Design of "Z" Drives for Propulsion of Hydrofoil 



DISCUSSION 



Henry M. Cheng 



Office of the Chief of Naval Operation 

 U.S.A. 



The author has discussed in the paper mostly what is involv- 

 ed in and the implications of a 'Z '-drive propulsion system. However 

 the technical information presented is very sketchy. I wonder if he is 

 purposely doing so to tease us. Figure 3, showing information on 

 thrust reduction, as he defined it, is only applied to a particular con- 

 figuration of the pod, that is the L over d equals 5, and the diameter 

 of the propeller (D) to the diameter of the pod d is I. 5. I wonder if any 

 more work is being done with other configurations for different L over 

 Ds and D over ds. 



Secondly, in the conclusions the author alluded to the fact 

 that the paper is to help us to choose the optimum designs among the 

 parameters, and yet the information presented is incomplete to help 

 us to accomplish this. If it is not already in the plan I am certainly 

 hopeful that this work is going to be pursued. 



The third point is that the title implies the whole propulsion 

 system of a 'Z '-drive, whereas the discussion of the other elements 

 of the system, which are very important and have pronounced effects 

 on the selection of the pod, the proportions and the propeller itself, is 

 done only briefly. The material presented is considered only a very 

 small portion of the total system. I wonder if the paper is intended for 

 a general discussion and some more information will be coming from 

 the author ? 



Lastly, the information presented is intended for hydrofoil 

 ships application. To my knowledge there are very few, if any, hydro- 

 foils with propulsion pod independent of or divorced from the foil. 

 This brings me to my comment : whether this information is useful at 

 all, if the pod is separated from the foil which is dependent of the load- 

 ing and the other components of the total system that we call the pro- 

 pulsion system here. 



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