Propeller Excitation and Response of 230000 TDW Tankers 



Examples of different kinds of signals are shown in Figure 21 for some 

 of the runs (results from the frequency analyser are also included). 



In Figure 18 the results obtained at atmospheric pressure cor- 

 respond closely to non-cavitating conditions, although limited cavita- 

 tion was present at some configurations. The diagram shows that, 

 under non-cavitating conditions, the difference between the amplitudes 

 for the different hull-propeller configurations is small. 



The diagram in Figure 18 further shows that, if the filtered 

 signals are taken as representative (they should correspond approxi- 

 mately to the first harmonic), the influence of cavitation on the ampli- 

 tudes of the signals of the five-bladed propeller is very small and in- 

 dependent of the model being fitted with fins or not. Especially in the 

 case of the model without fins the difference between the amplitudes 

 of the five -and six-bladed propeller in cavitating condition is ap- 

 preciable, in spite of the fact that the cavitation patterns are very si- 

 milar, see Figures 22-23. This situation is not changed very much if, 

 instead, the maximum values of the filtered signals are compared. 



If, however, the maximum amplitudes, obtained without filter, 

 are compared, a more reasonable relation is obtained between the 

 different configurations, see Figure 20. This diagram shows the bene- 

 ficial influence of the fins for the five-bladed as well as the six-bladed 

 propeller, which has been confirmed in full scale, both by vibration 

 measurements and measurements of pressure fluctuations. Both with 

 and without fins the five -blades propeller is better that the six-bladed, 

 in agreement with the tendency of the corresponding full scale mea- 

 surements. According to the vibration measurements in full scale the 

 two propellers should, however, be roughly equal. 



The amplification of the signals, caused by cavitation, is appre- 

 ciably higher for the six-bladed than for the five-bladed propeller. 

 The same tendency, although less pronounced, was found at systema- 

 tic tests carried out at Wageningen, using a dummy model Q 1J . The 

 maximum amplification factors found at the present tests were 4 

 (filtered signals) and 17 (max non-filtered/filte red without cavitation). 



Apart from being observed from the non-filtered signals, the 

 difference between the five- and the six-bladed propeller can be seen 

 in the diagrams obtained from the frequency analyser, see Figure 21. 

 It is evident that the content of higher harmonics is larger for the 

 five-bladed than for the six-bladed propeller. Results of comparisons 

 of higher harmonics have not been included here as, in several cases, 

 it was difficult to determine the amplitudes with reasonable accuracy. 



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