THE VIBRATORY OUTPUT OF 

 CONTRAROTATING PROPELLERS 



R. Wereldsma 

 Netherlands Ship Model Basin 

 Wageningen, The Netherlands 



ABSTRACT 



For high-powered ships unconventional propulsion arrangements such 

 as contrarotating propellers have proved efficient. The fluctuating 

 forces generated by this propeller combination were determined ex- 

 perimentally. The wake has a dominant influence on thrust as well as 

 torque fluctuations of both propellers. The fluctuations, expressed as 

 a percentage of their respective mean values, are comparable with the 

 fluctuating output of single propellers. The lateral forces are consid- 

 erably reduced. The thrust eccentricity shows a high-frequency dis- 

 turbance introduced by the mutual blade encounter of the propellers. 



Guidelines are given for the dynamic analysis of the axial and torsional 

 vibrations of the shafting for two types of gearings in combination with 

 a turbine engine: conventional and epicyclic gearing. For the epicyclic 

 gearing it is expected that the frequency modulation, introduced by load 

 variations due to ship motions and orbital water motions, is stronger 

 than for a conventional gearing. For the high-frequency analysis a 

 number of unknown coefficients describing the mutual propeller inter- 

 action have to be estimated to determine whether they are significant. 



NOMENCLATURE 



A = gear ratio 

 A = aft perpendicular 



Ap = developed blade area . , ., 



Aq = disk area of propeller ,n: . ;> ■. - ..s: • 



B = gear ratio 

 CWL = construction waterline 



D = disturbing torque 



F = torque-rpm characteristic of propeller 



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