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Are propelled by water drawn in by a centrifugal water pump and then expelled again through a steerable nozzle above the waterline. The discharge of a high-velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft's hull, propelling it forward. The first Seacats were propelled in this manner.

The principle is an old one. It was first patented by two Englishmen in 1661 who proposed using a bellows to force out water as a means of propelling a vessel forward. With the coming of steam propulsion the number of experiments increased, but none was particularly successful. However, in 1785 an American, James Rumsey of Virginia, built a boat with a steam pump which drew water in at the bows and forced it out at the stern, and took a voyage on the Potomac River in it.

In the modern jetboat, the jet unit is mounted inboard in the aft section of the hull. Water enters the jet unit intake—via a vent flush in the bottom of the boat—at boat speed, and is accelerated through the jet propulsion unit and discharged through the transom at a high velocity. Reverse is achieved by lowering an astern deflector into the jetstream after it leaves the nozzle.

This method, which has proved highly successful, was the invention of a New Zealander, Sir William Hamilton (1899–1978), who began experimenting with marine jets in the early 1950s, following the lead of the American Hanley Hydrojet. Hamilton's breakthrough came when he modified the Hydrojet so that the jet stream was expelled above the waterline—as with some Seacats—instead of below it. This design eliminates all underwater appendages, which makes it ideal for travelling over very shallow, fast-running water.

Hamilton's first jetboat had a 3.6-metre (12-ft) plywood hull with a 100 E Ford engine which is now in the Auckland Maritime Museum. Improvements in the pumping system followed, and by the 1990s waterjets were being built for craft up to 60 metres (197 ft) and for ones with speeds of 60–65 knots.

Nowadays jetboats are widely used for flood relief, hydrography, and recreation. They are also employed as patrol boats and fishing vessels, and the jet propulsion units have been sold to many navies.

Subjects: Maritime History.

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