Hydrofoil Motions in a Random Seaway 



DISCUSSION 



A. Silverleaf 



National Physics Laboratory 



Teddington, England 



This paper is probably the most thorough account yet available of the over- 

 all development of the design of a seagoing hydrofoil ship of unorthodox and ad- 

 vanced foil configuration. Among the many significant points which it rr ises is 

 the clear indication that fixed surface-piercing foils may yet have an important 

 and useful role to play in such craft in spite of many recent statements to the 

 contrary. The authors have naturally emphasised the value of analogue computer 

 studies in investigating the motions in a seaway of a craft of this kind. It is, of 

 course, important to simulate correctly the performance characteristics of 

 ventilated foils in such analogue calculations, particularly if the motions of the 

 craft may cause ventilation to be intermittent, alternating with short periods 

 during which the foils are either fully or partly wetted, in which case their force 

 characteristics will be very different. 



Some of the early experiments at N.P.L. with a 1/8 scale skeleton model of 

 the complete craft, free to heave and pitch, showed that intermittent ventilation 

 of the bow foil unit could occur in certain sea conditions. In these circumstances 

 there were disturbing differences between the analogue computer calculations of 

 the craft motions and those measured on the large model in the high speed tow- 

 ing tank at Feltham. However, when steps were taken by the authors to ensure 

 that ventilation was continuous, the motions of the model were very considerably 

 improved and there was then good agreement with the calculated values. This 

 episode well illustrates the need to simulate the correct physical conditions in 

 any computer calculations; if the hydrodynamics are incorrectly reproduced it 

 is unlikely that useful conclusions will be obtained. 



The authors have pointed out that many of the model experiments have been 

 carried out at N.P.L.; as mentioned in Table 1 these include not only measure- 

 ments to determine hydrodynamic performance but some very unusual experi- 

 ments to investigate hydroelastic characteristics. All these experiments have 

 been and are being made as one aspect of a most interesting three-part approach; 

 analogue computer studies in Toronto, trials with a manned craft in Halifax, and 

 model experiments at Feltham have proceeded simultaneously and in parallel. 

 It is I think fair to say that, particularly during the early development stages, 

 each of these three approaches identified and resolved problems which at first 

 sight appeared more than daunting. This comprehensive and thorough attack 

 emphasises the need for such procedures if advanced high speed marine craft 

 are to be successfully developed. 



221-249 O - 66 - 45 689 



