Asymmetrical Shaft Power Investigation via Free-Running Self-Propelled Ship Model

Marine propulsion plants can experience large power fluctuations during tight manoeuvres. During these critical situations, dramatic increases of shaft torque are possible, up to and over 100% of the steady values in straight course. In the case of a twin-screw ship turning circle and mainly on naval surface ships, the two shaft lines dynamics can be completely different in terms of required power and torque. On the basis of the outcomes of these preliminary analyses, it is believed that this phenomenon, if not correctly considered, is potentially dangerous, especially for propulsion plants with two shaft lines powered via a unique reduction gear, which can be subject to significant unbalances. This kind of propulsion plant, despite not very common, has been recently proposed as a solution for particular applications, such as fast naval ships. From another point of view, effect of asymmetrical shaft power increase during manoeuvres (and of different strategies of the automation plant) may result in different manoeuvring behaviour of the ship, with effect on macroscopic parameters such as tactical diameter in turning circle. In order to better analyse the physics related to this phenomenon, a series of dedicated free running model tests has been carried out, increasing the number of measurements with respect to what is usual for this kind of tests (for example measuring also thrust on the two shafts), and performing trials with different configurations of propulsion system (e.g. constant shaft revolution, limitations on torque or power). Results of these trials are presented in present paper and discussed, in order to provide a further insight to better understand this phenomenon, and how it can affect ship propulsion plant behaviour and ship manoeuvrability. These results are considered very important in order to allow a fine tuning of ship propulsion system and manoeuvrability simulators.