Rousetsky 



information. It is most important, because if we are going to put a 

 right-angled drive, with its complexity of gears, shafts and double 

 gears down in the pod, and so on, the real problem becomes, as 

 Henry Cheng mentioned, a matter of optimising a system of the pod 

 and the propulser. So long as we have a conventionally loaded or light- 

 ly loaded propeller there is really no problem in that the optimum 

 combination can be found, this being very close to the optimum pro- 

 peller or may be one could say it is a trade-off between weight and the 

 propeller itself, but as we come to the very lightly loaded propeller, 

 as we find them on these craft, hydrofoil boats and so on, it is neces- 

 sary to take the total system into account in optimising the hydrodyna- 

 mics. We have to have both the pod track in the picture and the pro- 

 peller. 



It turns out that the optimum combination is a trade-off be- 

 tween the two. It is not the optimum propeller but we have to have a 

 much higher shaft rpm in order to get high efficiency. I wonder if the 

 author has any comments or could add something about the kind of 

 work being carried out in the Sovied Union today. I think this was also 

 pretty similar to what Henry Cheng mentioned. 



Finally, I should like to ask a question related to Figure 2. 

 There we see the wake for the pod. Of course the wake is very small. 

 It is suggested from this figure that the diameter of the propeller to 

 the diameter of the pod is really of minor importance, and in a way I 

 can only agree. On the other hand, I think it might well leave us with 

 the impression that the diameter of the propeller is of no significance 

 at all, and that is certainly not so. It is quite clear that as the pro- 

 peller diameter to pod diameter increases we see the wake being re- 

 duced, and we ought to see a set of curves where the uppermost curve, 

 for instance, can be for a smaller propeller diameter to pod diameter 

 ratio and then other curves coming further down. So I am suggesting 

 that the propeller diameter is a parameter that ought to have been in 

 that curve and which will show up as soon as we cover a wider range 

 of propeller diameter to pod diameter ratios. Are any curves available 

 at present ? That is my last question. 



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