The Nautical Glossary
One structured, source-checked entry per term — every definition laid out the same way and cross-linked into the reference it belongs to.
At right angles to the fore-and-aft line of the ship; on the beam.
Towards or at the stern of the ship.
Resting on the bottom; a vessel that has run onto the seabed.
Marks, lights, buoys and beacons established to help a mariner fix position and follow a safe course.
A transponder system that broadcasts a vessel’s identity, position, course and speed to other ships and shore stations.
The middle of the ship along its length; also the order to centre the rudder.
A watch kept at anchor to ensure the anchor is holding and the ship is not dragging.
The angle a ship with negative initial stability settles to, heeled over rather than upright.
The ECDIS function that warns when the planned route or the ship’s track crosses the safety contour or a charted danger.
The wind felt aboard a moving vessel — the vector sum of the true wind and the ship’s own motion.
The first stage of passage planning: gathering and reviewing all information relevant to the voyage before any plan is drawn.
Behind the ship; also the movement of a vessel going backwards.
Across the ship, at right angles to the fore-and-aft line.
Weight, usually seawater in tanks, carried to improve stability, trim or draught.
A shoal of sand or mud across a harbour mouth or river entrance.
The breadth of a vessel at its widest point; also a direction at right angles to the keel.
The horizontal direction of one object from another, expressed in degrees from north.
A scale relating wind force from 0 to 12 to observed sea and land conditions.
A place where a ship is moored or anchored; also a crew member’s bunk.
The turn of the hull where the bottom meets the side; also the lowest internal space where water collects.
A strong post on a quay or deck to which mooring lines are made fast.
The forward part of a vessel.
The compartment from which a ship is navigated and controlled.
A vertical partition within a ship, often watertight, dividing it into compartments.
A ship’s fuel oil; bunkering is the act of taking fuel aboard.
A system of floating marks indicating channels and dangers; the IALA system defines lateral and cardinal marks.
An IALA buoy showing the direction of safe water relative to a danger, named for the cardinal point on which it lies.
A diagram showing the distribution and stowage of cargo throughout the ship.
The updating of a chart from Notices to Mariners to keep it current and safe to use.
The reference level to which charted depths and drying heights are measured, usually Lowest Astronomical Tide.
The depth of water below chart datum printed on the chart at a given position.
The International Regulations for Preventing Collisions at Sea, 1972 — the rules of conduct, lights, shapes and signals between vessels.
The combined effect of variation and deviation; the angle between compass north and true north.
The act of directing the steering of a ship through helm and engine orders.
The intended horizontal direction of travel, measured in degrees.
The actual track achieved over the ground after the effects of tide and leeway.
Closest Point of Approach and Time to CPA — the predicted nearest passing distance to a target and the time until it occurs.
A reference level or surface for measurement — vertical for depths and heights, horizontal for position.
Estimating position from a known fix using only the course steered and distance run, without external references.
The total weight a ship can carry — cargo, fuel, stores, crew and water — at her load line.
The compass error caused by the ship’s own magnetic field, varying with the ship’s heading.
The weight of water displaced by a floating ship, equal to the ship’s own total weight.
A situation of grave and imminent danger requiring immediate assistance.
The vertical distance from the waterline to the lowest point of the hull — how deep the ship sits.
The height a feature dries above chart datum, shown underlined on the chart; covered only when the tide exceeds it.
The falling tide, when the tidal stream runs seaward.
Electronic Chart Display and Information System — a type-approved navigation system using official electronic charts.
Electronic Navigational Chart — the official vector chart data issued by hydrographic offices for use in ECDIS.
A dead-reckoning position corrected for the estimated effect of tide and leeway.
Emergency Position-Indicating Radio Beacon — a device that transmits a distress alert and position via satellite.
A unit of depth equal to six feet (1.83 m), used on older charts.
A cushion hung over the ship’s side to protect the hull when alongside.
A position established from external references — bearings, ranges or GNSS — at a known time.
The country in which a ship is registered and whose laws and regulations she must follow.
The rising tide, when the tidal stream runs landward.
A sea running in the same direction as the ship’s heading.
The forward part of the upper deck; historically the crew’s quarters.
The loss of stability caused by liquid moving freely in a slack tank, which raises the effective centre of gravity.
The vertical distance from the waterline to the freeboard deck — the reserve buoyancy above the water.
The opening or ramp by which people board or leave a ship.
Under COLREGs, the vessel required to keep clear and take early, substantial action to avoid the other.
The vertical distance between a ship’s centre of gravity and its metacentre — the measure of initial stability.
Global Maritime Distress and Safety System — the international framework for distress alerting and communications.
Global Navigation Satellite System — the umbrella term covering GPS, GLONASS and Galileo.
The shortest route between two points on the globe, lying on a circle centred on the earth’s centre.
A compass using a spinning gyroscope to indicate true north, unaffected by magnetism.
The horizontal distance between the lines of weight and buoyancy at a given heel — the lever that rights the ship.
The highest tide level predicted under average conditions; the datum for charted vertical clearances.
The part of the bow where the anchor cables run out through the hawsepipe.
The direction in which a ship’s bow is pointing at a given moment.
To hold a vessel nearly stationary in heavy weather by balancing engine, helm and sea.
The transverse inclination of a ship caused by an external force such as wind or turning.
The height of the sea surface above chart datum at a given moment; added to charted depth to find actual depth.
The wheel or tiller by which a ship is steered; also the order to apply rudder.
The main watertight body of a ship.
