Rouse t sky 



REPLY TO DISCUSSION 



Vjacheslav N. Treshchevsky 



Rpyloff Research Institute 

 Leningrad U.S.S.R. 



I am thankful to Mr. Strom-Tejsen for his comments. He is 

 right pointing out that the wake must decrease with increasing D/d 

 ratio. For instance, for L/D = 5 and D/d = 2, 5 the value of the 

 wake is about 0, 02-0, 025. But D/d = 2, 5 is too large for practical 

 realization, and Figure 2 corresponds to D/d ratios which are of 

 interest for 'Z 1 -drive design. I also share Mr. Strom-Tejsen's point 

 of view that taking into account the pod resistance and weight charac- 

 teristics the efficiency of the whole complex is more important than 

 the maximum fo the propeller efficiency. But the increase of the 

 number of revolutions usually correlates with the loss of the effective 

 thrust corresponding to take-off conditions. Thus when power is res- 

 tricted these considerations cannot always be realised. 



In connection with Mr. Leopold questions I can say that at a 

 given pod diameter and assuming L/d > 5 we have as a consequence 

 the decrease of efficiency. Increase of the L/d ratio may be neces- 

 sary when speed increases and general design considerations are 

 taken into account. As for better conditions for the after propeller of 

 the two propeller 'Z' -drive construction the free vortex density of the 

 forward propeller must be relaxed and the ratio D for/D aft > 1 

 chosen. 



Answering Mr. Morgan's question concerning Figure 3, I am 

 to tell that x means the cavitation number, ap = -c — rTwj " the l° a d 

 factor. The increase of the thrust deduction due to propeller cavitation 

 can take place only when after the propeller some construction ele- 

 ments are present. 



I can also inform in connection with Mr. Novae ki question 

 that in formula (2) the effective potential wake, and on Figure 2 - the 

 nominal one are used. 



To conclude, I can say that the method described in the paper 

 was used in design of hydrofoils, so that considerations concerning 

 the construction of the hydrodynamic complex in general were taken 

 into account. Information on one such hydrofoil - Typhoon was published 



416 



