Nautical Glossary

Off the side of the vessel; perpendicular to the keel.

Toward or near the stern of the vessel.

In the direction the vessel is heading; forward of the bow.

At or near the middle of the vessel, lengthwise or athwartships.

Behind the vessel; in the opposite direction of travel.

At right angles to the centreline of the vessel.

Buoys, beacons, lights, and other devices placed to assist mariners in determining position or course.

Automatic Identification System — a VHF transponder that broadcasts vessel identity, position, course, and speed for collision avoidance and traffic monitoring.

Automatic Radar Plotting Aid — radar processing that automatically tracks targets and computes CPA / TCPA for collision avoidance.

The horizontal angle of a celestial body or other object measured clockwise from north.

A change in heading; under Rule 8 must be positive, made in ample time, and large enough to be readily apparent to other vessels.

A light showing an unbroken light over an arc of 360° on the horizon.

An all-round white light shown by a vessel at anchor; vessels under 7 m may omit if not in or near a narrow channel.

The order to leave a vessel in distress and take to a life-raft or other survival equipment.

A knot used to attach a line to an anchor ring; a round turn followed by two half hitches with the second hitch passed through the round turn.

The static angle a vessel takes up when its centre of gravity is above the metacentre (negative GM); a sign of imminent capsize.

The widest part of a vessel; also a direction at right angles to the keel.

The lowest internal compartment of a vessel where water collects and is pumped out.

The forward end of a vessel.

The vessel's command station — where the helm, controls, and navigation equipment are located.

A vertical partition inside a vessel; watertight bulkheads divide the hull into compartments for damage control.

The raised section of a vessel's side above the deck; prevents persons or gear from going overboard.

The horizontal direction of one terrestrial point from another, expressed as an angle measured clockwise from a reference direction (true, magnetic, or relative).

A 0–12 wind-force scale tied to observable sea state; force 6 (~25 kt) is the typical small-craft warning threshold.

The change in compass bearing of an approaching vessel over time; if the bearing is constant and range is decreasing, risk of collision exists.

A pump that removes water from the bilge.

A loop knot that does not slip or jam; the most useful general-purpose knot on board.

A charter where the vessel is hired without a crew; the charterer assumes full responsibility for operation.

An enclosed compartment inside the vessel for accommodation or shelter.

The intersection of the hull side and bottom; can be hard (sharp) or soft (rounded).

A T-shaped fitting on a deck or dock used to secure a line.

The open recessed area in the after part of a vessel, typically where the helm or fishing chairs are located.

The vertical reference for charted depths and tidal predictions; in US waters typically Mean Lower Low Water (MLLW).

The actual track made over the ground; differs from heading when set or drift is acting on the vessel.

A circle on a chart marked with directions in degrees true and magnetic, with the local variation noted.

The minimum predicted distance between two vessels on their current courses and speeds.

The intended direction of travel, expressed as a bearing — typically true (relative to true north).

Short, steep, irregular waves caused by local wind blowing over a fetch.

Constant Bearing, Decreasing Range — the textbook indicator of risk of collision under Rule 7.

See CPA. The minimum predicted distance between two vessels on their current courses.

International Regulations for Preventing Collisions at Sea, 1972 — the 38 rules governing vessel conduct on the high seas and connected waters.

Two power-driven vessels crossing so as to involve risk of collision; the vessel with the other on her starboard side is the give-way vessel (Rule 15).

A day-shape exhibited by a vessel constrained by her draft (COLREGs Rule 28).

Fire involving ordinary combustibles (wood, paper, cloth). Extinguished by water, foam, or dry chemical.

Fire involving flammable liquids and gases (gasoline, oil, propane). Extinguished by foam, dry chemical, or CO₂.

Fire involving energised electrical equipment. Extinguished by CO₂ or dry chemical.

The standard method of securing a line to a cleat — a turn around the base, two figure-eight crossings, and a locking hitch.

A simple hitch for securing a line to a piling or post temporarily; not load-rated for permanent moorings.

The point through which the total weight of the vessel acts vertically downward; raised by adding topside weight.

