Munk and Prohaska 



DISCUSSION 



Dr. P. C. Pien 

 Naval Ship Research and Development Center 

 vj., , Washington, D. C. , ; 



This paper by Professor Prohaska and Mr. Munk is very interesting. At 

 the Naval Ship Research and Development Center we have been experimenting 

 with the overlapping propeller stern arrangement for the last couple of years. 

 The authors use the term "interlocking propellers." Six models with such an 

 arrangement have been tested for powering performances. In each case, a 

 large saving in power requirements has been achieved as compared with a con- 

 ventional twin-screw arrangement. At several other towing tanks, similar ex- 

 periments have been conducted. All the available experimental results are 

 quite comparable with those given in the paper. 



Despite the simplicity and the high hull efficiency of a single- screw stern 

 arrangement, there are many cases where such an arrangement cannot be 

 used. In most cases a conventional twin-screw arrangement has been chosen 

 as an alternative. In the light of the results given in this paper, as well as 

 other published and unpublished test results, it can be stated that the over- 

 lapping propeller stern arrangement is a better alternative. Besides saving 

 power, another advantage is the possibility of using with it either a single or a 

 twin powerplant. If two powerplants are chosen, an overlapping propeller ar- 

 rangement is simply a matter of installing the propellers in the proper loca- 

 tions to a single- screw ship hull. Two shafts are inclined to have propellers 

 overlapping each other. By choosing a different number of blades between the 

 two propellers, each propeller is essentially independent of the other as far 

 as the hull vibration problem is concerned. Since the power absorbed by each 

 propeller is only one-half of the total, the risk of propeller cavitation and 

 propeller-induced vibration would be greatly reduced. 



In view of this discussion and their own experience with the overlapping 

 propeller stern arrangement, what reservations would the authors have in 

 recommending such a stern arrangement to the shipping industry? 



DISCUSSION 



J. Strgim-Tejsen 



Naval Ship Research and Development Center 



Washington, D. C. 



I would like to congratulate the authors on a most interesting paper, and in 

 particular on their measurements of the fluctuating-blade bending forces, which 

 to my knowledge are the first measurements of this kind carried out for the 



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