Lang 



REPLY TO DISCUSSION 



Thomas G. Lang 

 Naval Undersea Research and Development Center 

 San Diego , California U.S.A. 



As pointed out by Mr. Hook, vehicle efficiency sould be com- 

 pared in both calm water and rough water. The objective of the paper, 

 however, was the semisubmerged ship concept ; consequently, full 

 comparisons were not made between hydrofoils and SES. 



In regard to Mr. Hook's comment on Figure 6, it should be 

 recalled that all curves in this figure pertain to 3, 000-ton vehicles. 

 Conventional hydrofoils of this size are generally considered im- 

 practical ; hydrofoil weight tends to be high, and the design Froude 

 number tends to be too low. Design tradeoffs for a conventional 

 3, 000-ton hydrofoil between structures, cavitation, and hydro- 

 dynamic drag would tend ro result in reduced hydrodynamic efficiency 

 of the order shown in Figure 6. On the other hand, the value of 

 1 L./D for semisubmerged ships, and displacement hulls in general, 

 increase with displacement, for a given speed. This result should in 

 no way detract from the good efficiency exhibited by small, high- 

 speed hydrofoils and their excellent performance in rough water. 



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