The Vibratory Output of Contrarotating Propellers 



frequencies below blade frequency being lower multiples of the rps can be de- 

 fined as low-frequency interaction of both propellers. The frequencies of mu- 

 tual blade encounter are high-frequency interactions. 



RESULTS OF THE MEASUREMENTS 



The frequencies generated by the propeller combination can be estimated 

 as follows. The four-bladed propeller will generate blade frequencies and its 

 multiples, i.e., 4 x rps, 8 x rps, etc. The five-bladed propeller will generate in 

 a similar way 5 x rps, 10 x rps, etc. From previous experience it is reasonable 

 to neglect the wake- generated components, having frequencies higher than 3 x 

 blade frequency. Also very low frequencies from the low-frequency interaction 

 can be expected as well as very high frequencies due to the mutual blade encoun- 

 ter. 



The records of the thrust and torque fluctuations of the forward and the aft 

 propeller are shown in Figs. 5 and 6. The peak-to-peak values of the thrust 

 fluctuations of the four-bladed forward propeller amounts to 36% of the mean 

 thrust. The peak-to-peak values of the torque fluctuations amounts to 28% of the 

 mean torque. For the five-bladed propeller the peak-to-peak thrust fluctuations 

 amount to approximately 23% of the mean thrust. The peak-to-peak torque var- 

 iations equal 20% of the mean torque. These percentages for both propellers are 

 not unusual for four-bladed and five-bladed propellers behind this ship type, 

 which has sharp wake peaks. 



Fig. 5 - Recording of the thrust fluctuations of (top) 

 the four-bladed forward propeller and (bottom) the 

 five-bladed aft propeller 



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