Jetboat

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Jetboats move by blasting a jet of water from the back of the boat. Instead of a propeller in the water, a jetboat takes water in from under the hull, pushes it through a pump inside the boat, and expels it through a nozzle at the stern. The modern jetboat was developed in the mid-1950s by New Zealand engineer Sir William Hamilton to run in shallow rivers that were too shallow for propellers.

Waterjets have proven useful for many vessels, from ferries and rescue craft to patrol boats and high-speed ships. Jetboats are highly maneuverable and can stop quickly, sometimes within their own length, in a move called a crash stop. A famous maneuver, the Hamilton turn or jet spin, involves cutting the throttle, turning the steering sharply, and reapplying throttle to spin around with a spray of water.

Steering is done by deflecting the jetstream with the nozzle or small side gates. To go in reverse, a reverse deflector redirects thrust forward instead of backward. Because steering depends on the jetflow, a jetboat needs the engine running to steer, and many jets can hold a position or “hover” in place by balancing forward and reverse thrust.

Jetboats use a conventional planing hull, riding on top of the water. They work best at higher speeds and in shallow water, and, when moving fast, can operate in surprisingly little depth. A crucial breakthrough was placing the jet outlet above the waterline, which reduced hull drag and improved performance in shallow water.

Jet propulsion isn’t limited to small craft. Very large jet-powered ships exist, including some military frigates and fast ferries. However, at low speeds, traditional propellers can be more efficient, while waterjets excel at high speed, shallow water, and quick maneuvering.

Applications include passenger services, coastguards, police, navy ships, adventure tourism, pilot boats, rescue operations, fishing and exploration, and racing on rivers or purpose-built tracks. Jetboats can be small and trailerable, and their lack of external rotating parts makes them safer for swimmers and marine life, though debris can still pose hazards. They are sensitive to how well the engine and jet unit match, and disruptions to water flow can reduce efficiency.