On modal analysis of ship strength

Abstract An approach is formulated by which structural dynamics of ships may be analysed in a linear modal form. By employing the principal modes of the ship in vacuo, simple orthogonality relations can be retained without dependence on the necessarily approximate techniques used to estimate fluid forces. It is also possible to identify modal contributions to mass, damping and stiffness for the hull and for the hydrodynamic actions separately. Those contributions of hydrodynamic origin may depend significantly on time history effects which can be measured by means of a model test; these effects can be admitted into the ship strength analysis, it is believed for the first time. It is shown how existing modal theories of ship strength and theories of seakeeping (i. e. of ‘rigid body’ motion in a seaway) fit into this more general analysis.