CRUISE LINER TECHNOLOGY Operational aspects of manoeuvring with pods
Podded propulsion has the potential to give many benefits,but can its advantages be exploited in the operational environment? Simon Burnay, manager - manoeuvring simulation products,at BMT SeaTech Ltd, a subsidiary of BMT Ltd, examines a recent suggestion for improving manoeuvring performance through the novel use of pods and looks at some of the practical aspects in relation to a master or pilot's perspective.
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HE advent of podded propulsion systems - such as Azipod or Mermaid - has the potential to provide ships with many benefits, including fuel economy, greater internal space, and improved manoeuvrability. Improved manoeuvrability is achieved by the ability to direct the thrust of the propeller through 360deg and as such, greater forces can typically be applied when compared with conventional propeller-rudder-thruster configurations. A typical 'pod' can generate very large thrust forces in any direction, giving an efficient means of providing control forces. However, where there are large forces, there is potential for problems, and as such, careful consideration must be given to the application of these thrust forces in relation to other factors such as the speed of the ship, engine shock loading, and possible structural implications. There has been a recent suggestion for an alternative - and perhaps novel - method of using pods to reduce the stopping distance of a ship in emergency manoeuvres, such as a crash stop. The idea is to simply rotate the pods to180deg (so they are pointing astern) and use ahead revolutions with pods reversed to provide net astern thrust. Because thrust from the pod is directional, and noting that the thrust due to ahead revolutions is greater than that generated by the equivalent astern revolutions, it is reasonable that a reduction in stopping distance is possible by rotating the pods to 180deg. From a pure hydrodynamic perspective, this would be true: the resultant astern thrust is greater and it would be quicker to rotate the pods in this manner than reduce the revolutions from ahead and build the astern power thereafter. However, to control the large forces generated by the pod, a ship is normally provided with a complex control system that assists in reducing noise and vibration and provides restrictions on:
• the power available to each pod • the angle through which the pod can be turned
• whether the pods may be rotated individually or are synchronised
• engine load management defining the response of the propeller revolutions.
These controls have significant impacts on a suggested manoeuvre and are described in the following sections.
Mode of operation
As part of a podded propulsion installation, there are normally two distinct modes of operation that broadly relate to open-sea (high speed) and manoeuvring (low speed) usage. Each mode has a
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Fig 1.Typical limitations with pod operation modes (vessels may have alternative configurations).
Fig 2. Typical normal operating and emergency engine load management programs, when applied to a 'podded' ship with diesel-electric propulsion.
different set-up that maximises the efficiency of pod control and ensures safe operation. The open-sea mode, often called open sea, cruise or transit, typically allows full power to be applied to each pod but limits the steering angle of the pods to 35deg. In this mode, the pods are linked, and steering is achieved through a central helm wheel or autopilot. The open-sea mode is therefore used at higher speeds in open water where maintaining a course is the most important factor. Manoeuvring modes typically limit the power to 40%-60% of the maximum and allow the pods to be rotated individually through 360deg once the ship's speed is less than a predefined value (typically 10knots). The 'manoeuvring' mode is
therefore normally used only at slower speeds and in close-quarters manoeuvring situations. Fig 1 shows typical operational limitations placed on a pod according to the mode of operation (open sea or manoeuvring). These limitations are designed to limit the loading on the pod stock and the ship's hull. If the pods are rotated to angles greater than 35deg (once the speed has reduced), then the power is automatically limited to avoid structural damage caused by large thrust loads at high azimuth angles.
Therefore, the suggested crash stop manoeuvre of rotating the pods to 180deg whilst employing ahead revolutions would incur an immediate power limitation as 'manoeuvring' mode is
THE NAVALARCHITECT FEBRUARY 2006
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