Application of the describing function technique to nonlinear rolling in random waves

In this paper we examine the nonlinear equations of ship rolling from a control engineering viewpoint by considering the ship as represented by a feedback system. This enables the powerful describing function technique to be brought to bear on the problem. We discuss, in detail, the extension of this technique due to West et al. [2] which can account for so-called “frequency distortion of the spectrum” in nonlinear systems and point to evidence which appears to show this happening in practice.