Maritime Terms and Acronyms: A Seafarer's Essential Glossary

Maritime has its own language — and it is not just jargon for its own sake. Precise terminology exists because on a vessel, ambiguity can be dangerous. Knowing the correct terms also marks you as a professional who belongs in the environment. This glossary covers the most essential navigation acronyms, vessel terms, rank titles, and regulatory abbreviations used across the UK maritime industry.

Navigation Terms

• ARPA: Automatic Radar Plotting Aid — a radar system that automatically tracks other vessels and calculates their course, speed, CPA, and TCPA.
• AIS: Automatic Identification System — transponders fitted to vessels that broadcast name, MMSI, position, speed, and course. Visible on ECDIS and dedicated AIS receivers.
• BRG: Bearing — the direction to an object, measured in degrees true (T) or magnetic (M) from north.
• COG: Course Over Ground — the actual direction the vessel is moving relative to the earth, accounting for current and leeway.
• CPA: Closest Point of Approach — the minimum distance a target vessel will pass if both vessels maintain current course and speed.
• DR: Dead Reckoning — estimating current position based on a known previous position, speed, course, and elapsed time, without external position fix.
• ECDIS: Electronic Chart Display and Information System — the electronic chart navigation system, now mandatory on most internationally trading vessels.
• ENC: Electronic Navigational Chart — the digital chart data used by ECDIS, produced to IHO standards.
• ETA: Estimated Time of Arrival.
• GNSS: Global Navigation Satellite System — the collective term for all satellite positioning systems including GPS (USA), GLONASS (Russia), Galileo (EU), and BeiDou (China).
• GPS: Global Positioning System — the US-operated satellite navigation system, the most widely used GNSS in maritime.
• HDG: Heading — the direction the bow of the vessel is pointing, measured in degrees true or magnetic.
• SOG: Speed Over Ground — actual speed relative to the earth (vs STW — Speed Through Water).
• TCPA: Time to Closest Point of Approach — how long until the CPA is reached.
• WPT: Waypoint — a defined position in a route plan.
• XTE: Cross Track Error — the distance the vessel has deviated off the planned route line.

Safety and Regulatory Abbreviations

• COLREGS: Collision Regulations — the International Regulations for Preventing Collisions at Sea (1972), the maritime rules of the road.
• EPIRB: Emergency Position Indicating Radio Beacon — activates on immersion to transmit distress signal via Cospas-Sarsat satellite system.
• GMDSS: Global Maritime Distress and Safety System — the integrated communications system for maritime distress, urgency, and safety.
• ISM Code: International Safety Management Code — the SOLAS-mandated framework for shipowner SMS documentation and safety culture.
• ISPS Code: International Ship and Port Facility Security Code — the post-9/11 framework for vessel and port security.
• LSA: Life-Saving Appliances — lifeboats, liferafts, lifejackets, immersion suits, EPIRBs, SARTs.
• MAIB: Marine Accident Investigation Branch — the UK independent body that investigates maritime accidents. Reports are publicly available and widely read for safety learning.
• MARPOL: International Convention for the Prevention of Pollution from Ships — covers oil, chemicals, sewage, garbage, air emissions, and ballast water.
• MCA: Maritime and Coastguard Agency — the UK government body responsible for maritime safety, regulation, and Coastguard operations.
• MLC: Maritime Labour Convention 2006 — the international instrument establishing minimum employment and welfare rights for seafarers globally.
• MOB: Man Overboard — person in the water, triggers immediate rescue procedure.
• MRCC: Maritime Rescue Coordination Centre — coordinates search and rescue operations. UK MRCC is at HM Coastguard.
• PSC: Port State Control — the system by which port state authorities inspect foreign-flagged vessels for compliance with international conventions.
• SAR: Search and Rescue.
• SART: Search and Rescue Transponder — responds to 9GHz radar with a distinctive pattern to aid location of a survival craft or MOB.
• SMS: Safety Management System — the documented safety procedures required by the ISM Code.
• SOLAS: Safety of Life at Sea — the primary IMO convention covering vessel construction, equipment, and operational safety.
• VDR: Voyage Data Recorder — the maritime equivalent of an aircraft black box.

Vessel Types and Size Measurements

• GT (Gross Tonnage): A measure of the overall internal volume of a vessel. Used for regulatory thresholds (SOLAS, MLC, STCW). Not a weight measurement.
• DWT (Deadweight Tonnage): The total weight a vessel can carry — cargo, fuel, water, stores, crew, and passengers.
• LOA: Length Overall — the maximum length of the vessel.
• Ro-Ro: Roll-on/Roll-off — vessel with vehicle decks accessed via ramps (ferries, car carriers, freight ferries).
• VLCC: Very Large Crude Carrier — large oil tanker (200,000–319,999 DWT).
• LNG Carrier: Vessel designed to transport liquefied natural gas at -162°C.
• SOV: Service Operation Vessel — large offshore wind support vessel that stations at a wind farm for extended periods.
• CTV: Crew Transfer Vessel — fast small vessel used to transfer wind turbine technicians to offshore installations.
• DSV: Dive Support Vessel — specialist vessel supporting subsea diving and ROV operations.
• OSV: Offshore Support Vessel — general term for vessels supporting offshore oil, gas, and wind operations.

Vessel Parts and Spaces

• Port: The left side of a vessel when facing forward.
• Starboard: The right side when facing forward.
• Bow: The forward (front) end of the vessel.
• Stern: The aft (rear) end of the vessel.
• Amidships: The midpoint of the vessel's length.
• Bulkhead: A vertical partition (wall) within the vessel structure.
• Deckhead: The ceiling of an internal compartment.
• Forecastle (fo'c'sle): The forward section of the upper deck, traditionally crew accommodation, now usually mooring equipment storage.
• Draft: The depth of water from the waterline to the lowest point of the vessel's hull.
• Freeboard: The distance from the waterline to the main deck — a measure of reserve buoyancy.
• Trim: The difference between forward and aft draft. A vessel trimmed by the stern has a greater draft aft than forward.

Ranks, Qualifications and Organisations

• AB: Able Seaman (Able Seafarer) — qualified rating with STCW Reg II/5 or III/5 certificate.
• OOW: Officer of the Watch — the officer on duty, responsible for safe navigation of the vessel.
• C/O: Chief Officer or Chief Mate — second in command of the vessel.
• CoC: Certificate of Competency — the MCA qualification authorising a seafarer to hold a specific rank.
• CoP: Certificate of Proficiency — an STCW endorsement certificate for a specific skill or training.
• DPA: Designated Person Ashore — the company representative under ISM Code who has direct access to the highest level of management for safety issues.
• IMO: International Maritime Organization — the United Nations agency responsible for international shipping regulation.
• MNTB: Merchant Navy Training Board — the UK body that approves cadet training programmes and oversees qualification standards.
• TRB: Training Record Book — the cadet's documented evidence of competencies achieved at sea, required for CoC application.
• SED: Seafarers' Earnings Deduction — the UK tax relief available to qualifying seafarers working internationally.
• SMarT: Support for Maritime Training — the UK government funding scheme subsidising cadet sponsorship costs for qualifying companies.