CRUISESHIP TECHNOLOGY Podded propulsors: learning from experience


A November technical presentation at the Institute of Marine Engineering, Science and Technology by John Carlton, of Lloyd’s Register*, and attended by The Naval Architect, outlined some of the problems experienced by high-power podded propulsors – partly, the author suggested, resulting from too rapid a rise in power output. He proposed ways in which operators could achieve the designer’s intention and how a unit could be correctly maintained to the best advantage.


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ZIMUTHING propulsors have revolutionised some sectors of our industry,


most notably cruise ships, where they are now commonly fitted to many new vessels. As many readers will be aware, it has not been roses all the way, despite the fact that azimuthing technology has been known since 1878. The mini crisis has largely arisen as a result of a massive jump in output during the latter half of the 1990s allied to the inclusion of an integral motor in the hub; up till then, powers generated had been quite modest, being mainly in operation on smaller types of ship such as ferries and workboats using mechanical Z-drive configurations. As early as 1992, Finnyards – as the company


then was – was able to install twin 7500kW Aquamaster Rauma mechanical steerable thrusters in the new dual-role icebreaker Fennica and her sister. However, that volume of power was probably around the maximum for mechanical trains. From that point, in association with integral electric motors, the power curve increased rapidly – perhaps too rapidly, believes Mr John Carlton, from Lloyd’s Register, although


Loads on pod components are a most important aspect of design and operation. Shown here is a typical variation in thrust generated at different azimuthing angles as a propeller rotates through one revolution. This should be contrasted with nearly constant thrust and torque signature produced at a zero azimuthing angle.


he was careful not to criticise individual designs, and indeed experience has apparently shown no special distinction between models. For the record, the success of ABB’s Azipod


design with icebreaking tanker retrofits, followed by a trial installation of two 14,000kW ABB Azipods on Carnival’s Elation (a member of an already successful series of cruise ship hulls, the Fantasy class) boosted prospects and led to larger units and other designers entering the market; these included Alstom/Kamewa with its Mermaid design; two 19,500kW Mermaid units went to sea onMillennium in 2000 and four Mermaid pods onQueen Mary 2 (4 x 21,500kW) in 2003. Such rapid increases in power have led to


* ‘Podded propulsors: some results of recent research and full-scale experience’, by J S Carlton, from Lloyd’s Register, presented on November 7, 2006 at the Institute of Marine Engineering, Science and Technology, London. Readers may also like to know that two


associated papers, both by Wendy Ball, from QinetiQ, and John Carlton, were, at the time of writing, in the process of publication, to appear in the Journal of Maritime Engineering as transactions of The Royal Institution of Naval Architects. These are: ‘Podded propulsor shaft loads from free- running model experiments in calm-water and waves’, and ‘Podded propulsor shaft loads from model experiments for berthing manoeuvres.’ Further, readers may like to know that


John Carlton and N Rattenbury presented a paper at the T-Pod conference, held on October 3-5 2006 in Brest, France; this was entitled ‘Aspects of the hydro-mechanical interaction in relation to podded propulsor loads’.


THE NAVAL ARCHITECT FEBRUARY 2007


several technical problems, including some early seal failures (seal problems generally show a very low incident rate). To investigate the causes of these, and particularly the loads on various components, Lloyd’s Register undertook a major study, although several national and international programmes had been completed years earlier. The results have been used to update that society’s propulsor rules, originally published in 2003, and to develop procedures for surveys which embrace the practical problems of initial manufacture and assembly as well as drydocking maintenance activities – the latter, Mr Carlton notes, can sometimes take place in very undesirable conditions. Hydrodynamists and metallurgists would


have found Mr Carlton’s presentation highly instructive; many of the problems seem to be common, regardless of manufacturer, however, mechanical failure numbers appeared to be higher than electrical ones. Possibly the most important lesson, suggested the speaker, was that designers perhaps lost track of industry experience in the rush to build ever-larger units. A factor of great interest for twin-pod and


quadruple-pod designers was the interaction that occurred at large manoeuvring angles and the


increased mechanical and bearing loads that can result at these angles – different for each pod. Sea trial manoeuvring programmes should therefore be carefully planned. Crash-stop operations must also be carefully considered, likewise sailing in poor weather when shaft bending moments can be significant. Mr Carlton said that CFD techniques can help in these matters but were not totally reliable, and more effort was still needed to examine the scale effect. Other problems have occurred with roller


bearing raceways in the thrust bearings (damage can be partly generated by bearings having to absorb bending moment loads while a pod is turning) when failure damages such as morphology, cracks, and so-called butterflies were apparent, including subsurface cracking. Normally, sterntube bearings can be expected to last out a ship’s life, but roller bearings in a pod probably have to be changed after 65,000hours. Here, correct lubrication – allied to careful regular monitoring of oil quality – and avoiding the creation of debris is considered of the utmost importance. In connection with cleanliness, it is most


interesting to note that ABB, designer of the Azipod, has built special enclosures for wrapping about pods during drydock maintenance, so that work can continue in a clean atmosphere. An innovative technique that could be useful in detecting impending bearing failure is what is known as acoustic emission technology; however, care has to be taken to avoid confusion with other noise such as that generated by cavitation and boundary layer turbulence. Finally, Mr Carlton predicted that in


approximately five years’ time, design and manufacturing technology will have caught up with the dramatic service experience of the last few years. Oh, and he also noted that pods on icebreakers were much stronger than those on cruise ships. Maybe there is a lesson to be learned there.


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