Maritime Glossary

A

Abandon ship

The order to leave a vessel in an emergency, requiring all personnel to muster at their assigned lifeboat stations and launch survival craft in the correct sequence. Governed by SOLAS Regulation III and the vessel's Muster List. Practice: Abandon Ship Drill →

AIS (Automatic Identification System)

A VHF transponder system that broadcasts a vessel's MMSI, name, position, course, speed, and type on channels 87B (161.975 MHz) and 88B (162.025 MHz). Mandatory on SOLAS vessels over 300 GT and all passenger vessels. Practice: Harbor Control →

Anchor watch

A continuous watch maintained when a vessel is anchored to detect dragging — monitoring bearings on charted objects or anchor alarms, and keeping the engine in a state of immediate readiness. Practice: Anchor Watch →

B

Beam

The maximum width of a vessel, measured at the widest point of the hull. A vessel with a large beam-to-length ratio is generally more stable but may be limited by harbour and lock widths. Practice: Docking Master →

Bearing

The direction to an object, measured in degrees clockwise from north (true bearing) or from the ship's head (relative bearing). Three simultaneous bearings taken on charted objects give a position fix. Practice: Sat-Nav Fix →

Beaufort scale

A numerical scale from 0 (calm, less than 1 kt) to 12 (hurricane force, over 64 kt) describing mean wind speed and associated sea state. Force 6 (22–27 kt) is a strong breeze; Force 8 (34–40 kt) is a gale. Practice: Heavy Weather →

Bunkering

The process of loading fuel oil (heavy fuel oil, marine gas oil, or LNG) onto a vessel from a bunker barge or shore terminal. Governed by MARPOL Annex I; requires a Bunker Delivery Note and an Oil Record Book entry. Practice: Bunkering →

Buoyage (IALA system)

The International Association of Lighthouse Authorities system of navigational marks. Region A (red port, used outside North America) and Region B (red starboard, used in North America) define lateral mark colours. Also includes cardinal, isolated danger, safe water, and special marks. Practice: Buoyage →

C

Cardinal mark

A buoy or structure indicating safe water on the named side of a hazard — north, south, east, or west. Identified by yellow-and-black banding, two cone-shaped topmarks, and a white light with a specific rhythm of quick or very quick flashes. Practice: Buoyage →

COG (Course Over Ground)

The direction of actual travel of a vessel over the seabed, incorporating both the course steered and the set and drift of the current. COG differs from heading when there is a current or leeway component. Practice: Chart a Course →

COLREGS

The International Regulations for Preventing Collisions at Sea, published as IMO Resolution A.1108(29). The 38 rules and 4 annexes govern all vessel encounters on the high seas and connecting waters navigable by seagoing vessels. Practice: COLREGS Quiz →

Compass deviation

The error in a magnetic compass caused by the magnetism of the vessel's own steel structure and equipment. Deviation varies with the ship's heading and is recorded on a deviation card; it is corrected by adjusting soft-iron correctors and permanent magnets in the binnacle. Practice: Compass Swing →

CPA (Closest Point of Approach)

The minimum distance between two vessels if both maintain their current courses and speeds. CPA and TCPA (time to CPA) are the primary risk-of-collision criteria under COLREGS Rule 7. A CPA of zero means the vessels will collide if nothing changes. Practice: Bridge Watch →

D

Dead reckoning

Estimating current position by applying the vessel's course, speed, and time elapsed to the last known fix. Subject to cumulative error from current set and drift, leeway, and helmsman error; must be updated with regular position fixes. Practice: Chart a Course →

Deviation card

A table or graph showing compass deviation on each heading, compiled after a compass swing. Used to convert compass bearing to magnetic bearing by applying the residual deviation. Practice: Compass Swing →

Draft (draught)

The vertical distance between the waterline and the bottom of the keel. A vessel's departure draft must be calculated from the load distribution before sailing to ensure stability limits are met. Practice: Stability Calculator →

DSC (Digital Selective Calling)

A digital calling system using VHF Channel 70 (and MF/HF equivalents) that allows vessels to send and receive distress alerts, urgency calls, and routine position reports. A DSC distress alert transmits MMSI, position, time, and nature of distress automatically. Practice: Distress Relay →

E

EPIRB (Emergency Position-Indicating Radio Beacon)

A 406 MHz distress beacon that transmits a coded identification signal to the COSPAS-SARSAT satellite constellation. Category I EPIRBs float free and activate automatically when submerged; Category II are manually activated. Both must be registered with the national authority linked to the vessel's MMSI. Practice: EPIRB Test →

