Propeller cavitation occurs when the pressure on a propeller blade’s suction side drops below the vapor pressure of the surrounding water, causing water to vaporize into bubbles — which then collapse violently as pressure recovers moments later.
The Damage Mechanism
When a cavitation bubble collapses (implodes), it generates a localized, intense pressure shockwave directly against the blade surface. Repeated over millions of rotation cycles, these micro-implosions progressively erode the blade material — visible during drydock inspection as pitted, roughened blade surfaces, particularly near the blade tips and leading edges where cavitation is most severe.
Recognizable Signs
- Vibration originating from the propeller/stern area
- Unusual noise from the propeller
- Reduced thrust efficiency despite increased RPM — the engine is working harder but the ship isn’t moving proportionally faster
- Visible blade erosion confirmed during drydock inspection
Common Causes
- Excessive RPM for the current draft condition — particularly relevant in light/ballast condition, where the propeller may be operating closer to the surface than its design intends
- Operating outside designed parameters generally, whether from damage, incorrect loading, or fouling changing the propeller’s effective performance
Prevention
Operating within the propeller’s designed RPM range for the vessel’s actual draft condition, and correct blade area ratio selection during original propeller design, are the primary preventive measures — cavitation is a genuine design and operational consideration, not something eliminated entirely but managed within acceptable limits.
Exam Relevance
Cavitation causes, damage mechanism, and prevention are core Ship Knowledge and Marine Engineering topics in deck and engineering officer competency exams.