Stability of autopiloted ships in a following sea.

Abstract

The object of this thesis was to investigate the stability of a ship with automatic steering in a following sea using the nonlinear equations of motion and to compare these results with the results of linear approximations. The solution of the nonlinear equations was obtained by a technique of treating nonlinear terms as linear terms with non-constant coefficients. Results showed that when drift angles become 0.1 radians or greater the nonlinear solution differs substantially from the linear solution. But the nonlinear solution has a greater range of stability in the wave profile than the linear solution. This leads to the conclusion than a system designed using linear theory provides sufficient stability for the ship in a following sea. The most important effect which this approach neglected was the coupling effect of the roll or heel motion on the yaw motion. Before a complete and accurate solution to the steering problem can be found the coupling effect must be investigated and some method of expressing it analytically must be produced.