Reduction of propeller cavitation induced hull exciting pressure by a reflected wave from air-bubble layer

• Propeller cavity induced hull surface pressure was reduced by phase-reversal reflection from artificially injected air-bubbles.
• This enables a conventional air-injection system to be in a simplified design.
• Full-scale measurement proved the vibration reduction up to 75%.
• Physical proofs for a pressure reduction are theoretically described.

Cavitation generated by marine propellers is one of the main sources of vibration and noise in commercial ships. By taking advantage of the compressibility of air, there have been many attempts to form an air-bubble layer underneath the stern-hull surface above the propeller, and consequently to isolate the cavity induced pressure wave across the layer. However, this approach is not widely used because the cost to deliver a sufficiently large amount of air for isolation is prohibitive. In this study, full-scale ship measurements proved that the pressure amplitude can be significantly reduced outside an air-bubble layer where the isolation effect has not been applied. A hull-vibration reduction of approximately 75% was achieved. Only a small amount of air is needed to reduce the cavitation-induced pressure amplitude, so the system to produce the air layer becomes simple. The purpose of this study was to provide physical evidence that this phenomenon was achieved. The solution of acoustic scattering from a bubble was approximated to show that the main reason for the reduced pressure outside the layer is phase reversal reflection, which provokes destructive interference.