Maritime glossary

Terms, acronyms, and jargon used by working seafarers, surveyors, and crewing managers. Each entry has a one-line definition for the messroom search and an expanded explanation for the page itself, plus cross-links to related entries and a citation for the source. Where a term is defined by an IMO convention (SOLAS, MARPOL, COLREGs, STCW, MLC, IMDG, IBC, IGC), the relevant article or regulation is named.

607 terms indexed. Browse by letter or category, or search.

• 'Tween Deck
  An intermediate deck between the upper and lower decks of a cargo hold, providing separate stowage tiers for break-bulk cargo.
• 15 ppm Bilge Alarm
  An automatic alarm fitted to an oily water separator that stops overboard discharge when oil content exceeds 15 parts per million.
• Able Seaman (AB)
  Certified deck rating with a minimum of 18 months sea service, competent in watchkeeping, mooring, and cargo handling.
• Abrasive Blast Cleaning
  Surface preparation by abrasive grit; standards Sa 1, Sa 2, Sa 2.5, Sa 3 per ISO 8501.
• Aframax
  Crude tanker 80,000-120,000 DWT — historically the AFRA (Average Freight Rate Assessment) midpoint; now the most common medium-haul crude size.
• AFS Convention
  International Convention on the Control of Harmful Anti-Fouling Systems on Ships (2001).
• AIS Aid to Navigation (AIS-AtoN)
  AIS station fitted on a buoy/light/synthetic mark broadcasting position and identity for ECDIS/radar overlay.
• AIS Class A
  The mandatory AIS transponder fitted on SOLAS ships transmitting vessel data at regular intervals and responding to interrogations.
• AIS Class B
  A lower-power AIS transponder used by smaller non-SOLAS vessels, transmitting position and identity data at reduced reporting rates.
• AIS-SART (AIS Search and Rescue Transmitter)
  Survival-craft device that broadcasts AIS Class A position reports when activated, with an MMSI beginning 970, enabling rescuers to plot its position on AIS displays.
• AIS-SART
  Survival craft locator that transmits a synthetic AIS Class A signal pattern to nearby ship and aircraft AIS receivers.
• Ammonia as Marine Fuel
  Carbon-free fuel under development for two-stroke engines; key challenges are toxicity, NOx, and ammonia slip.
• Anchor Handling Tug Supply (AHTS)
  Offshore vessel that handles MODU anchors during rig moves and supplies installations with deck cargo, mud, water, fuel.
• Anchor Handling Tug Supply Vessel (AHTS)
  An offshore support vessel designed to tow and position mobile drilling rigs and handle anchors in deepwater operations.
• Anchor Handling Tug Supply Vessel (AHTS)
  OSV that anchors and tows mobile offshore units and supplies rigs; high bollard pull and stern A-frame.
• Anchor Watch
  Continuous bridge / deck watch monitoring position, swing, and chain tension to detect dragging.
• Anchorage
  A designated area with suitable holding ground where a vessel anchors to wait for a berth, tide, or clearance.
• Anemometer
  An instrument for measuring wind speed, with the cup anemometer and sonic anemometer being the most common types aboard ships.
• Angle of Loll
  The angle at which a vessel with negative initial stability comes to rest on her side as she cannot return to upright.
• Angle of Vanishing Stability
  Heel angle at which the GZ righting arm becomes zero — beyond which the ship will capsize.
• Annual Survey
  A classification inspection conducted each year to confirm that a vessel's hull and equipment remain in a satisfactory condition.
• Antifouling Coating
  Hull coating slowing or preventing biofouling; modern systems are silicone-based (foul-release) or copper/zinc biocidal.
• articulated tug-barge (ATB)
  Tug rigidly locked into a purpose-built notch barge via a specialised coupling, operating as a single integrated unit while retaining separate vessel registration.
• Automatic Identification System (AIS)
  VHF transponder system that automatically broadcasts vessel identity, position, course, and speed to nearby ships and shore stations.
• Autonomous Underwater Vehicle (AUV)
