Fundamentals · 7 minute read
How marine autopilots work
Learn how marine autopilots work: heading sensors, course computers, steering drives, rudder feedback and control heads connected through a marine network.
Every marine autopilot, from a $1,200 tiller pilot to a $30,000 racing system, does the same four things. It measures which way the boat is pointing, compares that with where you asked it to point, decides how much rudder to apply, and moves the rudder. The differences between products are in how well each step is done and how many extra inputs (wind, position, speed, rudder angle) the system can use.
Understanding the four parts makes every other decision easier: which pack suits your steering, why a rudder reference matters, what an installer means by gain, and why the pilot that was perfect on Pittwater wanders in a following sea off Coffs Harbour.
Key facts
- Four parts
- Sensor, course computer, drive, control head
- Heading source
- Fluxgate, nine-axis solid-state or satellite compass
- Drive types
- Tiller, wheel, linear, rotary, hydraulic, windvane
- Network
- NMEA 2000, SeaTalkng, SimNet, NMEA 0183
- Extra inputs
- Wind angle, GPS position, speed, rudder angle
Autopilot heading sensors
The sensor tells the computer which way the bow is pointing. Older units used a , a magnetic sensor that works well on a level boat and less well as the boat heels and pitches, which is why an ST2000+ wanders on a rolling reach. Modern systems use a nine-axis solid-state sensor: three magnetometers, three accelerometers and three rate gyros, fused into a heading that stays stable while the boat moves. Raymarine's EV-1, Garmin's inside the Reactor 40 and B&G's Precision-9 are all of this type.
Steel and alloy boats, and vessels where radar overlay accuracy matters, use a , which derives heading from two GPS antennas and ignores the hull's magnetism entirely. Whatever the type, the sensor must be mounted away from engines, speakers, steel and heavy cables, and the installer must run a calibration so the computer knows the boat's own magnetic signature.
Autopilot course computers
The course computer is the brain. It holds the target heading, reads the sensor many times a second, and decides how much rudder to apply and for how long. The classic control settings are gain (how much rudder per degree of error), (how much opposite rudder to apply to stop the turn) and a deadband or sea-state setting that ignores small yaw so the drive is not working constantly.
Modern computers tune most of this themselves. Raymarine calls it Evolution AI; Garmin, B&G and Furuno have their own adaptive schemes. They learn how the boat responds and adjust as sea state changes. The adjustment is not magic, and every experienced skipper still nudges response up in a seaway and down on flat water.
The computer also hosts the modes. Auto holds a compass heading. Wind holds an angle to the apparent or . Nav or Track follows a route from a plotter, correcting for set and drift. holds a course over the ground. Pattern modes steer circles and zigzags for fishing. Each mode needs a data source, which is where the network comes in.
Autopilot steering drives
The drive is the muscle and it must match the steering. Tiller pilots push the tiller from a cockpit socket. Wheel pilots turn the wheel through a belt. Linear drives push a on the rudder stock below deck. Rotary drives turn a sprocket in chain-and-cable steering. Hydraulic pumps push fluid into the steering cylinder of a hydraulically steered boat. Windvanes replace the whole electronic chain with a wind vane and a water-driven servo or auxiliary rudder.
Drives are rated by thrust and by the displacement of the boat they can steer. That rating assumes a balanced helm and moderate conditions; Australian following seas demand margin.
- Tiller pilot: to about 4.5 t, cockpit-mounted
- Wheel pilot: to about 7.5 t, above deck
- Linear drive: 6 t to 35 t, below deck on a tiller arm
- Rotary drive: chain-and-cable steering
- Hydraulic pump: sized to steering cylinder volume
- Windvane: no power, steers to wind only
Rudder reference and feedback
The computer steers better when it knows exactly where the rudder is. A is a small potentiometer linked to the that reports rudder angle. Raymarine requires one with its Type 1 and larger drives; B&G includes one in its packs. Garmin's hydraulic packs, Furuno's and Simrad's infer rudder position from the boat's response instead, which makes outboard installations much simpler at a small cost in precision.
Autopilot control heads and marine networks
The control head is where you engage, adjust and disengage. It may be a dedicated head (Raymarine p70s, Garmin GHC 50, B&G Triton2 Pilot Controller, Simrad AP44), a chartplotter page, a wireless remote, or all three. The one key that matters is Standby: it must be one press away from wherever the helm stands.
All the parts talk over a network. is the industry standard; Raymarine's and Navico's SimNet are compatible variants with different connectors. The network carries heading, wind, position, speed, depth and AIS between the pilot and everything else, and it is why a good installation includes a properly terminated backbone with its own power.
What the pilot cannot see
Autopilot components connected on a boat
On a 38-foot cruising yacht with a Raymarine EV-200: the EV-1 sensor lives under a bunk, the ACU-200 sits in a locker near the batteries, a Type 1 bolts to a in the lazarette with a rudder reference beside it, a p70s head is on the pedestal and an Axiom at the helm shows the route. The backbone links them to the wind sensor at the masthead and the AIS. Engage Auto and it holds the heading; press Wind and it holds the angle; press Track from the Axiom and it follows the route to the next waypoint, with an alarm at each turn.
Learning what each of those modes does on your own boat, in your own waters, is what a good instructor teaches in an afternoon and what most owners otherwise learn by accident over years.
Questions
Frequently asked
Does an autopilot need GPS to work?
No. Auto mode needs only the heading sensor. GPS is needed for Track, Nav and No Drift modes, and a wind sensor is needed for Wind mode.
What is the difference between heading and course over ground?
Heading is where the bow points; course over ground is where the boat is actually going once tide and leeway are added. Auto mode holds heading; No Drift and Track modes hold a course over the ground.
Why does my pilot make constant small corrections?
Gain or response is set too high for the conditions, or the deadband is too small. It wears the drive and drains the battery. Turn response down on flat water.
Can I add a pilot to a boat with no instruments?
Yes. A basic pack with its own sensor and control head works alone. Wind and route modes need a wind sensor and a plotter on the same network.
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.