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.

Minitufts are increasingly used for flow visualisation on static wind tunnel models because they can be applied in larger numbers for increased detail and with less interference compared with conventional tufts. This Memorandum describes an extension of their use to continuously rotating models where the heavier conventional tufts are inadequate.

Tests on two combat aircraft models in the 4 m x 2.7 m Low Speed Wind Tunnel are described. Measured variations of rolling moment are explained with the aid of the minituft photographs.

Propeller power absorption and thrust have been measured over a range of rotational speeds up to 1400 rev/min at mainstream airspeeds up to 48 m/s for a wide range of blade setting. In addition, cruise conditions have been simulated by maintaining the appropriate ratio of propeller tip speed to the airspeed. Generally good agreement is shown between these full scale experiments with results obtained at 1/5 scale though the effect of Mach number is observable when the propeller is highly loaded.

Noise analysis demonstrates the predominance of multiple tones whose number and intensity increase with helical-tip Mach number. An empirical formula shows that the fundamental tone varies with tip speed and power loading in a similar manner to that observed on a 1/5 scale model.