The Vibratory Output of Contrarotating Propellers 



DISCUSSION 



J. Strom- Tejsen 



Naval Ship Research and Development Center 



Washington, B.C. 



Dr. Wereldsma's paper is most interesting and timely. According to his 

 measured results, the fluctuating thrust and torque of either of the contrarotating 

 propellers are roughly the same with or without the presence of the other pro- 

 peller, since the dominating cause of propeller fluctuation forces is the spatial 

 variation of the wake field behind a ship. I would like to ask Dr. Wereldsma 

 whether such an interesting result also would be valid for the overlapping pro- 

 peller arrangement? 



Since the development of the overlapping propeller arrangement at the Naval 

 Ship Research and Development Center a few years ago, several towing tanks in 

 the United States and in Europe have conducted powering tests with this arrange- 

 ment. However, to my knowledge no investigation of the vibratory forces similar 

 to Dr. Wereldsma's has been conducted. 



In most cases it has been anticipated that the two overlapping propellers 

 would be driven with exactly the same rpm through the same gear box. By 

 proper phase adjustment of the propellers, any of the three vibratory forces can 

 be eliminated. 



At the present time, however, consideration is being given to installation of 

 an overlapped propeller arrangement, using two power plants independent of 

 each other. This would, for instance, be the case when using two Diesel engines 

 without any phasing of the two propellers. This configuration would, in principle, 

 be very similar to the contrarotating propeller arrangement from a vibratory 

 point of view. It would be of great interest if Dr. Wereldsma would extend his 

 investigation to such an overlapping propeller arrangement. 



REPLY TO DISCUSSION 



R. Wereldsma 



It is difficult to extrapolate from one extraordinary case to another, par- 

 ticularly in the field of propeller vibrations. If neglect of the interaction of both 

 overlapping propellers is allowed, the relative vibratory shaft output will be 

 comparable to that of the single screw. The bending moments in the individual 

 blades, however, will be considerably increased because of the increased lower 



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