1. Introduction. By definition a hovercraft or air-cushion vehicle is a vehicle capable of being operated so that its weight is partly or wholly supported by a self-generated cushion of air that is dependent for its effectiveness on the proximity of the surface over which the vehicle operates. Thus from the safety aspect of navigation a hovercraft must be treated as a surface vehicle or craft; for any collision risks involved are proportional to the amount of conventional surface vehicles or craft and static obstructions, in the area of operation. The clearance of minor obstacles less than the hoverheight is practicable; but on present experimental craft with an undersurface clearance of three feet, these clearances are limited to small buoys, waterlogged obstructions and rock outcrops of limited height.