AERODYNAMIC AND HYDRODYNAMIC ASPECTS OF HIGH-SPEED WATER SURFACE CRAFT

High speeds on water are being attained in racing sport and in attempts on world speed records in various classes. Success, safety and stability of these craft depends upon the favourable interaction of their aerodynamic and hydrodynamic characteristics under the influence of two media, one about 800 times denser than the other. Speed on the straight course and in turns is important. As its velocity increases, a craft experiences increasing dynamic pressure in water and to maintain the balance, the ‘wetted’ area of the craft reduces as it rises up (‘planing’). Modern fast craft have ‘tunnel-hulls’ and lifting areas to generate aerodynamic lift and to assist the craft to attain planing attitudes rapidly. The ‘lifting’ areas may not necessarily be in the correct locations however. An example often seen is that of a power boat riding virtually on its propeller and ‘wallowing’ in an unstable manner. In this case the variation in riding height alters the relative positions of centres of aero- and hydro-lift such that the ‘stability-margins’ are near critical in both the longitudinal and the lateral sense.