Hydrojet

A hydrojet (also called by its English name toilet-jet or waterjet ) is a maritime system of Propulsion: water is pumped under the Bateau then expelled at high-speed behind this one thanks to a turbine at high-speed of rotation. This system is effective at high-speed (more than 25 nodes) and is used on the ferries with high speed, on the boats of service to engines such as boats controls and on the Jet Ski S (Scooter of the sea).

The hydrojet is presented in the form of a kind of ducted axial flow pump (turning inside a conduit).

the entry of water is done under the hull of the ship,

a bent conduit brings water to the pump fixed at the back table of the ship,

the jet of water is accelerated by the pump,
the exit of the water jet is done with the free air, in the dynamic base of the back table:

at high speed the filaments of water almost horizontally prolong the shape of the hull; the back table is not any more in water.

the direction is obtained by deviating the jet laterally, behind from the pump,
step back is obtained by deviating the jet forwards using a removable deflector.

Certain models include an orientation system of the jet, which makes it possible to control the direction of the ship even at low speed, an advantage compared to the traditional system Hélice and saffron. It is also possible to reverse the direction of the jet in order to go into reverse; however, the power of the jet directed towards the bottom can lead to a fast erosion of this one and to environmental problems.

Compared to the traditional system: , chair propeller shaft of tree, Propeller, Rudder, this system propelling does not require any body external with the hull generator of trail.

In addition the containment of the propeller in a conduit decreases the losses of output related to the free flow in blade tip. The propelling output of the hydrojets is generally higher than that of the traditional propellers, especially at high speed (more than 25 nodes). Beyond 30-35 nodes, the problems of cavitation of the propellers impose the hydrojets or possibly (and more rarely) the propeller of surface.

The absence of Gouvernail is an advantage for the reduction of the trail and the reduction with the draft, but it is a disadvantage for the stability of road (loss of surface stabilizing with the back of the ship).

Primary source

Prof Anthony Molland, Ship Resistance & Propulsion , notes of course, M.Eng Ship Science , University of Southampton

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