Lindgren, Johnsson and Dyne 



90 



Contrarot propeller (84 RPM] 

 Forward prop No. PI250 Aft prop. No. PI25f 



27(f 



Single propeller (1 12 RPM) 

 Prop. No. PU72 



180" 



180 



27Cf 



Fig. Z3 - Tanker project. Results of cavitation tests in irregular flow. 



7.3. Local Pressure Fluctuations Induced on the Afterbody by 

 Conventional and Contrarotating Propellers 



In many cases studies have been carried out at SSPA in order to determine 

 the pressure fluctuations on the afterbody induced by the propeller. Unfortu- 

 nately, experiments of this kind have not yet been carried out with the two proj- 

 ects discussed above. To throw some light on these problems, two diagrams 

 are, however, presented, Figs. 25 and 26. The first diagram illustrates the in- 

 fluence of the number of propeller blades on the pressure fluctuations at some 

 critical spots in the aperture. This influence is obvious. Thus an increase 

 from 4 to 6 blades decreases the amplitude by more than half of the original. 

 From Fig. 26 it is evident that the pressure amplitudes from the contrarotating 

 propellers are somewhat lower than those from a conventional, single propeller. 

 With the conventional twin-screw arrangement still lower amplitudes were re- 

 corded. 



1296 



