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 a pump-jet inside the boat, then expels it through a nozzle at the stern.

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.

Jet boats are 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 Hamilton turn.

How jet boats work

A conventional screw propeller accelerates a large volume of water by a small amount, in a manner similar to the way an aeroplane's propeller 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 force gives rise to an equal and opposite force.

In a jetboat, by pumping a small volume of water and accelerating it by a large amount, useful thrust can be obtained. This is achieved using multiple impeller stages to accelerate the water. The key to success for a jet boat is to have the "jet" exit above the waterline. Early efforts tended to eject below the waterline, perhaps in the mistaken belief of their designers that the jet needs to "push" against the water to create thrust. In fact, the drag this creates negates almost all of the thrust available — ejecting into air rather than water allows thrust to be developed far more efficiently. Steering is accomplished by small vanes that direct the water jet to turn the boat.

Unlike hydrofoils, which use 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 majority of 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 loses most of its maneuverability and promptly 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 12 inches (30 cm) of water.

Manufacturers

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 performance car manufacturer.

Applications

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 specially designed racecourses. Most jetboats are small enough to be carried on a trailer and towed by car.

Queenstown, New Zealand claims the title of jetboat capital of the world.

See also

• Sport
• Jet sprint boat racing

References

• History of jet boats
• Jet boating on the Waiau
• Jet boat origins at Irishman Station