Abstract
This paper deals with non-linear periodic motions of a ship running in following and quartering seas with low encounter frequency. It aims at providing a methodology for more comprehensive understanding of broaching. The motions discussed here include surge, sway, yaw and roll with an auto pilot. A manoeuvring mathematical model in waves is transformed with a mean-velocity inertia axis system and then an averaging method is applied to the transformed model. As a result, local stability and outstructure of periodic motions are discussed with numerical results derived using a purse seiner. These cannot be directly assessed by conventional linear models or purely simulation based approaches. Comparisons with experimental results are also shown.
