Tank full: a large number of modes between 25 and 80Hz Tank empty: a large number of modes between 40 and 80Hz Harmonic excitation at 20 and 30Hz Strong response at natural frequencies at 30 Hz and above 40 Hz Shaft speed (rpm) Vibration energy (mm/s/Hz) Fig. 8 Stability diagram of a tank bulkhead. The analysis results shown are for an empty and a full tank Fig. 9 Frequency distribution of vibration energy of a tank bulkhead during normal operating. A waterfall plot showing the frequency distribution of vibration energy of a bulkhead of a filled tank is shown in Figure 9 with the shaft rate and propeller blade rates shown as red lines. There is no substantial vibration of the bulkhead up to about 25 Hz and two areas of high energy are seen around 30 Hz and 40 Hz. These areas correspond to frequencies that the aft peak is passing energy through as shown in Figure 6. The highest peak in the waterfall plot of the tank bulkhead (Figure 9) is present at 73 RPM at 29 Hz. At this frequency the hull is excited by the fourth blade order and the aft peak tank has a mode as shown by both the modal analysis (Figure 6) and the waterfall plot (Figure 7). Therefore, a large amount of energy is passed from the hull to the water tanks at this frequency. This explains why the largest vibration level of tank bulkhead 1 in Figure 2 occurs at 73 RPM rather than at the maximum shaft speed when propeller excitation is most energetic Analysis of further measurements confirmed that the largest excitation at a particular frequency in the tank bulkheads occurs when the third, fourth or fifth order of the blade rate coincides with a local natural frequency of the aft peak tank and a local natural frequency of the tank bulkhead. Lower, but non-negligible, excitation will occur when these nearly coincide. Negligible excitation energy from the hull pressure will pass through to the tank when they do not coincide 285
RkJQdWJsaXNoZXIy MTMzNzEzMQ==