Energy Losses Related to Automatic Steering of Ships in Waves—Part I: Losses Under Conventional Autopilot Control

Results are presented relating to energy losses due to ship steering in waves. Propulsion losses related to yawing and rudder activity of ships during open-seas course-keeping are evaluated. Two representative tankers of 250,000 and 400,000 dwt, and an 880 ft (268 m) long containership are examined, using hydrodynamic data resulting from captive model tests. The approach presented involves time-domain computer simulation studies of the yaw-sway-surge-rudder coupled motions of the ships. Evaluation of losses due to both yawing of the uncontrolled ship and those resulting from yawing and rudder effects in the automatically steered case are made. On the basis of the results presented it is shown that under the action of waves yawing of a ship results in significant energy losses. It is also shown that a substantial increase in energy losses occurs under automatic steering control with commonly accepted autopilot specifications.