Adaptive autopilot

A marine autopilot having adaptive gain control. The autopilot operates under a proportional plus derivative (PD) control law during course change operation (i.e. when the difference between a desired course and the actual vessel heading is greater than a predetermined value) and a proportional plus integral plus derivative (PID) control law during course keeping operation (i.e. when the difference between the desired course and the actual heading is less than the predetermined value). A gain value associated with the derivative term of the PID control law is adaptively controlled as a function of the roll frequency of the vessel. More particularly, since the damping provided by the derivative term is ineffectual in maintaining the desired course when the vessel is heading into the seas (i.e. when the roll frequency is greater than a predetermined value), the derivative term is nulled in such conditions. In this way, ineffectual rudder movement and thus wear on the rudder and associated drive apparatus is reduced.