Code of Federal Regulations — codified federal rules. 33 CFR governs Navigation; 46 CFR governs Shipping.

USCG application form for Merchant Mariner Credential (MMC).

USCG medical certificate form — physical exam required for every credential issuance and renewal.

USCG drug-test form — pre-employment test required with every credential application.

Sea-service form documenting time aboard small vessels for credential applications.

A USCG-issued document for inspected passenger vessels stating maximum passenger count, route, manning, and other operating restrictions.

A cylindrical green port-side aid in the US lateral system (red right returning).

An IALA buoy indicating the safe side of a hazard relative to a cardinal point of the compass; rare in US waters but standard internationally.

A for-hire fishing trip on a vessel licensed for paying passengers; OUPV is the typical credential.

A horizontal surface inside or on top of a vessel; the main deck is the principal weather deck.

The vertical distance from the waterline to the lowest point of the hull or keel.

A method of determining position by applying course steered and distance run from a known starting point, without external fixes.

The angle between magnetic north and compass north, caused by the vessel's own magnetism.

Differential GPS — augmented GPS using land-based reference stations for sub-metre accuracy.

Range to a target measured directly by the radar; combined with a visual or radar bearing produces a fix.

A running-fix technique: when the relative bearing to an object doubles, the distance run equals the distance to the object.

The speed at which a current is moving; the magnitude component of set & drift.

Lines published in 33 CFR Part 80 marking the boundary between Inland Rules and COLREGs waters.

Digital Selective Calling — the VHF protocol that lets a vessel send a single-button MAYDAY transmitting identity and position via the radio.

A black diamond day shape shown where it can best be seen. On the OUPV / Master rules exam, it usually signals either a tow exceeding 200 meters or the middle shape in a restricted-in-ability-to-maneuver ball-diamond-ball display.

Any of the signals listed in Annex IV of the Rules indicating that a vessel is in distress and requires assistance.

A pre-packed grab-bag containing emergency supplies (EPIRB, water, signal devices, first aid) brought when abandoning ship.

The carrying capacity of a vessel — fuel, cargo, water, stores, persons.

The weight of water displaced by the vessel; equals the vessel's total weight under Archimedes' principle.

Material placed under or between cargo to keep it dry, secure, and well separated.

An engine that continues running after the ignition is shut off, due to hot carbon deposits igniting fuel.

A fixed (non-floating) aid to navigation on a piling or platform; an unlit version of a navigational light.

The outgoing tidal current — water flowing away from shore as tide falls.

An official vector chart database produced by national hydrographic offices for use in ECDIS systems.

Position derived from a DR plot adjusted for known set & drift; less reliable than a fix.

A falling tide — the period between high and low water.

Emergency Position-Indicating Radio Beacon — a 406 MHz satellite-monitored distress beacon that transmits the vessel's identity and position when activated.

A unit of depth equal to 6 feet (1.83 m). Older charts give soundings in fathoms; modern US charts use feet or metres.

The compartment in the extreme bow of a vessel.

Toward the bow of the vessel.

The vertical distance from the waterline to the lowest point of the weather deck or gunwale.

A position on the earth determined by two or more lines of position from independent observations.

The incoming tidal current — water flowing toward shore as tide rises.

The distance over which wind blows uninterrupted across the water; longer fetch produces larger waves.

A rising tide — the period between low and high water.

A light flashing at regular intervals at a frequency of 120 or more flashes per minute (Rule 21(f)).

A sound signal required by Rule 35 in or near restricted visibility, irrespective of whether vessels are in sight of one another.

The three elements required for combustion — fuel, heat, and oxygen. Removing any one extinguishes the fire.

A document filed before departure detailing the vessel, route, persons aboard, and expected return; used by SAR if the vessel is overdue.

A stopper knot tied at the end of a line to prevent it from running through a block.

The reduction in stability caused by liquid in a partially-filled tank or compartment shifting as the vessel heels.

The kitchen of a vessel.

The top edge of a vessel's side, where the hull meets the deck. Often used as a handhold or rod-holder mount.

