What is cavitation in a marine propeller and how can it be mitigated?

• A. Water leakage due to hull rivets.
• B. Vapor bubble formation due to low pressure causing damage; mitigated by adequate NPSH, proper blade design, and avoiding high loaded conditions.
• C. Formation of electrostatic charge in propeller bearings.
• D. Increase in hull draft when propeller cavitates.

Answer: (B)

Cavitation on a marine propeller happens when the pressure on the blade surface, especially on the suction side, drops below the water’s vapor pressure. Tiny vapor bubbles form in these low-pressure regions and then collapse violently as they move into higher-pressure areas. This collapsing action causes shock loads, erodes material, and leads to noise, vibration, and loss of thrust and efficiency. The best way to prevent it is to keep the pressure from dipping too low: provide adequate Net Positive Suction Head (NPSH) at the propeller intake, design the blades to maintain favorable pressure distribution (appropriate pitch, camber, and hub/blade geometry), and avoid operating under high-load conditions that push the blade into deep low-pressure regions.

The other options describe phenomena that aren’t about cavitation—water leaks from hull rivets, electrostatic charge in bearings, or hull draft changes tied to cavitation don’t explain the bubble formation and collapse that cavitation entails.