Glossary

Marine autopilot glossary

The same terms appear as tooltips throughout the site. Here they are in one place, with the longer explanation.

AHRS: Attitude and heading reference system: a nine-axis sensor giving heading, pitch and roll.; An attitude and heading reference system fuses magnetometers, accelerometers and rate gyros into a stable heading that is not disturbed by the boat's motion. Garmin's Reactor 40 has one built in; Raymarine's EV-1 and B&G's Precision-9 are equivalent nine-axis sensors.
Apparent wind: The wind felt on the moving boat: true wind plus the wind of the boat's own motion.; Apparent wind swings forward as the boat accelerates. Pilots steering to apparent wind under spinnaker chase that swing, which is why fast boats steer to true wind downwind.
Auto mode: Heading hold: the pilot steers the compass heading it had when engaged.; The basic mode on every autopilot. It needs only the heading sensor and knows nothing about tide, wind or route.
Backbone: The main cable of an NMEA 2000 network, with a terminator at each end.; Devices connect to the backbone by drop cables. Missing terminators, multiple power feeds and over-long drops are the commonest network faults.
Bar: A shallow sandbank across a river or harbour entrance where waves break.; Australian bars such as Wide Bay, the Gold Coast Seaway, Swansea and Lakes Entrance are crossed by hand-steering only. Never on autopilot.
Counter rudder: Opposite rudder applied to stop a turn before the heading overshoots.; A tuning parameter alongside gain. Too little and the boat overshoots each course change; too much and it feels sluggish.
Course computer: The autopilot's brain: reads the sensor, decides on rudder, drives the actuator.; Raymarine's ACU, Garmin's Reactor 40, Navico's NAC-2 and NAC-3 and Furuno's processor are all course computers. Called the actuator control unit by Raymarine.
Course over ground (COG): The direction the boat is actually moving over the seabed.; COG includes the effect of tide and leeway. Heading is where the bow points; the two differ in any current. No Drift and Track modes steer to COG.
Cross-track error (XTE): The distance the boat is off the planned route line.; Track and Nav modes steer to reduce XTE. The XTE alarm warns when the pilot cannot hold the line.
Deadband: The heading error the pilot ignores before correcting.; A wider deadband, or sea-state setting, stops the drive working constantly in a seaway and saves power.
Drive: The actuator that moves the rudder: tiller, wheel, linear, rotary, hydraulic or solenoid.; Drives are matched to the steering system and rated by displacement or cylinder volume. The drive is the part most often wrongly sized.
Fantum Feedback: Furuno's method of steering without a rudder reference transducer.; The NavPilot learns the steering system's response and infers rudder angle, simplifying outboard installations.
Fluxgate compass: A magnetic heading sensor used in older autopilots.; Works well level, less well heeled and pitching. Superseded by nine-axis solid-state sensors in modern systems.
Gain: How much rudder the pilot applies per degree of heading error.; Higher gain means quicker, larger corrections. Too high causes hunting or S-turns; too low lets the boat wander. Sometimes called rudder gain or response.
Gust response: B&G feature that bears away in a puff and comes back up as it eases.; Keeps a sailing yacht on its feet in wind mode without human intervention. Comparable in purpose to Raymarine's Hydro-Balance for downwind stability.
Hard-over time: Time for the drive to move the rudder from full one side to full the other.; Cruising boats aim for 10–15 seconds; planing hulls faster; heavy vessels slower. Pump and drive sizing set it.
Heading: The direction the bow points, from the compass.; Differs from course over ground in any current or leeway. Auto mode holds heading.
Hydro-Balance: Raymarine Evolution setting that dampens downwind and following-sea wander.; Adjusted on the p70s head; worth setting up with an instructor on a trade-wind run.
Jackline: A strong line or webbing along the deck to clip a tether onto.; Rigged inboard so a fall keeps you on deck. Essential on any boat sailed under autopilot.
Linear drive: An electric ram below deck pushing a tiller arm on the rudder stock.; The standard below-deck drive on yachts from 6 to 35 tonnes. Quiet, powerful, clutch-disengaged. Raymarine Type 1–3 and Jefa are common.
Lookout: The Rule 5 duty to watch and listen for danger at all times.; Applies whoever or whatever is steering. An engaged autopilot never relieves it.