The body that defines the international buoyage system of lateral and cardinal marks.
An insulated, waterproof suit that protects a survivor against hypothermia in cold water.
The International Maritime Organization — the UN agency regulating shipping safety, security and pollution.
The International Safety Management Code — requiring a documented safety management system aboard and ashore.
The lowest tide level predicted under average meteorological conditions; the usual chart datum.
An IALA buoy marking the side of a channel — port or starboard — to be left on a given hand.
Two lights or marks in line giving a safe approach bearing into harbour.
The side or direction sheltered from the wind; the opposite of windward.
The sideways drift of a vessel caused by wind — the angle between the heading and the course made good.
An enclosed or open survival craft for evacuating a ship in emergency.
An inflatable survival craft stowed in a canister and launched when abandoning ship.
A persistent transverse inclination caused by an internal imbalance such as off-centre weight or flooding.
The mark on a ship’s side showing the maximum permitted draught for a zone and season; the Plimsoll mark.
An instrument measuring a ship’s speed and distance through the water; also the official record book.
The direction indicated by a magnetic compass, differing from true north by the variation.
The International Convention for the Prevention of Pollution from Ships.
The spoken radio distress signal indicating grave and imminent danger requiring immediate assistance.
The average high-water level at spring tides; the datum for charted heights of land and structures.
A great circle passing through both poles; a line of longitude.
The point about which a ship’s buoyancy force effectively acts as she heels through small angles.
Maritime Mobile Service Identity — the unique nine-digit number identifying a vessel in DSC and AIS.
Securing a vessel to a berth, buoy or anchor; also the lines and equipment used.
The station bill assigning every crew member their duties and stations for fire, abandon ship and man-overboard emergencies.
A unit of distance of 1,852 metres, equal to one minute of latitude.
The tide of least range, occurring near the first and last quarters of the moon.
An area marked on the passage plan that the vessel must not enter because the water is too shallow or dangerous.
Official notices issued to correct charts and publications and to warn of changes affecting safe navigation.
A radar technique for monitoring a vessel’s track relative to a fixed object along a pre-set line.
The berth-to-berth plan covering the four stages: appraisal, planning, execution and monitoring.
A mariner with local knowledge who advises on the navigation of a ship in confined waters.
Navigation in confined or coastal waters by reference to visual marks and local knowledge.
The fore-and-aft rotational motion of a ship in a seaway; also the advance of a propeller per revolution.
The load line marked on a ship’s side indicating the maximum legal draught.
The left-hand side of a ship when facing forward.
A line on the chart somewhere along which the ship is known to lie, from a bearing, range or transit.
A system using radio waves to detect the range and bearing of other vessels and objects.
The distance to an object; also the difference in height between successive high and low waters.
The opposite direction to a given bearing or course, 180 degrees away.
The direction of an object measured relative to the ship’s head rather than to north.
A line crossing all meridians at the same angle; a constant-course track on a Mercator chart.
The transverse rotational motion of a ship from side to side in a seaway.
The hinged underwater foil at the stern used to steer the ship.
A position obtained by transferring an earlier position line to cross a later one, allowing for the run between.
The depth contour set in ECDIS that separates safe from unsafe water for the vessel; it drives the anti-grounding alarms.
Search and Rescue Transponder — a device that marks a survival craft’s position on a rescuer’s radar.
The direction (set) and rate (drift) of a tidal stream or current acting on the ship.
An instrument measuring the angle between two objects, used for celestial position fixing.
The brief period of little or no tidal stream around the turn of the tide.
The International Convention for the Safety of Life at Sea — the principal treaty on ship safety, construction and equipment.
A measured depth of water referenced to chart datum; also the act of measuring depth.
The tide of greatest range, occurring near new and full moon.
The bodily sinkage and change of trim a ship undergoes in shallow water, reducing under-keel clearance.
A ship’s tendency to return upright after being heeled by an external force.
Under COLREGs, the vessel required to keep her course and speed while the give-way vessel takes avoiding action.
The right-hand side of a ship when facing forward.
The Standards of Training, Certification and Watchkeeping for Seafarers.
The foremost structural member of the bow.
The aftermost part of a vessel.
Long, regular waves that have travelled out of the area in which they were generated.
The horizontal movement of water caused by the rise and fall of the tide.
The periodic rise and fall of the sea caused by the gravitational pull of the moon and sun.
A measure of a ship’s size, gross or net, based on enclosed volume rather than weight.
A routeing measure separating opposing streams of traffic into lanes to reduce collision risk.
Two fixed objects seen in line, giving a precise position line or a leading line into harbour.
The difference between the forward and after draughts; the longitudinal attitude of a ship.
A bearing measured clockwise from true north.
The depth of water beneath the keel — the actual depth of water less the ship’s draught.
A vessel that is not at anchor, made fast to the shore, or aground.
Coordinated Universal Time — the time standard used at sea for navigation and communications.
The angle between true and magnetic north at a place, changing with position and slowly over time.
Very High Frequency radio — the principal medium for short-range ship-to-ship and ship-to-shore communication.
Vessel Traffic Service — shore-based monitoring and management of ship traffic in busy or confined waters.
The track of disturbed water left astern by a moving vessel.
A division of the day during which part of the crew is on duty; also the personnel on duty.
The maintenance of a ship’s structure and closures to keep water out and preserve buoyancy and stability.
A position marking a point on the planned route where a course alteration or event occurs.
The deck machine used to heave in and pay out the anchor cable.
The side or direction from which the wind blows; the opposite of lee.