Cavitation occurs when the local water pressure around the propeller blades falls below the vapour pressure of water, causing vapour bubbles to form. These bubbles, when carried into regions of higher pressure, collapse violently against the blade surface.

Progressive nature of damage:

1. Formation of vapour bubbles: Low-pressure zones, usually on the back (suction) side of the blade, form vapour cavities.
2. Collapse of bubbles: As the bubbles move to higher-pressure regions, they implode.
3. Shock waves and micro-jets: Each implosion generates high-energy shock waves and micro-jets that strike the blade.
4. Surface pitting: Repeated bubble collapses remove tiny particles of metal, producing small pits.
5. Material loss: Continuous pitting deepens the damage, causing roughness and erosion of the blade surface.
6. Propeller imbalance: Progressive material loss changes blade shape and weight distribution, causing vibration and reduced efficiency.

To minimise cavitation, propeller design focuses on maintaining sufficiently high blade surface pressures and ensuring smooth water flow. Key design considerations include:

• Higher blade area ratio (BAR): Larger blade area reduces loading per unit area, preventing extreme pressure drop.
• Optimised pitch and camber: Ensures smooth acceleration of water across the blade, avoiding sudden low-pressure regions.
• Well-shaped and thinner blade sections: Streamlined profiles reduce turbulence and local suction peaks.
• Improved blade tip design: Rounded or optimised tip geometry lowers tip vortices, which are common cavitation zones.
• Reduced propeller RPM: Lower rotational speed reduces pressure fluctuations.
• Adequate propeller submergence: Keeping the propeller deep enough prevents suction peaks near the surface.

At least four major negative effects include:

1. Erosion of blade surfaces → leads to material loss and surface roughness.
2. Reduced propeller efficiency → disturbed flow causes reduction in thrust.
3. Vibration and noise → collapsing bubbles and imbalance generate noise and structural vibration.
4. Damage to propeller and shafting → prolonged cavitation may lead to blade cracks, imbalance, and stress on shaft bearings.