1. Hovercraft Environment. With the exception of the sidewall craft which is unlikely to achieve speeds in excess of 30 knots, all hovercraft depend on their ability to rise clear and free from water drag in order to achieve their high economical speeds. Nevertheless a hovercraft will tend to conform with the surface over which it is travelling. A rough sea produces a rough ride which at speed produces a motion not unlike that of a car going over a series of hump bridges with vertical accelerations in the order of 1½ g—one needs to be strapped into a seat. The most efficient means of propulsion is achieved by using air screws; these are noisy and necessitate crew members using an intercom. Control surfaces, rudder and airscrews are aerodynamic and to reduce air drag the hovercraft is likely to be low in silhouette and give poor all-round vision to the command. The biggest single factor is drift; being aerodynamic and free of water drag, the hovercraft is completely wind borne and in moderate beam winds can track with as much as 45° of drift angle on and turns out of wind can be alarmingly large.