Application of System Identification for the Improvement of Manoeuvrability Prediction for Twin-Screw Ships

Interest in ship manoeuvrability has increasingly grown during last decades, both for merchant and naval ships resulting in the adoption by IMO of RESOLUTION MSC137 (76) and in publication by NATO of ANEP 70. Ship manoeuvrability characteristics prediction is strongly related to the accuracy in determination of the hydrodynamic coefficients which specify the mathematical models used in simulation tools. Hydrodynamic coefficients are usually estimated, in preliminary design stages, from regression formulas based on existing data, which may lead to inaccurate predictions for non conventional vessels or ships exceeding experimental data base parametric range. In this work techniques already developed by the authors have been applied systematically to a series of seven twin screw naval ships of different type, in order to improve the current prediction capabilities of the available simulators. From the comparison of results before and after system identification, it can be noticed that a considerable improvement in the simulations quality has been experienced, and a simple procedure for calibrating the existing regressions on the basis of results obtained is proposed. Moreover, the very important influence of the hull appendages configurations for this kind of ships is underlined, and a tentative method for its assessment is described.