measure. For example, deeper hull submergence (greater draft) can raisethe ambient pressure on the suction side, and changes in salinity or speed can affect cavitation risk somewhat, but they’re not the primary, guaranteed safeguard. Lower propeller speed may reduce the tendency to cavitate by reducing flow velocity and associated pressure drops, yet it also reduces propulsion. The principle most directly linked to preventing cavitation is maintaining adequate NPSH.