Twin-screw vessel manoeuvrability: The traditional twin-rudder configuration vs pod-drives

The employment of twin pod-drives is spreading nowadays through the fleet of modern twin-screw vessels. The adoption of this steering device is of interest for both new-buildings and for retrofit because of the high manoeuvring effectiveness in restricted waters, free-sailing efficiency, hybrid-electric optimal engine control room space optimization and noise-comfort. In terms of manoeuvring, in particular, the control system offers great side-stepping and turning ability. On the other hand, the pod configuration could result in poorer course-keeping and yaw checking abilities at high-speed due to the more open stern shapes, together with the reduced appendages extension. In this respect, the IMO yaw-checking ability, represented by the usual overshoot angles of the zig-zag manoeuvre, should be carefully checked since an early stage. In this work, a simulation tool specifically developed to deal with twin rudder or twin pod ship configurations is described and validated against experimental results. Then, a first application of the tool is shown, presenting the results of a comparative study of spade and flap rudder setups against pod-driven units for a specific group of ships, allowing to show that, in this case, a proper design of pod units is not detrimental of the yaw checking ability.