Energy Losses due to Limit-Cycle Behavior of a Large Tanker Under Automatic Steering Control

The problem of limit-cycle behavior of a 250,000-dwt tanker in full-load and ballast conditions under automatic steering control in calm water is addressed. The approach presented involves digital computer time domain simulation studies of the yaw-sway-surge-rudder coupled motions of the ship emanating from nonlinearities in the steering system. It is shown that the amplitude of limit cycle in yaw remains, in general, within acceptable limits for open-seas navigation for a fairly wide range of autopilot bandwidths. Propulsion losses resulting from limit-cycle behavior in calm water are shown also to be, in general, small relative to the losses experienced in some conditions in waves. It is shown, however, that whereas increasing bandwidth reduces limit-cycle behavior in calm water, it can be expected to increase propulsion losses in heavy weather. The problem this poses in. design of steering gear controls and autopilot for this type of ship is discussed.