If you have ridden on a Fast Ferry (one that runs at speeds that may reach 50 knots), operated a "JetStick-controlled Hinckley Picnic Boat, run an outboard with no visible propeller or driven a Personal Watercraft (PWC), you have been water-jet propelled.
Today's water jets are the end result of developments that date back to the first century B.C., when the Greek mathematician and scientist Hero of Alexandria built his aeolipile to demonstrate a principle of jet propulsion. (That's force = mass x acceleration to all you Top Gun heroes.) William Hamilton (later, Sir Hamilton) developed the modern water jet in New Zealand in the early 1950s by modifying an American invention, the Hanely water jet, relocating the jet nozzle so that the stream of fast-moving water was expelled above the waterline, thereby eliminating all drag-inducing underwater appendages. Location was everything-the boat took off.
A water-jet drive provides a number of advantages when compared with the conventional screw propeller. The absence of propellers, prop shafts, support brackets and rudders reduces drag, making jet drives particularly efficient at medium and high planing speeds. Since the drive train can be contained entirely within the hull, a jet-drive boat can, when necessary, operate in water only a few inches deep. The water jet's ability to propel a hull without an exposed propeller is what makes the PWC practical and can be a significant safety factor for people swimming near a boat, as well as for marine life.
Jet drives steer the boat with vectored (aimed) thrust, providing unmatched maneuverability, including the ability to precisely hold a boat in a fixed position against the effects of current and wind. The jet drive avoids the asymmetric thrust produced by propellers mounted on inclined shafts. The vectored thrust capability of a jet drive also provides efficient astern thrust (typically 60 percent of forward thrust) and instantly available reverse thrust for very rapid braking at any speed. The load imposed on the driving engine does not vary with boat speed or wave impact, protecting the engine from possible overspeed or overload. In high-speed applications jet drives can be quieter and smoother than propellers.
Today's yacht may depend entirely on water-jet propulsion or it may use a conventional propeller drive with a jet providing an auxiliary power source for high-speed dash operation. One of the notable examples of this application was seen in the yacht Fortuna, built by Palmer Johnson for King Juan Carlos of Spain in 1979. Twin diesel-driven props were augmented by a gas turbine-powered water jet to propel this 100-foot yacht to more than 50 knots. In such installations the jet nozzle is fixed in position to deliver forward thrust. Water jets intended for use over the entire speed range of the vessel are fitted with steerable discharge nozzles and flow diverters to provide the vectored thrust needed for maneuvering. Some yachts are propelled by three jet drives, the two outboard units equipped with thrust vector control for steering, and the center fixed jet used when maximum speed is required.