Editing Jetboat (section)

==Function==
[[Image:Rogue jetboat M D Vaden.jpg|thumb|right| Jetboat on the Rogue River by Grants Pass, Oregon]]

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 [[acceleration|accelerating]] 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 [[Isaac Newton|Newton]]'s [[Newton's laws of motion|third law]]— every action has an equal and opposite reaction.

In a jetboat, the waterjet draws water from beneath the hull, where it passes through a series of [[impeller]]s and [[stator]]s – known as stages – which increase the velocity of the waterflow.  Most modern jets are single-stage, while older waterjets may have as many as three stages. The tail section of the waterjet unit extends out through the [[transom (nautical)|transom]] 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 the flow of water through the nozzle for control, it is not possible to steer a conventional jetboat without the engine running.

[[File:Shotover Jet, Jet Boating the Shotover River Canyons, Queenstown, New Zealand.jpg|thumb|left|A jetboat on [[Shotover River|Shotover Canyon]] in [[New Zealand]], the country for which jetboats were originally invented]]

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 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.

Unlike [[hydrofoil]]s, which use underwater [[wing]]s or struts to lift the vessel clear of the water, standard jetboats use a conventional [[Planing (sailing)|planing hull]] 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 [[planing (sailing)|planing]] 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 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&nbsp;cm (3&nbsp;inches) of water.

[[File:Whirlpool Jet Boat Tours in Devil's Hole Rapids in Niagara River Gorge.jpg|thumb|260px|A jetboat powers through the rapids of [[Niagara Gorge]], near [[Niagara Falls]]]]
One of the most significant breakthroughs, in the development of the waterjet, was to change the design so it expelled the jetstream ''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, while also providing a "clean" hull bottom (i.e. nothing protruding below the hull line) to allow the boat to skim through very shallow 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.{{citation needed|date=April 2014}}