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Jetboat
From Wikipedia, the free encyclopedia
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 Sir William Hamilton (who invented the waterjet in 1954) for operation in the fast-flowing and shallow rivers of New Zealand, specifically to overcome the problem of propellers striking rocks in such waters, although Italian inventor Secondo Campini had demonstrated a similar vessel as early as 1931 in Venice.
The difference between Campini's and Hamilton's inventions is that Campini's waterjet had a very short lifetime in operation due to some unsolved material problems. Hamilton, unlike Campini, filed for a patent.
Jet boats are highly maneuverable, and many can, from full speed, be reversed and brought to a stop within their own length, in a maneuver known as a Hamilton turn.
There is no engineering limit to the size of jet boats, though the classic prop-drive is more economical than the jet-drive. Thus, the biggest jet-driven vessels are found in military use. South Africa's Valour class frigates (approximately 120m long) are the biggest jet-propelled vessels so far. Even these German-built vessels are capable of performing the Hamilton turn.
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A conventional screw impeller accelerates a large volume of water by a small amount, 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, pumping a small volume of water and accelerating it by a large amount delivers the thrust. The acceleration of the water is achieved by using either single or multiple impeller stages. Steering is accomplished by a movable nozzle at the outlet of the turbine, or less commonly, small vanes that direct the water jet. Because the jet boat relies on the flow of water through the nozzle for control, it is not possible to steer a conventional jet boat without the throttle open.
Unlike hydrofoils, which use underwater wings or struts to lift the vessel clear of the water, jetboats normally plane across the water surface, 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, greatly enhancing speed and maneuverability, so jetboats are normally operated at planing speed. Any slower, the jetboat loses most of its maneuverability and quickly slows down even more 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. At speed, jetboats can be safely operated in less than 3 inches to (7.5 cm) of water.
The jet of water is expelled above the water line, contrary to many people's intuition. Hamilton discovered early on that this greatly improved performance, compared to expelling below the waterline. From a physical point of view, this is understandable because it is the momentum of the expelled jet which provides the equal and opposite thrust. By expelling below the waterline, the momentum is rapidly absorbed by the surrounding stationary water; the jet does not "push against" the surrounding water.
Queenstown, New Zealand, where jetboats are used extensively for adventure tourism, claims to be the jetboat capital of the world.
- Westend marine, Wakefield Uk.http://www.westendmarine.co.uk/
- The New Zealand company Hamilton Jet designs and builds jetboats and manufactures the "Hamilton Jet Unit", the patented device that produces and controls the water jet. Jetboats owned by private citizens are normally powered by one or two V8 petrol engines.
- Sword Marine
- Bombardier of Canada makes many models of both jetboats and personal water craft (PWCs).
- Yamaha makes many models of both jetboats and personal water craft (PWCs).
- Cabora builds lightweight, high-powered jetdrive inflatables also known as BlowBoats.
Applications for jetboats include adventure tourism, surf rescue, farming, fishing, marine law enforcement, exploration, pleasure boating, and other water activities where motor boats are used. Jetboats can also be raced for sport, both on rivers and on specially designed racecourses. Many jetboats are small enough to be carried on a trailer and towed by car. Recently there has been increasing use of jetboats in the form of rigid-hulled inflatable boats and as luxury yacht tenders.
One very important feature of the jetboat is the fact that it has no external rotating parts; it is essentially safe for swimmers and marine life, excluding being struck by the hull. This in itself can be reason enough to use this type of propulsion.