Azimuth Thruster Propulsion Systems

Author

A.N. Other

Subjects

Ship design and development, Naval technology

Tags

Warship design

RAN Ships

HMAS Choules

Publication

December 2012 edition of the Naval Historical Review (all rights reserved)

By Warrant Officer Hugh Johnson

One of the enduring features of naval engineering is a desire for reliability, which may lead to conservatism. In some respects warship design has changed slowly in the past century whereas our merchant cousins, driven by commercial imperatives, have progressed much more rapidly in the propulsion systems now largely employed in merchant ships and gaining acceptance by naval shipbuilders.

Azimuths and Pods

The first azimuth thrusters, where transmission is sent directly to a pod fitted beneath the hull containing the drive and propeller, were introduced into small craft in the 1950s. In essence this eliminates traditional shafts and gearboxes and, as the pod can rotate on its axis in any horizontal direction, it also eliminates the need for rudders. These systems were introduced to harbour craft and then gradually to coastal ships. It was the passenger ship which had all but died in the 1970s, when the cruise industry evolved, that created a market for much larger and less expensive engineering plants, giving rise to a new generation of azimuth thrusters.

Azimuth thrusters were first installed in cruise ships in 1998 and are now the accepted choice of propulsion system for all new construction. Three engineering companies which dominate the market in supplying these systems are: ABB of Finland which uses the brand name of ‘Azipod’, Britain’s Roll-Royce with its ‘Mermaid’ series and the German consortium of Siemens-Schottel.

Azipod and Mermaid are similar, using a single forward facing propeller which pulls the ship through the water. Siemens-Schottel uses two propellers, one at the front and another at the rear of the pod, with a pull and push effect which is said to be more efficient. Earlier azimuth propulsion systems placed the motor inside the ship’s hull and linked it to the propeller through shafts and gearboxes. In later systems the electric motor is inside the pod with the propeller connected directly to the motor shaft. Because fixed pitch propellers are used, the power to the system is fed through a variable frequency drive that allows speed and direction control to the propeller motors.

Power Train

The azimuth thrusters system comprises an electrical power plant, switchboard, transformers, frequency converters and propulsion motors. The transformer and converter are placed in a shipboard pod room with the motor in the pod. A slip ring is used to transfer electrical power from the converter to the freely rotating pod. The system usually consists of two sets of pods with exactly the same configuration but independent of each other. Originally DC motors were used for propulsion but in nearly all cases these have been replaced by AC drives. The marine application of the electric motors has been greatly influenced by the development in frequency converters which makes it easier to control the speed of the motors. Transformers are used to divide the system into several parts in order to obtain different voltage levels but also for phase shifting. Frequency converters control the speed and torque of the motor by changing constant frequency into variable frequency. An electrical motor is used for conversion of electrical power to mechanical power; the motor usually used is known as a Synchronous Motor because of its efficiency in the high power range.

While azimuth thrusters are now an accepted means of propulsion in many types of ships they are not without critics. A number of cases have arisen when ships have had to unexpectedly dry dock to repair bearings failing under load. In one celebrated case in 2006 the Queen Mary 2, which has four Mermaid pods (two rotating azimuths, two fixed), struck an underwater object, damaging one of the fixed pods which had to be removed for repair. However she continued operations for the remainder of the year using three pods and was still able to make 27 knots, in lieu of her normal maximum speed of 29 knots.

Benefits and Disadvantages

The major claimed benefits derived from azimuth thrusters are:

• Increased efficiency with lower fuel consumption
• Excellent manoeuvrability, usually eliminating the need for tugs
• Space saving and compact design
• Quiet and low vibration operation
• Standardisation of components which are available worldwide

A negative is that pods are placed quite low, in some instances increasing draught and increasing the risk of possible damage from striking underwater objects. While now used extensively, the technology in naval terms has yet to be completely proven.

Impacts to the RAN

This new propulsion system came to the RAN almost by accident when a sudden lack of amphibious capability resulted in the acquisition of the Bay class Royal Fleet Auxiliary Largs Bay, which was commissioned as HMAS Choules in December 2011. The Bay class are the first large British MOD vessels built without conventional propulsion systems of propellers and rudders. They are powered by a diesel-electric system comprising Wartsila diesels and Siemens electric motors which drive two John Crane Lips Defence azimuthing stern thrusters and a twin propeller Brunvoll bow thruster. They also have a Brunvoll D-P (Dynamic Positioning) system. However the purists could argue that the RAN has already accepted this technology with the introduction of the Adelaide class FFGs. Although these vessels are only fitted with a single propeller they also have a two retractable Auxiliary Propulsion Units (APUs) which are capable of being trained through 360 degrees. The APUs can be used as auxiliary propulsion or for manoeuvring.

Almost in the same timeframe as the acquisition of Choules a second acquisition was made of an offshore support vessel which entered naval service in June 2012 as Australian Defence Vessel (ADV) Ocean Shield. These ships will provide a much needed training ground for the new LHDs being built for the RAN, based on the Spanish Juan Carlos I class. The hull of the lead ship (Canberra), which was built in Spain, was transported by heavy lift ship to Williamstown, Victoria, and berthed on 17 October 2012. The fitout is being undertaken by BAE Systems Australia with Canberra expected to enter service in 2014.

There has been considerable publicity surrounding a failure of a transformer set on board Choules, the reasons for which remain undetermined. Replacement machinery has been ordered and repairs are expected to be shortly completed.

The technology in the production of azimuth thruster propulsion systems now being introduced into the RAN is largely taken from commercial applications which have been adapted to marine propulsion. As technological development continues and with improved practical knowledge in their application these should prove of significant benefit to the fleet.