The distance at which a light is visible based on the height of the light, the height of the observer, and the curvature of the earth (visibility range, not luminous).

Global Positioning System — satellite-based positioning service operated by the US, accurate to ~5 m without augmentation.

A wind of 34–47 knots (Beaufort 8–9); triggers a USCG gale warning.

Long, regular waves generated by distant weather systems.

The vessel directed by the rules to keep out of the way; takes early and substantial action under Rule 16.

One of the distress signals — a gun fired at intervals of about one minute (Annex IV).

The vertical distance between the centre of gravity (G) and the metacentre (M); a positive GM indicates stable equilibrium.

A measure of a vessel's overall internal volume, used to determine credentialing thresholds (OUPV is for vessels <100 GT carrying ≤6 passengers).

The marine toilet; also the upper edge of a sail.

The wheel or tiller that controls the rudder.

The main body of a vessel — the watertight structure that gives it buoyancy.

The direction the vessel's bow is pointing at a given moment; differs from course when affected by set or drift.

Two power-driven vessels meeting on reciprocal or nearly reciprocal courses; both alter course to starboard to pass on the port side of the other (Rule 14).

A temporary lean of the vessel from the upright position, caused by external forces (wind, turn).

A line on a chart connecting points of equal magnetic variation.

Two vessels are 'in sight of one another' when one can see the other visually; Section II of the Steering and Sailing Rules applies.

33 CFR Part 83 — the US version of COLREGs that applies inside the demarcation lines.

A flexible rubber pump element used in raw-water cooling systems; degrades when run dry and requires periodic replacement.

A propulsion configuration where the engine is mounted inside the hull and connected via a shaft to a propeller.

The principal longitudinal structural member running along the bottom of the hull.

A unit of speed equal to 1 nautical mile per hour (~1.852 km/h or ~1.151 mph).

Away from the wind; the side or direction the wind is blowing toward.

A persistent lean of a vessel to one side, caused by uneven loading or a flooded compartment.

A weighted, marked line used to measure depth and obtain a sample of the bottom; the precursor to the depth sounder.

The lateral movement of a vessel downwind, caused by wind pressure on the hull and superstructure.

A line along which the vessel is known to lie based on a single observation (a bearing, a range, a sounding contour); a fix requires two or more LOPs.

Angular distance north or south of the equator, measured in degrees, minutes, and seconds.

Angular distance east or west of the prime meridian (Greenwich), measured in degrees, minutes, and seconds.

The maximum distance a light can be seen, considering only its intensity and prevailing visibility.

The downwind side of a vessel, island, or structure; sheltered from the wind.

A proper watch by sight, hearing, and all available means; required at all times by Rule 5.

The pattern (colour, period, on/off ratio) by which a navigational light is identified — e.g. Fl R 4s = flashing red every 4 seconds.

An inflatable raft carried as primary survival equipment on vessels making coastal or offshore passages.

Rope that has been put to a specific use on board — anchor line, dock line, halyard, sheet.

A cylindrical map projection where latitudes and longitudes are straight lines; rhumb lines (constant bearings) appear as straight lines.

Mean Lower Low Water — the average of the lower of the two daily low waters; the chart datum for soundings on US charts.

A vessel underway and moving through the water under her own power.

A white light placed over the fore-and-aft centreline showing an unbroken light over a 225° arc, 112.5° to either side of dead ahead (Rule 21(a)).

The international voice radio distress signal indicating grave and imminent danger requiring immediate assistance. Spoken three times.

A person who has fallen overboard. Standard responses include the crash-stop, Williamson turn, or Anderson turn depending on conditions.

A USCG credential authorising operation of inspected passenger vessels; tiers are 25, 50, 100, 200, 500, and 1,600 GT.

The USCG-issued credential booklet that contains all of a mariner's licenses, ratings, and endorsements.

1 minute of arc of latitude; equal to 1,852 m or about 1.151 statute miles.

The tide of smallest range, occurring at first and last quarter moon.

A channel where vessels are constrained to navigate near the outer limit on their starboard side under Rule 9.

