Wereldsma 



Fig. 6 - Recording of the torque fluctuations of (top) 

 the four-bladed forward propeller and (bottom) the 

 five-bladed aft propeller 



The high frequencies of mutual blade encounter cannot be distinguished in 

 the records. The influence of the wake dominates for the forward as well as 

 aft propeller. For the five-bladed propeller a small irregularity has been re- 

 corded, probably caused by the interaction with the four-bladed propeller. 



The records of the horizontal and vertical bending moments of the four- 

 bladed and the five-bladed propeller are shown in Figs. 7 and 8. From these 

 records the mutual blade encounter can be detected. Forty time per revolution 

 a disturbance occurs. Due to the fact that one blade at a time encounters an- 

 other, the effect in lateral direction is much greater than in axial direction. 



The peak- to- peak variations at blade frequency of the vertical as well as the 

 horizontal bending moment of the four-bladed propeller expressed as a percent- 

 age of the mean thrust times the propeller radius amounts to approximately 10%. 

 On these blade frequency fluctuations a high-frequency disturbance due to the 

 mutual blade encounter is superimposed. The peak to peak amplitude of this 

 high frequency output equals from 1% to 2.5% of the mean thrust times the pro- 

 peller radius. (These figures are corrected for dynamic amplification, as de- 

 scribed in the next section.) For the five-bladed propeller the vertical bending 

 moment fluctuations at blade frequency amount to 15% peak- to- peak and the 

 horizontal bending moment to 10%. Superimposed on these blade frequency 

 fluctuations there is an extra ripple of approximately 1 to 2.5% due to the blade 

 encounter. 



242 