F

Free surface effect

The reduction in effective metacentric height (GM) caused by the movement of liquid in a slack (partially filled) tank as the vessel heels. Free surface effect is proportional to the cube of the tank's breadth — wide tanks have a disproportionately large effect and should be kept full or empty. Practice: Stability Calculator →

G

Give-way vessel

The vessel required to take action to keep clear in a collision situation. In a crossing encounter (Rule 15), the vessel with the other on her starboard side is the give-way vessel. Rule 16 requires early and substantial action. Practice: COLREGS Quiz →

GM (Metacentric Height)

The distance between the centre of gravity (G) and the metacentre (M), measured along the vessel's centreline. Positive GM indicates an initial tendency to return to upright; negative GM means the vessel will loll to one side. Minimum GM requirements are set in the Trim and Stability Booklet. Practice: Stability Calculator →

GMDSS (Global Maritime Distress and Safety System)

The international radio communications system for maritime safety, comprising DSC on VHF/MF/HF, EPIRB, SART, Navtex, and Inmarsat satellite communications. Mandatory on all SOLAS vessels. Replaces the old Morse code distress watchkeeping requirement. Practice: Distress Relay →

H

Heavy weather

Conditions of Force 7 or above (near gale, 28+ kt) requiring changes to vessel course and speed to maintain safe motion. A vessel running in heavy following seas is at risk of broaching or synchronous rolling; a vessel in head seas risks slamming and green-water damage. Practice: Heavy Weather →

HW (High Water)

The maximum tidal height in a tidal cycle. The time of HW at a secondary port is found by applying the High Water Time Difference to the time of HW at the standard port. Practice: Tide Tables →

I

Ice navigation

The specialized seamanship of operating in sea ice — reading ice charts and SIGRID notation, identifying leads of open water, adjusting speed to minimize hull stress, and managing the risk of beset. Requires an Ice Navigator endorsement for Polar Code operations. Practice: Ice Navigation →

IMDG Code

The International Maritime Dangerous Goods Code, published by the IMO under SOLAS Regulation VII. Classifies dangerous goods into 9 classes, assigns stowage categories (on deck, under deck, prohibited), and specifies segregation requirements between incompatible goods. Practice: Stow It →

IMO (International Maritime Organization)

The United Nations agency responsible for the safety and security of international shipping and prevention of marine pollution by ships. Key instruments include SOLAS, MARPOL, STCW, the COLREGs, and the Polar Code. Practice: Inspection →

J

Juliett (NATO phonetic)

The ICAO/NATO phonetic word for the letter J. Note the spelling Juliett (two t's), specified in ICAO Annex 10 to ensure correct pronunciation by non-English speakers. Hoisting the single-letter flag J at sea means "I am on fire and have dangerous cargo on board; keep well clear." Practice: Phonetic Alphabet →

K

KG (Centre of Gravity height)

The height of a vessel's centre of gravity above the keel, in metres. KG rises when weight is added at a high position; it falls when weight is removed from above the current G or added below it. GM = KM − KG, so raising KG reduces GM and worsens stability. Practice: Stability Calculator →

Knot

A unit of speed equal to one nautical mile per hour (1.852 km/h). The term originates from the 16th-century chip log, where a knotted rope was streamed astern and the knots counted as they ran out over a sandglass. Practice: Chart a Course →

L

Lateral mark

A buoy or structure indicating the boundary of a navigable channel. In IALA Region B (North America), red lateral marks (even numbers) are kept to starboard when returning from sea ("red right returning"); green lateral marks (odd numbers) are kept to port. Practice: Buoyage →

LOA (Length Overall)

The maximum length of a vessel from the foremost point of the bow to the aftermost point of the stern, excluding anchor handling equipment. LOA determines berth length requirements and, along with beam and draft, the lock dimensions the vessel can transit. Practice: Docking Master →

Lock (canal)

A section of a waterway enclosed by gates at each end, used to raise or lower vessels between reaches of different water level by filling (lockage up) or emptying (lockage down) the chamber. Water level is equalised before any gate is opened. Practice: Lock Transit →

LW (Low Water)

The minimum tidal height in a tidal cycle. The tidal range is the difference between HW and LW heights at the same place. Low water is the moment of greatest concern for vessels navigating shallow or bar-restricted approaches. Practice: Tide Tables →

M

MARPOL

The International Convention for the Prevention of Pollution from Ships. Annex I governs oil discharges and spill prevention; Annex VI sets sulphur (ECA limits: 0.10%) and NOx emission limits. Bunkering operations must comply with Annex I procedures and be recorded in the Oil Record Book. Practice: Bunkering →

