Hydrodynamic coefficients regressions analysis and application to twin screw vessels

The problem of ship manoeuvrability has reached nowadays a significant consideration, both for merchant ships, with the adoption of IMO standards, and naval ships, with the production of various documents by NATO Specialist Teams. In literature many works regarding manoeuvrability of single screw slow/medium speed ships can be found, while a lack of information exists for what regards twin screw ships (cruise ships, RoRo ferries, megayachts, naval vessels), characterised by different form coefficients and more complex stern configuration due to appendages like skegs, shaft lines and brackets that can strongly affect manoeuvrability behaviour. In this hydrodynamic coefficients of a series of twin screw naval vessels have been evaluated considering different semi-empirical formulations proposed by various authors and standard manoeuvres (turning circle and zig-zag) have been simulated with the aid of a simulator tool. Discrepancies of predicted manoeuvres with respect to experimental full scale ones have been analysed considering parameters involving stern appendages and hull main dimensions in order to select most suitable regressions to study twin screw vessel manoeuvring performance in the earlier design stages. Moreover, linear hydrodynamic coefficients corrections related to stern appendages contribution have been developed on the basis of results of a system identification technique and of an analysis of a set of experimental data at PMM for a ship model equipped with different stern appendages configurations.