A hydroplane (or hydro, or thunderboat) is a fast motorboat, where the hull shape is such that at speed, the weight of the boat is supported by planing forces, rather than simple buoyancy. A key aspect of hydroplanes is that they use the water they are on for lift rather than buoyancy, as well as for propulsion and steering: when travelling at high speed water is forced downwards by the bottom of the boat's hull. The water therefore exerts an equal and opposite force upwards, lifting the vast majority of the hull out of the water. This process, happening at the surface of the water, is known as ''.