although this would be well short of the remarkable achievement of active fins in 

 reducing roll. Comparable improvement in pitch and heave could, in principle, be 

 obtained from active stabilization but at the expense of excessive size and weight of 

 the equipment. Thus for the present it is suggested that the search should be for 

 limited stabilization of surface ships rather than the ideal target of complete stabiliza- 

 tion. The wave barrier is of course completely avoided by the submarine when oper- 

 ating at deep submersion. 



Man in the urge for speed has succeeded in travelling nearly 200 knots on 

 calm water but only as a record sprint. Thirty to 35 knots is about the present limit 

 for large ships and the speed of intermediate and small vessels is commensurate with 

 40 knots. Confronted by ocean waves, speed is greatly reduced and very few ships have 

 succeeded in crossing the Atlantic at 30 knots. The price of speed is high, the real 

 physical barrier being the large density of water. None will deny the scope for 

 improvement both in steadiness and speed. The Author's paper and the wide net cast by 

 this Symposium encourages the hope that achievement will not fail for lack of skillful 

 effort. 



M. C. Eames 



Mr. Niedermair, and some previous speakers, have made reference to the hydro- 

 foil craft. As is likely to be better known by readers of popular picture magazines 

 than by students of technical transactions, the Defense Research Board of Canada is 

 sponsoring a research project on the development of a particular type of hydrofoil 

 craft at its Naval Research Establishment at Halifax. Since in the design of the craft, 

 an entirely new form of hydrodynamic "barrier" has to be considered, it is felt that 

 the following notes may be of interest. 



While the most ardent supporter of the hydrofoil craft would agree with Dr. 

 Allan and others, that in general if one wishes to attain high speeds, one should take 

 to the air completely, there nevertheless exists a regime in the speed-size field of trans- 

 portation where the hydrofoil craft can show a definite advantage in terms of power 

 economics. Moreover, certain military advantages are apparent outside this region 

 where the hydrofoil craft presents the most economical means of transport. 



The existence of such a regime, and the state of the art up to 1953 have been 

 comprehensively recorded in a paper by Buerman, Leehey and Stilwell before the 

 Society of Naval Architects and Marine Engineers in 1953, and the Institute of Marine 

 Engineers in 1954. In general it may be said that subsequent advances have been a 

 matter of detailed improvement and the collection of data rather than one of basic 

 changes of principle. 



Mr. Niedermair has referred to the "barrier" in speed which results from 

 cavitation of the lifting hydrofoils. Particularly in surface-piercing hydrofoil configura- 

 tions, however, a further "barrier" can present problems at a lower speed than that at 

 which the hydrofoils begin to cavitate. Drawing an over-simplified picture, it is clear 

 that cavitation will commence when the pressure on the upper surface of a hydrofoil 

 has dropped to the vapor pressure of the surrounding water. However, in general a 

 much lower speed will suffice to reduce the pressure on th upper surface to that of the 

 atmosphere, and once the pressure falls below this value the phenomenon of ventilation 

 can occur. This is the process by which air is drawn down the upper surface of the 

 hydrofoil, possibly via a supporting strut, resulting in much the same effect on per- 

 formance as a well developed state of cavitation. 



The mechanics of ventilation has received little fundamental study in the past, 

 and the process is not yet fully understood. While means of prevention and cure for 

 particular hydrofoil craft have been successfully developed, the "barrier" cannot be 

 said to have been truly overcome. The essential reason for this statement is that the 

 scaling laws applicable to the phenomenon remain incompletely understood, and until 

 they are fully appreciated the extrapolation of proven results and methods on small 

 test hydrofoil craft to full scale prototypes will be subject to a degree of uncertainty. 



152 



