Hull pressure energy is generally contained in harmonics of the blade passing frequency (Figure 4) and, in this case, the amplitudes of the pressure pulse at the second and third harmonic of the blade passing frequency were of a similar value as the blade rate component. This is relevant because natural frequencies of local structures can be typically found at these frequencies. This distribution of pressure energy suggests a violent cavitation collapse and is not uncommon for vessels where propeller excitation is high. Observations of propeller cavitation were performed through a borescope that was passed through an M20 hole in the ship’s shell plating just forward of the propeller. These holes were drilled while the ship was underway and did not disrupt the ship’s schedule. Two images of a propeller blade passing through the twelve o’clock position, Figure 5, showed cavitation phenomena which normally lead to significant pressure pulses and which are indicative of a severely retarded propeller inflow. It is likely that the severely retarded propeller inflow was a result of the large amount of stagnant flow that was injected into the ship’s boundary layer just upstream of the propeller by the steam turbine cooling water outlet. (rpm) pressure energy (kPa/Hz) Fig. 3 Hull pressure measured during one blade passage Fig. 4 Frequency distribution of pressure energy Disintegrated cavity from previous blade interacts with this blade suggesting very low flow velocities in the 12 o'clock position. propeller blade with sheet cavitation (white) flow direction Fig 5 Two images of a propeller blade passing through the wake peak showing various cavitation phenomena Hull Pressure at 82.5 rpm 1 pr opeler r evolut ion 1 blade passage 2nd, 3rd and 4th collapse possibly associated with tip vortex activity Pressure pulse from primary collapse of sheet cavity 283
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