Fundamentals · 6 minute read
Autopilot rudder reference sensors
What a rudder feedback unit does, which autopilots need one and which infer rudder angle, plus installation geometry, calibration and failure symptoms.
A , also called a rudder feedback unit, is a small sensor on a linkage that tells the course computer exactly where the rudder is. It is one of the least glamorous parts of an autopilot and one of the most common causes of odd steering behaviour, because it sits in the wettest part of the boat and nobody looks at it.
Not every system needs one. Tiller and wheel pilots know their own drive position. Several manufacturers now offer below-deck systems that estimate rudder angle instead of measuring it. Knowing which camp your pilot is in tells you what to install, what to calibrate and what to suspect when things go wrong.
Key facts
- What it is
- A sensor on a linkage reporting rudder angle to the pilot
- Who needs it
- Most below-deck systems with a remote drive
- Who may not
- Systems using virtual or estimated feedback
- Geometry
- Linkage parallel to the tiller arm, no over-travel
- Calibration
- Zero centred, and matched at hard over both ways
- Catamarans
- Sensor goes on one rudder, linked rudders must be true
Why the course computer wants to know the rudder angle
A pilot that knows the rudder angle can do three things it otherwise cannot. It can apply a precise amount of helm rather than a burst of drive and hope. It can take the helm off at the right moment so the boat settles on course instead of sailing through it, which is what does. And it can protect the steering by stopping the drive before the rudder hits its stops.
Without that information the computer is working open loop. It knows how long it ran the drive but not what the rudder actually did, and if the drive is slow, loaded or slipping, its estimate drifts away from reality. That is why traditional below-deck systems have always used a feedback unit.
The sensor is usually a potentiometer or a hall-effect device on a short arm, connected by a link rod to the or quadrant. It reports an angle continuously, and the course computer compares commanded angle with actual angle many times a second.
Systems that require one, and systems that infer it
Raymarine's Evolution systems driving Type 1, Type 2 and Type 3 drives are normally installed with a rudder reference, and Raymarine documentation treats it as part of the standard package for mechanical and linear drives. B&G and Simrad below-deck packs are usually specified with a feedback unit too, although Simrad's virtual rudder feedback can steer without one in some hydraulic installations.
Furuno's is the best known of the estimating approaches, allowing a NavPilot to run without a physical rudder sensor on suitable hydraulic steering. Garmin's Reactor 40 hydraulic systems are also commonly installed without a rudder feedback unit, relying on the sensor pack and .
Fitting one anyway is often worthwhile even where it is optional, because measured rudder angle gives cleaner steering, a useful rudder display and better drive protection. Ask your installer what the current recommendation is for your exact model and steering type, because the guidance changes between software versions.
Optional is not the same as unnecessary
Installation geometry
The geometry is the whole job. The transducer arm and the link rod must form a parallelogram with the or quadrant, so that equal rudder movement either side of centre produces equal sensor movement. Get that wrong and the pilot thinks the rudder is at eight degrees one way and twelve the other, and it will steer asymmetrically.
The sensor must never reach its own internal limit before the rudder reaches the boat's stops. Over-travel destroys feedback units, and it is the most common installation error. Move the rudder hard over both ways by hand during the install and confirm the arm still has room.
Mount the body on something solid that does not flex, keep the unit above normal bilge water, and support the cable so its weight is not hanging on the connector. A feedback unit mounted to a flexing panel gives a reading that changes with load, which produces a fault that appears only in a seaway.
- Link rod parallel to the tiller arm with the rudder centred
- Equal sensor travel port and starboard through the full rudder range
- Rudder stops, not the sensor or the drive, take the load at hard over
- Solid mounting to structure, not to a thin panel or locker side
- Above bilge water, with the cable supported and the connector protected
- Ball joints in good condition, with no play in the linkage
Alignment and calibration
Commissioning is simple but must be done properly. Centre the rudder physically, confirm the steering is truly amidships rather than approximately so, then set the zero on the control head. Turn hard over each way and check the displayed angle matches what the rudder is really doing.
Most systems then ask for the rudder limits so the drive stops short of the mechanical stops. Set them a couple of degrees inside and the drive will never fight the stops, which is worth real money over the life of the system.
Redo the zero any time the linkage, quadrant, steering cables or the drive mounts are disturbed. A rudder zero that is two degrees out will show up as a boat that tracks slightly to one side and a pilot that carries a standing helm angle to compensate.
Failure symptoms
A failing rudder reference rarely dies cleanly. Typical signs are a rudder angle display that sticks, jumps or reads non-zero when the helm is centred, a pilot that steers well on one tack and badly on the other, unexplained drive stop alarms, or a system that will not complete its calibration.
Diagnosis is quick. With the pilot in standby, move the helm slowly lock to lock and watch the rudder angle readout. It should move smoothly and continuously. Any flat spots, jumps or sudden reversals mean the sensor or its linkage is at fault.
Then check the mechanical side before you buy a new sensor. A loose ball joint, a bolt backing out of the or a bracket that has cracked will all produce the same symptoms, and all are cheaper to fix.
Twin rudders and catamarans
Boats with two rudders have one linkage between them and normally one feedback unit, fitted to whichever rudder gives the better geometry and access. The critical point is that the between the rudders must be solid and correctly adjusted, because the sensor only knows about the rudder it is attached to.
On a catamaran the sensor usually lives in one hull with the drive, which means the pilot is measuring one rudder and assuming the other follows. Any slop in the linkage across the bridgedeck turns up as vague steering that no tuning will fix.
Check the and its joints as part of the annual inspection, and re-zero the rudder reference after any work on the steering. On a cat, an afternoon spent getting the rudders genuinely parallel is the best steering upgrade there is.
Questions
Frequently asked
Do I need a rudder reference transducer?
If you have a below-deck system with a remote drive, almost certainly yes unless your manufacturer specifically supports running without one. Tiller and wheel pilots never need one because they know their own drive position. Furuno's Fantum Feedback, Simrad's virtual rudder feedback and Garmin's hydraulic systems can run without one in suitable installations.
What happens if the rudder reference fails at sea?
Most systems raise an alarm and either drop to standby or continue with degraded steering, depending on the model. Take the helm, check the linkage first since a dropped ball joint is the most common cause, and be prepared to hand steer or use a windvane. Carry a spare on a long passage if your system depends on one.
Can I fit a rudder reference myself?
The mechanical work is within the reach of a competent owner, but the geometry is unforgiving and over-travel destroys the sensor. If you are unsure about linkage layout, mounting strength or how to set the limits, have an installer do it or at least check it before first use. Ask them to run the calibration with you watching.
Why does my rudder angle display not match the real rudder?
Usually a zero that has drifted, a worn ball joint, or a linkage geometry that is not parallel so the readings are asymmetric. Re-centre the rudder physically, re-zero on the control head, then compare the display at hard over each way. If the two sides differ noticeably, the linkage geometry needs correcting.
More in fundamentals
Practise autopilot operation with an instructor
Habits are learned at the helm with someone watching. The training section explains what a good instructor covers and where to find one in your state.