Nav mode: Navico's name for route following from a plotter.; Same as Raymarine's Track and Garmin's Route. Steers to reduce cross-track error and turns at waypoints.
NMEA 2000: The standard marine data network.; A powered bus carrying heading, wind, GPS, AIS and more between devices. SeaTalkng and SimNet are compatible brand variants.
NMEA 0183: The older serial marine data standard.; Still found on older instruments and tiller pilots. Bridged to NMEA 2000 with converters.
No Drift: Mode that holds a course over the ground, cancelling tidal set.; Behaves like Auto but uses GPS to stay on a line from where you engaged it. The coastal cruiser's best friend in a current.
NSCV: National Standard for Commercial Vessels, administered by AMSA.; Sets construction, equipment and operational standards for domestic commercial vessels, including steering arrangements a surveyor assesses.
Pattern steering: Pre-programmed circles, zigzags, orbits and cloverleafs, mainly for fishing.; Available on power-boat pilots from Garmin, Simrad, Furuno and Raymarine.
Power Assist: Furuno NavPilot 300 feature giving power steering at low speed.; Uses the autopilot pump to reduce wheel effort on heavy outboard boats.
Rate gyro: A sensor measuring how fast the boat is turning.; Part of every nine-axis sensor; lets the pilot correct yaw before the heading has changed much.
Response: A combined sensitivity setting on many pilots, from calm to rough.; Raise it in a seaway, lower it on flat water to save power and wear.
Rotary drive: A below-deck motor turning a sprocket in chain-and-cable steering.; An alternative to a linear drive on pedestal-steered yachts without room for a tiller arm.
Rudder reference transducer: A sensor on the tiller arm reporting rudder angle to the course computer.; Required with Raymarine Type 1 and larger drives; included in B&G packs; replaced by inference on Garmin hydraulic, Furuno Fantum Feedback and Simrad VRF systems.
Rule 5: The COLREGs lookout rule.; Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means. Applies in Australia through the Navigation Act and state law.
Safe Helm: Furuno feature that pauses the pilot when you turn the wheel.; Furuno's answer to Garmin's Shadow Drive.
Satellite compass: A heading sensor using two GPS antennas, immune to the hull's magnetism.; The choice for steel and alloy boats and for accurate radar overlay. Furuno's SC series is the best known.
SeaTalkng: Raymarine's NMEA 2000-compatible network with its own connectors.; Adaptor cables join it to standard NMEA 2000 devices.
Servo-pendulum: A windvane type using a water blade's swing to pull lines to the helm.; Windpilot Pacific and Monitor are servo-pendulum gears. Power grows with boat speed.
Shadow Drive: Garmin's hydraulic sensor that disengages the pilot when you turn the wheel.; Re-engages after a pause. The most-praised autopilot feature on Australian fishing boats.
Solenoid control: Autopilot control of a valve block on a constant-running hydraulic pump.; Used on vessels with power-assisted steering. Supported by Raymarine ACU-300/400, Simrad NAC-3 and Furuno 711C.
Special Regulations: Australian Sailing's safety equipment rules for racing, by category.; Set what an offshore race boat must carry. Autopilot use is governed by class rules and sailing instructions; check the current edition.
Standby: The pilot state where the helm is yours.; The most important key on the control head. Find it blindfolded.
Tether: A short line from a harness to a jackline or hard point.; Clip on before leaving the companionway whenever the pilot is engaged.
Tie bar: The rod linking twin rudders on a catamaran.; Allows a single drive on one rudder stock to steer both rudders.
Tiller arm: A lever on the rudder stock below deck that a linear drive pushes.; Whether a boat has one, or room for one, decides whether a below-deck linear drive is possible.
Track mode: Raymarine's name for route following from a plotter.; Same as Navico's Nav and Garmin's Route.
True wind: The wind over the water, independent of the boat's motion.; Calculated from apparent wind and boat speed. The right reference for wind steering downwind and on fast boats.
VRF: Virtual Rudder Feedback: Simrad's method of steering hydraulic outboards without a rudder sensor.; Infers rudder position from the pump and compass. Simplifies installation on trailer boats.
Wind vane mode: Raymarine's name for wind steering.; Holds an apparent or true wind angle using data from a wind sensor.
Windvane self-steering: A mechanical gear that steers to the wind with no electricity.; Auxiliary-rudder types (Hydrovane) and servo-pendulum types (Windpilot) are the two families.

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