Ducted and Contrarotating Propellers on Merchant Ships 



DISCUSSION 



V. Silovic 



Hydro-og Aerodynamisk Laboratorium i- 



Lyngby, Denmark " ■ 



At Lyngby, we use Cheng's development of Pien's theory for lifting-surface- 

 effect calculations. To my knowledge, the authors use a simplified 0.8 mean- 

 line representation. My question would be: What is their experience as to the 

 comparison of the two? • - ■ i - •?'. ^; 



REPLY TO THE DISCUSSION 



H. Lindgren, C.-A. Johnsson, and G. Dyne 



After having listened to the discussion, the authors have come to the con- 

 clusion that one of the most important results of the paper might be that it has 

 initiated activities in different laboratories and brought a great deal of interest- 

 ing material to common knowledge. In the case of the Dutch Tank, these activi- 

 ties have been so intense that they went two days ahead of us in the end of the 

 race by reading a similar paper without any advance announcement. 



With regard to the oral discussion by Dr. English and Mr. Minsaas, the re- 

 sults they have presented are of the utmost interest and we regret that they have 

 not given their discussion in a written form. We look, forward, however, to 

 seeing their material published. 



Mr. Oosterveld raises the interesting question about the relation between 

 the ducted propellers designed in accordance with the SSPA design method and 

 the systematic series of ducted propellers tested by van Manen. The number of 

 unknown factors influencing the efficiency and the cavitation characteristics of a 

 ducted propeller is of course much greater than for a conventional propeller. 

 This means that the empirically based systematic series must be very extensive 

 to ascertain that all factors have been considered. With the aid of a reliable de- 

 sign method it is possible to come to a good result much faster and in a more 

 controlled way. The influence of a number of factors can be clarified purely 

 theoretically, and the experiments can be concentrated to investigate such diffi- 

 cult problems as, for example, duct flow separation behind the propeller. In 

 connection with this question, it is interesting to note that the values of geomet- 

 rical angle of attack (ao = 8.8") and camber ratio (f/)/L = 0.062) of NSMB noz- 

 zle no. 19A is much closer to the theoretical values for shock-free entrance 

 than the earlier NSMB ducts plotted in Fig. 15. 



1309 



