Hydrofoil Motions in a Randonn Seaway 



by the maximum design speed and by structural stiffness. In this respect there 

 is some conflict between hydrodynamic requirements for the thinnest possible 

 foil section, to avoid cavitation, and structural requirements for the thickest 

 possible section to avoid divergence and flutter. In some instances the maxi- 

 mum speed may well be decided by stiffness of the foil elements, as sections 

 below a certain thickness may suffer from hydroelastic problems. This mini- 

 mum thickness may not be sufficiently low to allow cavitation free operation at 

 the maximum design speed and a physical limit will be placed on the maximum 

 attainable speed. Stability is also adversely affected by cavitation. Foil loads, 

 however, are effectively limited by cavitation, which is beneficial in this respect. 



When the hydrodynamics, hydrostatics, hydroelastic s, structural integrity, 

 power and machinery requirement, operational roles, accommodation spaces, 

 etc., have been considered then the initial stages in the design of a practical 

 hydrofoil craft will have been completed. At this stage the dynamic stability 

 and the operational environment of the craft have to be considered in some de- 

 tail, Foilborne seakeeping in rough water is of paramount importance since the 

 craft must be stable under all sea conditions and must have acceptable response 

 characteristics from the standpoint of human tolerance to motion. Some factors 

 influencing craft motions are foil taper ratios (for surface piercing foils), rate 

 of change of lift with angle of attack and rate of change of lift with immersion 

 depth. The foil system should be insensitive to angle of attack changes (i.e., 

 low Cl^) to reduce the effect of wave orbital velocities but should be relatively 

 sensitive to changes in immersion depth (Cl^) to control foil broaching and hull 

 slamming. The ideal response would be with the craft platforming all waves 

 below those which would cause broaching or slamming and contouring all larger 

 waves. In practice this ideal is not attainable and the craft motions are between 

 platforming and contouring for all significant waves. 



To obtain the above characteristics some compromise is necessary. A low 

 Cl„ usually implies a low aspect ratio (Fig. 2) and this yields a low lift-drag 

 ratio which is detrimental to performance. For maximum performance the foil 

 system should have the highest possible L/D ratio. A good compromise in this 

 respect can be achieved with a canard system, in which 80-90% of the total lift 

 is provided by the main foil. The bow foil supplies only 10-20% of the total lift; 

 therefore its L/D ratio can be relatively low without contributing an unaccept- 

 ably high drag to the total. Thus the bow foil can be optimised to produce mini- 

 mum motions resulting in relatively small angle of attack excursions at the 

 main foil. The main foil can then be designed to have a high aspect ratio (low 

 drag) without incurring unacceptably high accelerations at the craft e.g. In 

 practice the main foil BCL/^a and BCl/BH lift-curve slopes have to be optimised 

 to produce satisfactory performance in both head and following seas. However, 

 the values so obtained do not differ greatly from those desirable for best per- 

 formance. The bow foil unit is optimised to produce minimum motions in a 

 seaway and to a large extent, controls the natural frequency of the craft in pitch 

 and gives adequate separation between the craft natural frequency and the domi- 

 nant frequencies of encounter in head seas which produce significant inputs of 

 energy to the craft (Figs. 3, 4, 19 and 20). It is considered that fully cavitating 

 bow foil sections are necessary to provide the required characteristics in a 

 surface piercing system. These sections give low- lift curve slopes and are not 



613 



