Check the listing for details. Autohelm Raymarine Z131 M81105 Rotary Rudder Reference Sensor Rudder Feedback. Condition: Used. Listed at 284.05 USD. Autohelm Raymarine Z131 (M81105) Rotary Rudder Reference Sensor Rudder Feedback. Works perfectly, fully tested with guarantee. This unit came from a working Autohelm ST6000 autopilot. Fits all Autohelm, Raytheon and Raymarine autopilot systems. **** Connected to the ACU Unit of a Raymarine autopilot system, or SeaTalk rudder angle indicator displays. A rudder angle reference sensor/transducer can be connected to the ACU to provide rudder angle information to the autopilot system. The connection of a rudder angle reference sensor/transducer is highly recommended to ensure optimum autopilot performance. A rudder angle reference sensor/transducer converts its movement information to rudder angle in +/- degrees. Certain environmental conditions such as cross-current can cause the autopilot system to steer persistently to port or starboard, even when the rudder is centred. With a rudder angle reference sensor/transducer connected to your autopilot system, you can use an autopilot control head to specify an offset angle in +/- degrees to compensate for inaccurate rudder angle information caused by such conditions. The more accurate the rudder angle information, the more accurately the autopilot system can keep to a correct course. It was a required component with an older black Autohelm ST4000 Wheel Pilot. The rudder angle transducer is recommended to improve rudder positioning with all Raymarine ACU Units; from the ACU-100 through the larger ACU-400.