A vessel which through some exceptional circumstance is unable to manoeuvre as required by the Rules and is therefore unable to keep out of the way (Rule 3(f)).

National Maritime Center — USCG body in Martinsburg, WV, that evaluates applications, issues credentials, and oversees the RECs.

A conical red starboard-side aid in the US lateral system (red right returning).

Coming up with another vessel from a direction more than 22.5° abaft her beam (Rule 13); the overtaking vessel keeps clear.

A self-contained propulsion unit (engine + lower unit + propeller) mounted on the transom.

Operator of Uninspected Passenger Vessels — the 'Six-Pack' license; authorises carrying up to 6 paying passengers on a vessel <100 GT.

An enclosed bridge or wheelhouse from which the vessel is steered.

The left side of a vessel when facing forward; marked by red lights and red navigation aids.

A rotating set of blades that propels the vessel by displacing water.

A pair of hinged straightedges used to transfer a course or bearing across a chart parallel to itself.

Navigation by reference to landmarks, aids to navigation, and soundings; visual and confirmed against charted features.

A blast of 4–6 seconds' duration on the whistle (Rule 32).

The international voice radio urgency signal indicating a serious situation that does not require immediate assistance.

An offshore life jacket — best buoyancy, designed to turn most unconscious wearers face-up. Required on commercial passenger vessels offshore.

A near-shore vest — less buoyancy than Type I but more comfortable; suitable for inshore charter use.

A flotation aid — good for conscious users in calm inland waters; common for water sports.

A throwable device (ring buoy, cushion). Required on board in addition to wearable PFDs.

A special-use device (work vest, hybrid inflatable) approved for a specific activity; restrictions printed on the label.

The international load line marked on a vessel's side indicating the maximum legal loaded draft for various conditions.

A hinged hook with a quick-release lever, often used on lifelines and life-raft tie-downs.

Personal Protective Equipment — hardhat, gloves, hearing protection, safety glasses; required on commercial vessels under 46 CFR.

A flat surface aft of the propeller, hinged to the hull, used to steer the vessel.

Two charted objects in line; passing through the range provides a precise line of position.

A line of constant bearing; appears straight on a Mercator chart but is not the shortest distance over the earth.

A fix obtained from two LOPs taken at different times, with the first advanced along the DR track to the time of the second.

A vessel which from the nature of her work is restricted in her ability to manoeuvre and therefore cannot keep out of the way of another vessel (Rule 3(g)).

Any condition (fog, mist, falling snow, heavy rain, sandstorms) where visibility is restricted; Rule 19 applies (Rule 3(l)).

A condition existing when an approaching vessel cannot be passed at a safe distance on present courses; if doubt exists, risk shall be deemed to exist (Rule 7).

The line and chain combination used to secure the anchor.

A hitch used to attach a line to a rope or rod under load; the hitch grips when load is applied.

The horizontal distance between the lines of action of buoyancy and gravity when the vessel heels; the moment arm that returns the vessel to upright.

An engine cooling system that pumps sea or lake water directly through the engine; simpler but more corrosive than closed-cooling.

Revolutions per minute — engine or propeller rotational speed.

Regional Examination Center — one of 17 USCG facilities that administer captain's license written exams and credential applications.

The mnemonic for the US IALA-B lateral system — keep red buoys on your starboard side when returning to port from open water.

A white buoy or sign marking restricted areas, controlled areas, danger areas, or information; defined in 33 CFR Subchapter C.

A drain in the deck, gunwale, or bulwark that lets water flow overboard.

A vertical post supporting lifelines around the deck.

The right side of a vessel when facing forward; marked by green lights and green aids.

The aft end of a vessel.

The direction toward which a current is flowing; the direction component of set & drift.

A shallow area, typically of sand, gravel, or rock, that may be hazardous to navigation.

Actual speed made good over the ground; differs from speed through the water when current is acting.

A measurement of water depth, usually shown on a chart at a given position.

Speed of the vessel relative to the surrounding water, measured by a paddlewheel or pitometer log.

