Seeds of potential in pods

Podded propulsion has proven itself and is winning converts. When the chairman and chief executive officer of the world?s largest cruise operator, Carnival?s Micky Arison, says that all its future newbuildings will have podded propulsion ? that is a big vote of confidence. Festival and Radisson have joined the pod club, and American Classic Voyages? newbuildings will also use azimuthing thrusters. Grafting podded propulsion onto existing hull designs has given propulsion efficiency gains of around 8%, and an optimised hull design may offer even better prospects. Pods also offer the prospect of ?dynamic positioning? for cruise ships, utilised by the offshore industry, and solve the spate of problems with sterntube bearings. In addition the oil seals and propeller end shaft bearings can be changed from outside the pod, while the pod itself can be detached, usually while afloat, if major work is required. The cruise industry is already sold on diesel-electric propulsion, so is a relatively easy convert for podded propulsion ? but other operators can see advantages too. Ferry operators are also benefiting from diesel-electric drive as the engines can be moved to areas where they do not obstruct the flow of vehicles or cargo on and off the vessel. Having made this move it is but a short step to using podded propulsion, as is the case with TT-Line. The engines are situated in the double hull and with the pod motors outside the hull, there is full access to the rear of the vessel. However, podded propulsion offers additional options for other standard and specialised tonnage. Wartsila NSD is carrying out detailed studies on the use of a pod behind a traditional shaftline. This gives the possibility of increased propulsion power, lower propeller loading, and the efficiency gains offered by contra-rotating propellers. While this would open up new areas for even bigger or faster container ships, which are currently limited by engine output, it also adds redundancy to the propulsion system which could be attractive to other vessel types ? especially tankers. Existing ships with contra-rotating propellers systems show superior fuel efficiency of around 15%, but the additional cost of a second propeller, along with the complex gearing and sealing arrangements have deterred other owners. One example of the retrofitting possibilities offered by podded propulsion is the recent contract for stretching Costa Classica at Cammel Laird in the UK. The 1991 built ship is to have a 44.8m section added in the centre, along with an additional deck. Currently the ship has a diesel-electric drive with twin propellers rated at 10.5MW (14,275 bhp) but this would leave the service speed too low on the enlarged vessel. To counter this an 8MW (10,875 bhp) propulsion pod will be added on the vessel?s centreline to give a service speed of 23 knots. Owners of newbuildings other than cruise ships are also looking to utilise the flexibility offered by podded propulsion. As a business decision it is clear that pods add some cost, but their use removes the need for a rudder and stern thrusters, where fitted. Stolt has led the move to diesel-electric propulsion for chemical tankers as it gave an increase in payload within the same hull lines. However, Donsotank has taken the next step by specifying a 5.1MW podded drive from SSP for its 19,500 dwt vessel under construction in China. Also being built in China are two semi-submersible heavy lift vessels that will use SSP propulsion. This gives the benefit of increased manoeuvrability and allows the aft end to be completely flat, giving maximum flexibility for loading. As there is no need for long drivelines through the hull this minimises void spaces, increasing the floodable areas and therefore the depth to which the vessel can submerge. In making the selection COSCO and HaCe Marine considered the attributes of the four podded drives currently on the market: Azipod, SSP, Dolphin and Mermaid.