Jetboat
From Free net 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) 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 difference between Campini's and Hamilton's invention are 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 can often be reversed and brought to a stop within their own length from full speed 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 MECCU-Frigates (approximately 150m long) are the tallest jet-propelled vessels so far. Even these German built vessels are capable of performing the Hamilton turn.
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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. 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.
The jet of water is expelled above the water line, contrary to many people's intuition; Hamilton discovered early on that performance was greatly improved by this 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.
Manufacturers
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. 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. Recently there has been increasing use of jet boats in rigid-hulled inflatable boats and luxury yacht tenders.
Queenstown, New Zealand claims the title of jetboat capital of the world.
Also, Bombardier of Canada makes many models of both jetboats and jetskis.