Mayday

The international voice distress signal, derived from the French "m'aidez" (help me). Transmitted three times on VHF Channel 16, followed by vessel name, MMSI, position, nature of distress, number of persons, and any other useful information. A Mayday takes absolute communications priority. Practice: Mayday →

MMC (Merchant Mariner Credential)

The single credential issued by the USCG National Maritime Center combining all endorsements — licence grade, STCW certificates, and ratings — on one document. Replaced separate licences and certificates in 2009 under 46 CFR Part 10. Practice: USCG Exam Prep →

Muster list

The official station bill specifying every crew member's station, duties, and signals for emergencies including fire, flooding, and abandon ship. Required by SOLAS Regulation III/37; must be posted throughout the vessel in the crew's language. Practice: Abandon Ship Drill →

N

NATO phonetic alphabet

The ICAO/ITU spelling alphabet assigning a word to each letter: Alfa, Bravo, Charlie, Delta, Echo, Foxtrot, Golf, Hotel, India, Juliett, Kilo, Lima, Mike, November, Oscar, Papa, Quebec, Romeo, Sierra, Tango, Uniform, Victor, Whiskey, X-ray, Yankee, Zulu. Used in all VHF radio communications to spell call signs and waypoints without ambiguity. Practice: Phonetic Alphabet →

Nautical mile

A unit of distance equal to 1,852 metres, defined as one minute of arc of latitude along a meridian. One nautical mile per hour equals one knot. Position on a nautical chart can be measured directly with the latitude scale on either side of the chart. Practice: Chart a Course →

NMC (National Maritime Center)

The USCG office in Martinsburg, West Virginia responsible for processing Merchant Mariner Credentials, administering written examinations, and maintaining the official NMC question bank from which all USCG deck officer exam questions are drawn. Practice: USCG Exam Prep →

NUC (Not Under Command)

A vessel unable to manoeuvre as required by the COLREGS due to some exceptional circumstance — engine failure, loss of steering, jammed anchor. Displays two all-round red lights (at night) or two black balls in a vertical line (day). Has right of way over power-driven and sailing vessels. Practice: Night Lights & Shapes →

O

OUPV (Operator of Uninspected Passenger Vessels)

Commonly called the "6-Pack" licence, the OUPV authorizes carrying up to 6 passengers for hire on uninspected vessels under 100 GRT. Requires passing the Rules of the Road, Chart Navigation, and Deck Safety exam modules plus a drug test and physical. Practice: USCG Exam Prep →

P

Pan-Pan

The international urgency signal, indicating a very urgent message concerning the safety of a ship, aircraft, or person. Transmitted three times on VHF Channel 16, it has lower priority than Mayday but requires all stations to listen. A typical Pan-Pan is a medical emergency or a vessel that has lost power. Practice: Distress Relay →

Port side

The left side of a vessel when facing forward (toward the bow). Marked at night by a red sidelight with a 112.5° arc, from dead ahead to 22.5° abaft the beam on the port side. Port lateral marks in North America (IALA B) are also red. Practice: Night Lights & Shapes →

PSC (Port State Control)

The inspection regime allowing flag-state-independent USCG or foreign coast guard officers to board and inspect foreign vessels in port. Deficiencies that render the vessel unseaworthy or a danger to persons on board can result in detention until rectified. Practice: Inspection →

R

RAM (Restricted in Ability to Manoeuvre)

A vessel that by the nature of her work is restricted in her ability to manoeuvre as required by the COLREGS — dredging, surveying, towing, laying cable, replenishment at sea. Displays a ball-diamond-ball day shape and red-white-red all-round lights at night. Practice: Night Lights & Shapes →

Rule of Twelfths

An approximation of tidal height change over a semi-diurnal tidal cycle. The tide rises or falls approximately 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of the tidal range in each successive hour of flood or ebb. Used to estimate the depth of water in tidal channels at any state of tide. Practice: Tide Tables →

S

SAR (Search and Rescue)

Maritime SAR is coordinated by the Joint Rescue Coordination Centre (JRCC) under the IAMSAR Manual. The On-Scene Coordinator is usually the first vessel to arrive. Search patterns (expanding square, sector, parallel track) are selected based on the uncertainty of the datum position. Practice: Mayday →

SART (Search and Rescue Transponder)

A 9 GHz radar transponder that, when activated, paints a line of 12 blips on any X-band radar within approximately 8 nm. Carried in survival craft; activated when a SAR radar is detected. Practice: Distress Relay →

