Design of "Z" Drives for Propulsion of Hydrofoil 



DISCUSSION 



Jorgens Strom - Tejsen 



Naval Ship Research and Development Center 

 Bethesda 3 Maryland U.S.A. 



I should like to make a few comments to supplement what 

 Henry Cheng has said. 



The application of the propulsion pod has certainly many ad- 

 vantages, as pointed out by the author, in particular the fact it is 

 possible to align the propulsion pod in such a way that the flow into 

 the propeller becomes cleaner and more uniform. It is possible to 

 reduce cavitation and vibration. On the other hand, the introduction 

 of a 'Z '-drive with its complexity of gearing and so on has certainly 

 discouraged its application by most. I would like to bring to your at- 

 tention the fact that this year wa shall see the development of a super- 

 conducting motor which would make it possible to use the propulser 

 pod eventually, at some time in the future. A simple conducting 

 motor would make it possible to have the motor itself mounted down 

 in the pod and still have a sufficiently small size so that the diameter 

 of the propeller to the diameter, of the pod would still be of the order 

 of magnitude mentioned by the authors. To what extent is work of 

 this nature being pursued in the Soviet Union today ? Looking through 

 the literature on hull-propeller interaction or propulser-pod-propel- 

 ler interaction, it is very clear it is difficult to find any really good 

 information about wake and thrust deduction. We have a lot of infor- 

 mation from submarines and torpedoes but for most of these cases 

 the propeller diameter is much smaller than what we are talking 

 about here. So I should like to compliment the authors for providing 

 this information ; it is very valuable. The effect of cavitation on 

 thrust deduction is extremely interesting. We have to realise that in 

 the net thrust efficiency for a propulser pod combination the thrust 

 deduction comes in as in any hull-propeller interaction with its full 

 weight, so that unless we can predict the thrust deduction with accura- 

 cy we cannot predict this combination. Two or three points means 

 quite a lot. Figure 3, for instance, shows very clearly how the effect 

 of cavitation might change the thrust deduction from, say, . 05 to . 02. 

 It is three percentage points.. 



I should like to add my plea to Henry Cheng's for additional 



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