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reverse thrust. Most highly developed reverse deflectors redirect the jetstream down and to each side to prevent recirculation of the water through the jet again, which may cause aeration problems, or increase reverse thrust. Steering is still available with the reverse deflector lowered so the vessel will have full maneuverability. With the deflector lowered about halfway into the jetstream, forward and reverse thrust are equal so the boat maintains a fixed position, but steering is still available to allow the vessel to turn on the spot – something which is impossible with a conventional single propeller.
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water. It makes no difference to the amount of thrust generated whether the outlet is above or below the waterline, but having it above the waterline reduces hull resistance and draught. Hamilton's first waterjet design had the outlet below the hull and actually in front of the inlet. This probably meant that disturbed water was entering the jet unit and reducing its performance, and the main reason why the change to above the waterline made such a difference.
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by lowering the reverse deflector slightly and increasing throttle – so an operator may increase thrust and thus control without increasing boat speed itself. A conventional river-going jetboat will have a shallow-angled (but not flat-bottomed) hull to improve its high-speed cornering control and stability, while also allowing it to traverse very shallow water. At speed, jetboats can be safely operated in less than 7.5 cm (3 inches) of water.
177:, Hamilton had a specific need for a propulsion system to operate in very shallow water, and the waterjet proved to be the ideal solution. The popularity of the jet unit and jetboat increased rapidly. It was found the waterjet was better than propellers for a wide range of vessel types, and waterjets are now used widely for many high-speed vessels including passenger ferries, rescue craft, patrol boats and offshore supply vessels.
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the water line, contrary to many people's intuition. Hamilton discovered early on that this greatly improved performance, compared to expelling below the waterline, while also providing a "clean" hull bottom (i.e. nothing protruding below the hull line) to allow the boat to skim through very shallow
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speed. At slower speeds with less water pumping through the jet unit, the jetboat will lose some steering control and maneuverability and will quickly slow down as the hull comes off its planing state and hull resistance is increased. However, loss of steering control at low speeds can be overcome
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of the hull, above the waterline. This jetstream exits the unit through a small nozzle at high velocity to push the boat forward. Steering is accomplished by moving this nozzle to either side, or less commonly, by small gates on either side that deflect the jetstream. Because the jetboat relies on
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Another disadvantage of jetboats appears to be that they are more sensitive to engine/jet unit mismatch, compared with the problem of engine/propeller mismatch in propeller-driven craft. If the jet-propulsion unit is not well-matched to the engine performance, inefficient fuel consumption and poor
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Unlike conventional propeller systems where the rotation of the propeller is reversed to provide astern movement, a waterjet will continue to pump normally while a deflector is lowered into the jetstream after it leaves the outlet nozzle. This deflector redirects thrust forces forward to provide
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There is no engineering limit to the size of jetboats, though whether they are useful depends on the type of application. Classic prop-drives are generally more efficient and economical at low speeds, up to about 20 knots (37 km/h; 23 mph), but as boat speed increases, the extra
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Applications for jetboats include most activities where conventional propellers are also used, but in particular passenger ferry services, coastguard and police patrol, navy and military, adventure tourism (which is becoming increasingly popular around the globe), pilot boat operations,
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a mass of water rearward. By contrast, a waterjet unit delivers a high-pressure "push" from the stern of a vessel by accelerating a volume of water as it passes through a specialised pump mounted above the waterline inside the boat hull. Both methods yield thrust due to
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A conventional screw propeller works within the body of water below a boat hull, effectively "screwing" through the water to drive a vessel forward by generating a difference in pressure between the forward and rear surfaces of the propeller blades and by
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to ride 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
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Previous attempts at waterjet propulsion had very short lifetimes, generally due to the inefficient design of the units and the fact that they offered few advantages over conventional propellers. Unlike these previous waterjet developments, such as
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The fuel efficiency and performance of a jetboat can be affected by anything that disrupts the smooth flow of water through the jet unit. For example, a plastic bag sucked onto the jet unit's intake grill can have quite an adverse effect.
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or "jet spin" is a high-speed manoeuvre where the boat's engine throttle is cut, the steering is turned sharply and the throttle opened again, causing the boat to spin quickly around with a large spray of water.
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Jetboats are highly manoeuvrable, and many can be reversed from full speed and brought to a stop within little more than their own length, in a manoeuvre known as a "crash stop". The well known
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held in Mexico, Canada, USA and New
Zealand) and on specially designed racecourses known as sprint tracks. Recently there has been increasing use of jetboats in the form of
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led a jetboat expedition, titled "Ocean to Sky", from the mouth of the Ganges River to its source. One of the jetboats was sunk by a friend of
Hillary.
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One of the most significant breakthroughs, in the development of the waterjet, was to change the design so it expelled the jetstream
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the flow of water through the nozzle for control, it is not possible to steer a conventional jetboat without the engine running.
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The biggest jet-driven vessels are found in military use and the high-speed passenger and car ferry industry. South Africa's
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are among the biggest jet-propelled vessels as of 2020. Even these vessels are capable of performing "crash stops".
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477:(Large print ed.). Leicester: Ulverscroft Large Print Books Ltd. p. 16.
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in the mid-1950s. His goal was a boat to run up the fast-flowing rivers of
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From the Ocean to the Sky: Jet
Boating Up the Ganges
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71:Learn how and when to remove this message
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447:"Race Division – Outlaw Eagle"
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522:Motorboats
516:Categories
456:31 January
433:References
283:hydrofoils
95:region of
93:West Coast
45:improve it
398:Drawbacks
343:position.
247:impellers
240:third law
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