A boat is designated as a jet boat when its means of propulsion is a stream of water jetted out from the boat. Some boats move atop the water by other methods of energy, such as wind, oars or mechanized propellers. Jet boats utilize a central pump system to draw water up from beneath the surface of the craft. The water is channeled through a designed route and then is expelled in streaming form through a nozzle. The nozzle is situated above the water's surface at the rear of the jet boat. Steering is largely accomplished through the repositioning of the jet stream.
The central pumping unit of a jet boat features a propeller system but not for propulsion through the water. The propeller action of a jet boat is specifically designed to draw water into the craft and force it into a powerful stream that moves the boat forward. Such propellers are properly designated as impellers and typically feature smaller blades and faster revolutions than standard propellers.
Developed in the early 1950s by New Zealander, Sir William Hamilton, as a shallow water propulsion solution, the jet boat has since undergone multiple transformations. The combination impeller and pump system have evolved into two basic types. The mixed flow is designed to maximize pressure and increase speed. It compresses the water by diminishing the diameter of the chamber plumbing, forcing the water to emit under pressure through a thin nozzle, resulting in greater speed. The axial flow type is designed to result in greater load carrying capacity. The plumbing chambers remain at a fixed diameter so that the emitting water does so at a greater volume than its mixed flow counterpart.
The engines used to power modern jet boats are available in both inboard and outboard configurations. In both instances, there is a measurable decrease in overall horsepower since anywhere from twenty-five per cent to thirty-five per cent (25-35%) is utilized to draw the water in, pump the water through, and emit the water out. For example, a one hundred and fifty horsepower (150HP) engine will produce the equivalent of one hundred horsepower to one hundred and five horsepower (100-105HP) at the point of emission (the nozzle).
Jet boats are not limited to popular recreational type vehicles. The jet boat design has been applied to racing boats, ferries and even military vessels. The increased maneuverability and quickness in starting and stopping have added to its growing popularity and use.