Unusual Two-Propeller Arrangements 



overlapping-propeller arrangement. The tests indicate that there should be 

 no reason to anticipate problems due to the close proximity of the two pro- 

 pellers to each other. In comparison with a single-screw arrangement, 

 vibration and cavitation problems should be greatly reduced, since the loading 

 carried by each propeller in the overlapping arrangement would be only half 

 of the load to be carried by the propeller in a single -screw arrangement. 



The authors have considered the application of the overlapping system to 

 a large tanker, and the test results show that the system would be most attrac- 

 tive due to the reduction in the horsepower requirement. Another typical ap- 

 plication would be the fast cargo ship, where, in particular, vibration problems 

 might become serious for a single- screw arrangement. Consequently, a contra- 

 rotating system might be considered. In this case, however, the overlapping 

 arrangement would seem to have an advantage over the contrarotating system, 

 since there should be no particular mechanical problems in comparison with 

 those to be faced in developing the shaft arrangement for the contrarotating 

 system. . .-, .^ • . . . . ; .■ ; . ^ ^.-..■ 



In the analysis of the test results, the authors have introduced a correction 

 of the wake coefficient due to scale effects. This correction makes a twin- 

 screw arrangement more favorable, whereas the single-screw is penalized. 

 As a result of this analysis technique, the optimum distance between the pro- 

 peller shafts in the overlapping-propeller arrangement is somewhat larger 

 than in the case where no scale-effect correction is applied. The correction, 

 however, is very small, and it seems that the difference in the method of analy- 

 sis would give only an insignificant difference in the optimum shaft distance. 

 I should like to know if the authors share this view. 



REPLY TO DISCUSSION 



C. W. Prohaska 



It has been a great pleasure for the authors to have the comments of Dr. 

 Pien and Dr. Str^m-Tejsen, who originally drew attention to the subject 

 treated in the present paper. 



Dr. Pien mentions that a further series of tests with overlapping propellers 

 have been carried out at the Naval Ship Research and Development Center and 

 that the results are quite comparable with those of this paper. The authors are 

 glad to learn this, as the advantages of the new propeller arrangements regard- 

 ing total efficiency are thus further confirmed. 



The authors agree with Dr. Pien that the interlocking or overlapping pro- 

 pellers should be a superior alternative to the heavily loaded single- screw 

 and to twin- screws. Until now, however, only a very full hullform has been 

 tested at HyA, and our experience is therefore limited; but for this type of ship 

 nothing has been found which could justify a rejection of the new systems. 



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