The tide of greatest range, occurring at new and full moon.

The brief period at the change of tidal current when the flow is essentially zero.

NWS advisory issued for sustained winds of ~22–33 knots or hazardous wave conditions.

NWS warning issued for sustained winds of 48–63 knots.

Wind-generated waves that have travelled out of the area where they were generated.

A speed appropriate to prevailing circumstances such that proper and effective action can be taken to avoid collision (Rule 6).

A blast of about 1 second's duration on the whistle (Rule 32).

The vessel directed by the rules to maintain her course and speed; may take action when collision cannot be avoided by the give-way alone (Rule 17).

A green light on the starboard side and a red light on the port side, each showing an unbroken light over a 112.5° arc (Rule 21(b)).

A white light placed as nearly as practicable at the stern, showing an unbroken light over a 135° arc, 67.5° from right aft on each side (Rule 21(c)).

The international voice radio safety signal preceding navigation or weather warnings.

Search and Rescue Transponder — a radar-band beacon that responds to a 9 GHz radar interrogation with a series of dots, used in life-rafts.

A signal made on the whistle, bell, or gong as required by the Rules — manoeuvring signals (Rule 34) or fog signals (Rule 35).

A reflective signalling device used to attract the attention of aircraft or distant vessels.

A daytime distress device producing dense orange smoke; one of the visual distress signals listed in Annex IV.

International Convention for the Safety of Life at Sea — the IMO treaty governing safety standards for international shipping.

The ratio of the length of anchor rode deployed to the depth of water plus the height of the bow above the water; typical 7:1 in good conditions.

A knot used to join two lines, especially of unequal diameter.

A short stretchy line attached to anchor rode to absorb shock loads.

A knot used to join two lines of similar diameter for non-load-bearing tasks; not safe for critical use.

A filter on the raw-water intake line that removes debris before water reaches the impeller.

A propulsion configuration with the engine inside the hull and a steerable lower unit through the transom; combines inboard power with outboard manoeuvrability.

46 CFR Subchapter C — regulations for uninspected vessels.

46 CFR Subchapter K — regulations for small passenger vessels carrying more than 150 passengers or with overnight accommodations for more than 49.

46 CFR Subchapter T — regulations for small passenger vessels under 100 GT carrying ≤150 passengers.

A red-and-white vertically striped buoy marking deep water (often a channel entrance from sea); also called a 'fairway' buoy.

A transverse seat or bracing in an open boat.

The flat or slightly curved aft surface of the hull at the stern.

The fore-and-aft balance of a vessel; bow-down or stern-down vs level (in trim).

The horizontal flow of water associated with the rise and fall of the tide.

The vertical rise and fall of the sea level due to the gravitational interaction of the moon, sun, and earth.

An IMO-adopted routing scheme with one-way lanes and a separation zone; Rule 10 governs conduct within.

A yellow light with the same characteristics as the sternlight (Rule 21(d)).

Transportation Worker Identification Credential — TSA-issued biometric card required for unescorted access to secure maritime facilities; mandatory for every USCG credential.

Not at anchor, made fast to the shore, or aground (Rule 3(i)). A vessel can be underway and not making way.

United States Coast Guard — the federal armed service responsible for maritime safety, security, and stewardship; issues all captain's-license credentials.

The angle between true north and magnetic north at a given location; published on charts and changes slowly with time.

A bearing obtained by sighting an object on land or another vessel using a compass or bearing circle.

USCG-regulated traffic management in select harbours; the Inland counterpart of TSS.

The international VHF distress, safety, and calling frequency (156.800 MHz). Required to be monitored when a VHF radio is on board.

Documented vessels must display the name and hailing port at the stern in characters of specified minimum size (33 CFR §67.123).

The line where the hull meets the water at a given load condition.

Toward the wind; the side or direction the wind is blowing from.

A sound-signalling appliance capable of producing the prescribed short and prolonged blasts (Rule 32(a)).

A man-overboard manoeuvre — turn 60° one direction, then full rudder the other way until on the reciprocal heading; used at night or in poor visibility.