Raymarine, E70139 Rotary Rudder Reference Rudder Feedback

The rotary rudder reference sensor gives real-time feedback of the rudder’s actual angular position, which is essential for closed-loop control in autopilot systems. Without actual rudder feedback, the autopilot would have to infer position, increasing the risk of drift, oscillation, or overshooting.
It can detect mechanical limits or stops, helping prevent the rudder from exceeding safe angles. In many systems, the control logic will use this feedback to clip or limit motion.
It improves steering precision, especially in rough sea conditions, where the vessel can move and the rudder can experience loads.
The sensor often mounts at or near the rudder stock, quadrant, or steering linkage, so that its rotation is directly proportional to rudder rotation.
The E70139 is the Actuator Control Unit (ACU-300) manufactured by Raymarine. It is used in solenoid-controlled steering / autopilot systems. The ACU-300 includes a rotary rudder reference transducer (sensor) — in Raymarine’s catalogs, that sensor is the M81105 Rotary Rudder Reference.
The rudder reference sensor (rotary) is used to measure and report the actual angle / position of the rudder (feedback) to the autopilot system / ACU. This feedback allows precise control and prevents overtravel, oscillation, or mis-steering.
Raymarine’s support documentation refers to the part number for its rotary rudder reference transducer as M81105.

Technical Specification

Type: Rotary potentiometer (angular position transducer).
Potentiometer value: 5 kΩ (single-turn). Midpoint ≈ 2.5 kΩ.
Wiring / pin-out (typical Raymarine colours):
- Red — one end of potentiometer (end terminal).
- Green — other end of potentiometer (end terminal).
- Blue — wiper (middle terminal / variable output).
  (Measure with an ohmmeter: resistance between Red–Green ≈ 5 kΩ; Red–Blue or Green–Blue varies 0 → 5 kΩ depending on angle.)
- Mechanical travel range: typically ±60° of the transducer arm (careful alignment during installation so arm is opposite cable entry when rudder amidships). Overtravel may damage the unit.
- Cable: Supplied with 10 m cable (standard). Longer runs may be available from aftermarket sources.
- Mounting: Mechanical bracket and hardware included; mount on rudder stock, quadrant, or linkage so rotation of sensor is proportional to rudder rotation. Proper mechanical alignment is critical for correct rudder angle reporting.