Experimental Investigation of Asymmetrical Propeller Behaviour of Twin Screw Ships During Manoeuvres

Twin screw ships may experience considerably asymmetrical propeller functioning during manoeuvres. This phenomenon may result in large power fluctuations during tight manoeuvres, with increases of shaft torque up to and over 100% of the steady values in straight course and considerable unbalances. A joint research project supported by Italian Navy has been set up in order to deeply investigate the phenomenon, potentially dangerous for ships propulsion systems with coupled shaft lines, by means of large scale model testing and related numerical simulations. In the present work, the extensive experimental campaign results on a free running model of a twin-screw ship are presented, allowing to obtain a deeper insight of the problem. In particular, tests have been carried out simulating different simplified control schemes, starting from the most common constant rate of revolution tests and including different control strategies (constant torque and power). Usual standard manoeuvres (turning circle, zigzag and spiral) have been carried out, providing results for asymmetrical shaft and ship manoeuvrability behaviour. Results from the present analysis, together with the previously presented results, allow to obtain the complete model for the time domain simulation of asymmetrical shaft functioning.