Hydrofoil Motions in a Random Seaway 



it is quite adequate to consider a fixed mean aspect ratio with the changes in 

 circulation being considered as being subject to the same delay whatever the 

 cause of change in circulation delay. 



Circulation delay effects become more marked at the higher aspect ratios, 

 therefore, an aspect ratio value biased toward the high side was chosen so that 

 the simulated effects would be more severe than in practice. An aspect ratio of 

 6 was assumed using the data from Ref. 16. 



The results of response studies on a hydrofoil craft using the exponentials 

 of Fig. 17 are given in Fig. 18. It can be seen that the effect of lift delay has a 

 minor effect on the overall hydrofoil craft motions. 



Virtual Inertia (Added Mass Effects) 



Virtual inertia is usually considered as the inertia of the body of water that 

 can be thought of as moving with the foils and which adds to the craft inertia 

 when the foils are imparting an acceleration to the surrounding water. There 

 are some instances, however, when the fluid mass opposes the craft inertia such 

 as in a seaway when the body of water surrounding the foils may be imparting a 

 disturbing acceleration to the craft. 



Some studies of this effect were carried out but the indications were that 

 the effect on the craft motions was small, producing only minor changes in peak 

 accelerations for the shortest and steepest seas (Fig. 18). 



As a result of these studies virtual inertia effects were neglected in all 

 subsequent simulations. 



MODEL TRIALS 



General 



Predicted hydrofoil ship characteristics require verification by model tri- 

 als, in a similar manner to the experiments usually conducted on conventional 

 ship and aircraft models. 



For the hydrofoil ship, stability in a seaway is the major consideration, the 

 measurement of model resistance and lift characteristics being important sec- 

 ondary problems. 



For a new vehicle concept, it is essential to verify scaling laws and provide 

 experimental proof of theoretical performance and stability predictions. 



The hydrodynamic design of the FHE-400 is supported by the results of a 

 series of model trials, mainly carried out at the National Physical Laboratory 

 and the Admiralty Research Laboratory in London, and at the Canadian Naval 

 Research Establishment, Halifax, and the National Research Council in Ottawa. 

 Initial trials to determine resistance and seakeeping were conducted by the 

 Davidson Laboratory, S.I.T. in Hoboken, New Jersey. The model programme 



221-249 O - 66 - 42 641 



