A pontoon is a flotation device with buoyancy sufficient to float itself as well as a heavy load. A pontoon boat is a flattish boat that relies on pontoons to float. Pontoons may be used on boats, rafts, barges, docks, airboats, hovercrafts, floatplanes or seaplanes. Pontoons may support a platform, creating a raft. A raft supporting a house-like structure is a houseboat. A fixed platform can be used as a dock. Common boat designs are a catamaran with two pontoons, or a trimaran with three. Pontoons may be simply constructed from sealed cylinders such as pipes or barrels, or fabricated as boxes from metal or concrete. Pontoon boat drafts may be as shallow as eight inches, which reduces risk of running aground and underwater damage. The pontoon effect is when a large force applied to the side capsizes a pontoon boat without much warning, particularly a top-heavy boat.