Propulsion – Azimuth Thrusters

Azimuth thrusters are electric podded drive systems, which are mounted on a 360 degree rotating shaft under the ship. There is no rudder required in an azimuth system and hence, the underwater dynamics are improved which not only increases manoeuvrability, but azimuth thrusters also include the advantages of combined engine systems. Tugs and supply boats, where precise directional control is required, often in tight spots, were the first proper ships that were fitted with azimuthing systems, where the propeller could be rotated around a 360 degree horizontal axis.

This required some quite complex shafting and bearings with the propulsion shaft having to be aligned from a horizontal axis, to a vertical axis (around which the unit rotates) to the horizontal axis of the propeller itself. The principle of what might be called “dynamic” positioning, with total control of a vessel’s movements, independent of external assistance from tugs is very attractive to cruise ship operators, and thus has emerged the development of the “podded” propulsion unit, which are very large azimuthing propulsion systems in which the whole propulsive power of big ships can be fully directional.

Cruise ships often have to get into confined ports where tug assistance is not necessary, or even lay off a port where the water is too deep to anchor. The total manoeuvrability which is available from azimuthing ?EURoepods?EUR? is hugely appreciated. Moreover, the giant rudders and separate steering gears which would normally be fitted to such ships are no longer necessary, and it may be possible to dispense with some of the lateral thrusters which would be required for a conventionally propelled cruise ship.

There are two major variants, based on the location of the motor:

1. Mechanical transmission, where the motor is inside the ship and power gets transmitted to the propeller via bevel gears. The motor may be either a directly mechanically connected diesel engine, or an electrical motor getting its power from generators run by an electricity producing engines (usually diesel engines) elsewhere in the ship.

2. Electrical transmission, where the electrical motor is in the pod itself, directly connected to the propeller without gearboxes. The energy used to drive the motor in the pod is produced by machinery inside the vessel, usually by diesel engines or gas turbines which drive electric generators, in a system comparable to that used by diesel-electric locomotives.

Invented in 1955 by Mr. F.W. Pleuger and Mr. Friedrich Busmann (Pleuger Unterwasserpumpen GmbH), ABB Azipod was the first product using this technology. The Azipod propulsion system improves the steering behaviour. It can be built for pushing or pulling operation, and low or high speeds. When Azipod units are installed in the stern and bow, the ship can do sideways manoeuvres (or azimuthing) the ship can go astern by changing the direction of rotation of the propeller or by rotating the azimuth angle 360 degrees. Additional manoeuvrability may be attained by changing the azimuth elevation. Using azimuth thrusters, the crash stop distance can be halved compared to conventional propeller systems.

sources: Ship Technology, BIMCO