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Podded drives have been developed and successfully introduced in the marked during the last two decades. Despite of some problems of single components, podded drives have proven their suitability and capability as the main propulsion and steering device. They are used mostly in the segment of cruise liners and ferries which need especially at low ship speed a good manoeuvrability. The installed power ranges from 5 up to about 20 MW per module. The most spectacular ship with podded drives will be launched this year: the Queen Mary 2 with four pods totalling about 80 MW driving power. The ship with a length over all of 345 m will have a speed of 30 kn. In contradiction to conventional rudder propellers, podded drives produce the lateral steering force not only by changing the propeller jet direction, but also by using a considerable area of the shaft like a conventional rudder. In this respect the pod can be regarded as the Sphinx of steering. Traditional rules can not be applied on a pod due to the mixed nature of steering device. Consequently, new approaches have to be developed based on CFD calculations and mechanical considerations. Since the traditional rules incorporate decades of positive experience a new approach should be based on traditional formulae or at least should show a connection or comparative philosophy. The current paper deals with the steering capability of a pod. As an example a SSP-type pod ( Siemens Schottel Propulsor) has been chosen. The calculation results of propeller in inclined flow obtained with different methods will be presented. Steering forces and moments are calculated with a RANSE and a Vortex- Lattice code and correlated to those of a conventional rudder. Traditional formulae are applied for both – conventional rudder and pod – and conclusions are drawn, how the gap could be closed with a view on further steps in the near future. |
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