HYDROFOIL MOTIONS IN A 

 RANDOM SEAWAY 



B. V. Davis and G. L. Gates 



De Havilland Aircraft of Ca)iada, Limited 



Doivnsview, Ontario, Carmda 



INTRODUCTIGN 



This paper outlines the analog simulations and the complementary model 

 test programmes conducted by De Havilland (Canada) during the design of the 

 200 ton FHE-400 Hydrofoil Ship for the Royal Canadian Navy. 



The equations of motion required to describe the motions of the hydrofoil 

 are discussed in detail, together with the simulation of the equations and the 

 seaway forcing functions. The model trials are also discussed and it is demon- 

 strated that good correlation has been achieved between predicted and actual 

 behaviour of a 1/4 scale model of the FHE-400 and between simulated and actual 

 seaways. 



The achievement of satisfactory dynamic stability requires an iterative 

 design procedure similar to that followed in aircraft design, first to establish 

 steady- state requirements and then to examine the dynamic behaviour. When 

 examining the hydrofoil system in a seaway, it is necessary to consider hydro- 

 dynamic and structural requirements in order to develop a balanced and practi- 

 cal design. This is illustrated in Fig. 1. 



Initial studies can be carried out using simplified equations with calculated 

 derivatives, as only "broad" outlines are required. Subsequent studies have to 

 be performed in greater detail as more accurate information becomes available 

 from calculations and model trials data. The initial studies should show up any 

 major shortcomings in the design. Some modifications are likely to result from 

 the initial simulations. Gnce a reasonable foil configuration has been derived, 

 then extensive model trials should be conducted and the results used for further 

 and more accurate dynamic stability studies. Sophisticated equations are then 

 required to take account of all significant nonlinearities. 



Because of the complex nature of both the ship and random seaway simula- 

 tion, model trials are necessary to verify theoretical predictions. While towing 

 tank trials of foil units are necessary to measure resistance and to provide foil 

 derivatives, it is even more important to evaluate "seagoing" models, prefer- 

 ably manned, in order to measure response in a scale seaway. By comparing 

 measured response with the mathematical model, the validity of the simulation 

 can be established. 



611 