Sécurité

The international safety signal, used to announce a navigational or meteorological warning — a floating object, a missing buoy, or a TSS crossing. Transmitted three times on VHF Channel 16 followed by the message on a working channel. Lower priority than Pan-Pan and Mayday. Practice: Distress Relay →

Sextant

A navigational instrument that measures the angular altitude of a celestial body above the visible horizon by reflecting the body's image onto the horizon using two mirrors. After applying index error, dip, and refraction corrections, the observed altitude (Ho) is compared to the calculated altitude (Hc) to produce a line of position. Practice: Celestial Fix →

Sight reduction

The mathematical process of converting a sextant altitude observation (Hs) to an observed altitude (Ho) by applying corrections, then comparing Ho to the calculated altitude (Hc) derived from the assumed position. The difference (intercept) and the azimuth define the line of position. Practice: Celestial Fix →

SOLAS

The International Convention for the Safety of Life at Sea, first adopted in 1914 after the Titanic. The current version (SOLAS 1974, as amended) is the principal maritime safety treaty, covering construction, fire safety, lifesaving appliances, radio, navigation, cargo, and nuclear ships. Practice: Abandon Ship Drill →

Stand-on vessel

The vessel required to maintain course and speed while the give-way vessel takes action. Under Rule 17, the stand-on vessel may take action once it becomes apparent the give-way vessel is not acting, and must take action under Rule 17(b) when collision cannot be avoided by the give-way vessel alone. Practice: COLREGS Quiz →

Starboard side

The right side of a vessel when facing forward. Marked at night by a green sidelight with a 112.5° arc, from dead ahead to 22.5° abaft the beam on the starboard side. Starboard lateral marks in North America (IALA B) are green. Practice: Night Lights & Shapes →

STCW (Standards of Training, Certification and Watchkeeping)

The IMO convention establishing minimum training, certification, and watchkeeping standards for seafarers. The 2010 Manila Amendments introduced Basic Safety Training (BST), Advanced Fire-Fighting, Medical First Aid, and Proficiency in Survival Craft as mandatory for all seagoing personnel. Practice: USCG Exam Prep →

SWL (Safe Working Load)

The maximum load that a piece of cargo lifting equipment — crane, derrick, shackle, wire — is certified to carry. SWL is reduced when a crane operates at a large radius or when two cranes lift together in a Union purchase configuration. Annual inspection by a competent authority is required. Practice: Cargo Gear →

T

TCPA (Time to Closest Point of Approach)

The time remaining until two vessels on their current courses and speeds will reach CPA. Used together with CPA to assess collision risk. A short TCPA combined with a small CPA indicates an urgent close-quarters situation requiring immediate action under COLREGS Rule 8 or 16. Practice: Bridge Watch →

Tide

The periodic rise and fall of sea level caused by the gravitational attraction of the Moon (primary) and Sun (secondary). Spring tides (large range) occur near new and full moon; neap tides (small range) occur near first and third quarter. Semi-diurnal tides have two highs and two lows per day. Practice: Tide Tables →

TSS (Traffic Separation Scheme)

A routing measure where vessels in opposite directions are separated into traffic lanes, typically in congested straits or approaches. Vessels shall use the appropriate traffic lane proceeding in the general direction of traffic flow for that lane (COLREGS Rule 10). Practice: Harbor Control →

V

VHF (Very High Frequency)

Radio frequencies from 156–174 MHz used for short-range maritime communications, typically 20–50 nm. Channel 16 (156.8 MHz) is the international distress, safety, and calling channel, monitored continuously. Channel 70 is reserved for DSC digital selective calling. Practice: VHF Squelch →

VTS (Vessel Traffic Service)

A maritime traffic monitoring and control service analogous to air traffic control, operated by port authorities or coast guards. VTS provides traffic information, navigational assistance, and in some ports, traffic organisation (mandatory routeing instructions). Practice: Harbor Control →

W

Watch (navigation)

A period of duty on the bridge or in the engine room. The standard pattern on deep-sea vessels is 4-on/8-off (three watches); on some vessels 6-on/6-off (two watches), though this conflicts with STCW rest-hour requirements and is discouraged. The officer taking the watch must receive a thorough handover before assuming the conn. Practice: Watch Handover →

WTD (Watertight Door)

A door fitted to a watertight bulkhead that, when closed and dogged, prevents the passage of water and maintains the watertight integrity of the vessel. SOLAS requires WTDs to be closed in heavy weather and in all emergency situations. Power-operated WTDs must be capable of local manual operation from both sides. Practice: Damage Control →