Analysis of twin screw ships' asymmetric propeller behaviour by means of free running model tests

Twin screw ships may experience considerably asymmetric 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; this, in its turn, may be potentially dangerous, especially in case of particularly complex propulsion plant configurations, such as those with coupled shaftlines. A joint research project supported by the Italian Navy has been set up in order to deeply investigate the phenomenon, 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 asymmetric shaft functioning and ship manoeuvrability behaviour. Results from the present analysis allow to obtain the complete model for the time domain simulation of asymmetric shaft functioning.