Michael Woodward

Operability of fast podded RoPax vessels in rough seas

Methods are described for the evaluation of the seakeeping performance of a fast RoPax vessel driven by podded propulsors in the North Sea environment. The evaluation procedure is based on the responses of the vessel in regular waves, the wave conditions encountered, and the limiting values associated with the vessels mission. The seakeeping performance is represented by the average attainable speed (AAS), which is defined as the ratio of the average speed of the vessel on a specified route to the calm water speed. Both the natural (due to the added wave resistance) and the voluntary (due to the excessive ship responses) speed losses are taken into account. The annual average speed of the fast RoPax design in the North Sea is evaluated and compared with that of a similar size conventional RoPax vessel. The results indicate that, despite its 20 per cent smaller displacement, the fast RoPax design, in general, has better seakeeping performance characteristics than that of the conventional design. The effect of podded propulsors on the average annual speed is also investigated and it is found that the presence of podded propulsors can have favourable effects on the seakeeping performance characteristics, particularly on the vertical plane responses.

Comparison of Stopping Modes for Pod-Driven Ships by Simulation Based on Model Testing:

Conventionally, the stopping of a ship is achieved by direct reversal of propeller rotation. However, the introduction of azimuthing pods presents other options. The following study examines the various modes that may be employed to stop a pod-driven ship. A continuous function is derived describing the hydrodynamic forces on both the propeller and the pod body for any load condition and helm angle, including fluid damping and added mass effects. The proposed function is validated through comparison with comprehensive open water model tests. Next, a time domain simulation algorithm is proposed to examine the dynamic effects including the mass inertia on both the propeller shaft and slewing stock. Finally, a simulation study for the proposed stopping modes is performed using a known design as a case study. Results and discussion are presented.