Azimuth thrusters are thrusters that can be rotated in order to produce thrust in any direction. The thrust is
controlled either by constant speed and CPP design, variable speed FPP design, or in rare cases with a
combination of speed and pitch control. Variable -speed FPP designs has a significantly simpler mechanical
underwater construction with reduced low-thrust losses compared to constant speed, CPP propellers.
In vessels with strict limitation of in-board height of the thruster room, the electric motor will normally be
horizontal, and the azimuthing thruster will then consist of a Z-type gear transmission. Due to a simpler
construction with less power transmission losses, vertically mounted motors and L-shaped gear transmission will normally be selected when the height in the thruster room allows for it.
A limitation of azimuth thrusters is their limited ability for producing thrust at negative pitch or RPM, because
they are designed and optimized for unidirectional thrust. If they have a certain degree of negative thrust
capability this should be utilized in order to maintain dynamic thrust capacity without performing continuous
azimuth rotation.
The conventional azimuth thruster was earlier used for station keeping and maneuvering, but has recently also
been taken in use as the main propulsion device in vessels with electric propulsion. In order to improve the
hydrodynamics and steering capability that is required for propulsion, the shape of the thruster has been adapted, such as the “mechanical pod”. This is an azimuthing thruster, which is powered from an in-board, typically a horizontal motor, and the mechanical power is then transferred to the propeller with a Z-shaped gear. The underwater shape is optimized for low hydrodynamic resistance at higher ship velocity, for higher propulsion efficiency.
Some vendors can supply thruster devices with dual propeller, either on the same shaft, or with contra-rotating propellers. Contra rotating propeller increases the hydrodynamic efficiency by utilizing the rotational energy of the jet stream from one propeller, to create thrust from the other that rotates the opposite direction.
Conventional azimuth thrusters are at present (2002) in use with power ratings up to 6-7 MW.
