A jetboat is a boat propelled by a jet of water ejected from the back of the craft. Unlike a powerboat or motorboat that uses a propeller in the water behind the boat, a jetboat draws the water from under the boat into an impeller pump, known as a jet unit, inside the boat then expels it through a nozzle at the stern. Steering is accomplished by small vanes that direct the water jet in the direction the boat needs to go. Because of this the jetboat is highly maneuverable and can often be reversed and brought to a stop within its own length from full speed in a maneuver known as a jetboat turn.
By analogy to aircraft, a conventional screw propellor accelerates a large volume of water by a small amount, in a manner similar to the way an aeroplane's propellor accelerates a large volume of air by a small amount. An aircraft's jet engine, by contrast, accelerates a small volume of air by a large amount. Both methods yield thrust due to Newton's third law - every action gives rise to an equal and opposite reaction. Thus in a jetboat, by pumping a small volume of water, accelerating it by a large amount, useful thrust can be obtained.
Unlike a hydrofoils, which uses underwater wings or struts to lift the vessel clear of the water, jetboats normally plane across the water surface at operating speed, with only the rear portion of the hull displacing any water. With the hull clear of the water, there is reduced drag and speed and maneuverability are greatly enhanced. Jetboats are normally operated at planing speed. Below planing speed the jetboat looses most of its maneuverability and promply slows due to greatly increased drag. For this reason a jetboat is difficult to operate at speeds other than very fast or dead slow. For stability, the jetboat has a very shallow-angled (but not flat-bottomed) hull. It is claimed that jetboats can be safely operated in less than a foot (30 cm) of water.
Jetboats were originally designed by inventor Bill Hamilton to operate in the fast-flowing and shallow rivers of New Zealand to overcome the problem of propellers striking rocks in such waters, although Italian inventor Secondo Campini demonstrated a similar vessel as early as 1931 in Venice.
The New Zealand company CWF Hamilton designs and builds jetboats and manufactures the Hamilton Jet Unit, the patented device that produces and controls the water jet. Jet boats are normally powered by a V8 petrol engine, often an adapted automotive engine from a high perfomance car manufacturer.
Applications for jetboats include adventure tourism, surf rescue, farming, fishing and marine law enforcement, exploration, pleasure boating and other water activities where a motor boat is used. Jetboats can also be raced for sport, both on rivers and on a specially designed racecourse. Most jetboats are small enough to be carried on a trailer that is towed by a car.
Queenstown, New Zealand claimed it was the jetboat capital of the world when jetboating was the only form of adventure tourism.