Roll-Heave-Sway Coupling In Beam Waves

To have reliable results by applying the modern methods of Non-Linear dynamics, a realistic mathematical model of ship motions is needed. In this paper we discuss, on an experimental basis, the large amplitude roll motion modeling, an issue of great interest in the studies of ship capsizing. In particular, the coupling of roll motion with the other motions in the transversal plane, heave and sway, is analyzed by means of a parameter identification technique used to discuss the goodness-offit of different mathematical models to the experimental data and to obtain values for the coefficients. The application of the modern methods of Non-Linear dynamics to the study of Non-Linear roll motion equation started during the late seventies (see Francescutto, 1997) and it is now almost mature to give design indications for safer and more seaworthy ships (Spyrou & Thompson, 2000). This is particularly evident by considering the IMO Weather Criterion (see IMO Res. A.749 and the underlying theory, Yamagata, 1959). Of course, these approaches cannot describe the last stages of motion leading to capsizing, but it is common opinion that capsize in beam waves is always due to the concurrent action of several causes, so that the conventional approach based on intact ship - rigid body - is not suitable because the physics of the phenomenon is different. In other words, a ship capsizes in beam waves because she has an extremely poor static stability (sub-standard) or because, in addition to waves, there is some additional phenomenon, water on deck, damage, flooding from upper deck openings, shifting of cargo, strong wind gusts acting on extended superstructures. The problem of the mathematical modeling of ship rolling is thus often reduced to that of the possibility of describing large amplitude motion leading to additional phenomena deteriorating the stability more than directly the capsizing.