WHEN the Queen Mary 2, the world’s largest passenger ship, sailed into Fort Lauderdale, Florida, at the end of its maiden transatlantic voyage, its sheer size was the big talking point. But equally impressive was the technology hidden beneath the waterline. Remarkably, the giant ship has no rudder to steer it. Yet it is so manoeuvrable that it was able to sail straight into harbour and dock without the aid of tugs.
Conventional ships are driven by propellers on shafts connected to an engine within the hull. But the QM2, like a growing number of large passenger ships, is propelled in a different way. Due to a progressive reduction in size, electric motors powerful enough to drive the enormous ship can now be housed in watertight pods suspended on arms beneath the stern. Four 20-megawatt motors each drive a propeller (see Graphic) with power coming from generators driven by diesel engines and gas turbines, located in an engine room in the hull. Together the pods can propel the ship at up to 30 knots (55 kilometres per hour). Two of the pods can rotate through 360 degrees, to steer the vessel.
One key advantage of this arrangement, known as podded propulsion, is that it improves the hydrodynamic profile of the hull. Anything that disturbs the smooth flow of water over the hull adds to the drag as the ship cuts though the water and creates noise. In a conventional ship, the propeller shafts are supported by brackets attached to the hull. These create turbulence in the water before it meets the propellers, which reduces their efficiency. In pod designs, however, the propeller can be mounted at the front of the unit, where water flow is relatively undisturbed.
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Pod drives were first used on icebreakers in the early 1990s. These work best if they can smash ice with both sterns as well as the bow. Pods allowed the propellers to be placed further forward, where they are far less vulnerable than conventional ones.
Naval architects soon spotted that the superior hydrodynamics, quietness and manoeuvrability provided by pods would have benefits for other kinds of ship, too, particularly those carrying passengers. And because the engine room delivers electrical rather than mechanical power, it no longer has to be in a direct line with the propellers, allowing more design flexibility. Although the pod system is more expensive than conventional propulsion at present, costs are expected to fall as the technology matures.
As well as cruise ships, there are now a handful of roll-on roll-off ferries with pods, and many more are likely to follow, as they are more fuel-efficient and quicker into port. “Ferries will be the next big market for pods,” predicts John Carlton of Lloyd’s Register, which undertakes structural surveys of many of the world’s ships.
The European Union is funding a €4 million research project, called Fastpod, to spread the technology to ferries, tankers and freighters. The researchers hope to develop a pod-propelled ferry with a top speed of 40 knots – nearly twice as fast as today’s ferries. “It’s well beyond today’s pod technology, but we think it’s reachable,” says Mehmet Atlar, who heads the Fastpod research programme at the University of Newcastle upon Tyne in the UK.
Trials carried out by Florida-based Carnival Cruises, the parent company of the QM2’s owner, Cunard, underline the advantages of pods. Carnival has eight cruise ships in its Fantasy class, of which six were built with conventional shaft propellers while the last two, built in 1998, had pods. Comparisons “showed a 7 per cent improvement in propulsive efficiency,” says Stephen Payne, Carnival’s chief naval architect and designer of the QM2. This translates into fuel savings.
The podded ships were also quieter. In a standard ship, the noise made by the propellers is transmitted up the drive shaft into the rear of the ship. “If you have the dining room aft, it’s very important to have as little noise as possible,” says Payne.
The trials also demonstrated the podded ships’ superior manoeuvrability. “You save about a third of the time needed for berthing – typically about 15 to 20 minutes,” says Payne. And you don’t need to pay for tugs.
Despite teething troubles with the pods such as leaking seals and seized bearings, the cruise-ship industry sees a bright future for podded propulsion. About 70 per cent of all cruise ships are now being built with pods. “We only build shaft propellers on old, established designs, where it is too much work to change the design,” says Payne.
