Jetboat

Previous attempts at waterjet propulsion had very short lifetimes, generally due to the inefficient design of the units and the fact that they offered few advantages over conventional propellers.

The vast majority of waterjet units are therefore installed in high-speed vessels and in situations where shallow draught, maneuverability, and load flexibility are the main concerns.

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.

In a jetboat, the waterjet draws water from beneath the hull, where it passes through a series of impellers and stators – known as stages – which increase the velocity of the waterflow.

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.

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.

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.

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.

Jetboats can also be raced for sport, both on rivers (World Champion Jet Boat Marathon held in Mexico, Canada, USA and New Zealand[1]) and on specially designed racecourses known as sprint tracks.

In 1977, Sir Edmund Hillary led a jetboat expedition, titled "Ocean to Sky", from the mouth of the Ganges River to its source.