  Untethered, programmable underwater vehicle for survey work — typically hydrographic, environmental, or military.
• Auxiliary Engine (AE)
  A diesel generator engine providing electrical power for shipboard systems independent of the main engine.
• Azimuth
  The angular bearing of a celestial body measured clockwise from true north, used to check compass error.
• Azimuth Stern Drive Tug (ASD)
  Harbour/escort tug with two stern-mounted azimuth (Z-drive) thrusters giving 360° thrust direction.
• Azimuth Thruster
  A propulsion unit that can rotate 360 degrees, providing both thrust and steering without a conventional rudder.
• Backing
  An anti-clockwise shift in wind direction in the Northern Hemisphere, often indicating the approach of a low pressure system or cold front.
• Bahamian Moor
  Mooring with both bow anchors set roughly 180 deg apart, restricting the swing circle in tight anchorages.
• Ballast Water Management Convention (BWM Convention)
  The 2004 IMO convention requiring ships to manage ballast water to prevent transfer of invasive aquatic species between regions.
• Ballast Water Record Book (BWRB)
  A mandatory log recording all ballast water uptake, treatment, exchange, and discharge operations under the BWM Convention.
• Ballast Water Standard D-1
  Ballast water exchange standard requiring at least 95% volume exchange in deep ocean water.
• Ballast Water Standard D-2
  Performance standard limiting viable organisms in discharged ballast; met by an approved ballast water management system.
• BAPLIE
  An EDI message format conveying the stowage positions of all containers aboard a vessel between terminals and ship operators.
• Bareboat Charter
  Demise charter where charterer takes full operational control, including crewing and maintenance, for a fixed period.
• barge
  Non-self-propelled flat-bottomed vessel pushed or towed by a tug, used on inland waterways and coastal routes to carry bulk, liquid, or project cargo.
• Basic Offshore Safety Induction & Emergency Training (BOSIET)
  OPITO-accredited 3-day course required for new offshore travellers; includes HUET, fire, escape, first aid.
• Basic Safety Training (BST)
  Mandatory STCW training covering personal survival, fire prevention, first aid, and personal safety for all seafarers.
• Bay Plan
  A grid diagram showing every container slot in a vessel's cross-section by bay, row, and tier with container data.
• Bearing
  The horizontal direction of one point from another, expressed in degrees from 000° to 359°.
• Beaufort Scale
  An empirical scale from 0 to 12 that relates wind speed to observed sea surface conditions and wave heights.
• Bergy Bit
  Medium fragment of glacier ice, 1–5 m above water and 5–15 m across.
• Berth
  A designated position at a wharf or quay where a vessel moors to load or discharge cargo or embark passengers.
• Best Management Practices 5 (BMP5)
  Joint industry guidance for protection against ship attacks in the Western Indian Ocean and Gulf of Aden / Arabian Sea.
• Bilge
  The lowest interior space of a vessel where leakage, condensation, and drainage water collects; must be pumped out regularly.
• Bill of Lading (B/L)
  A legal document issued by a carrier acknowledging receipt of cargo for shipment, serving as a receipt, title document, and contract of carriage.
• Bill of Lading — Types
  Order, bearer, straight, sea waybill — distinguishes how title/possession transfers and whether the document is negotiable.
• Biofouling Guidelines
  IMO guidelines for the control and management of ships' biofouling to minimise transfer of invasive aquatic species.
• Bitter End
  Inboard end of the anchor chain secured in the chain locker; from which 'to the bitter end' is derived.
• Block Coefficient (Cb)
  Ratio of underwater volume to a rectangular block of length × beam × draft.
• BMP West Africa
  Counter-piracy guidance for vessels transiting the Gulf of Guinea (current 'Global Counter Piracy Guidance for Companies, Masters and Seafarers').
• Boatswain's Mate
  Senior deck rating, deputy to the Bosun on larger ships; supervises deck crew, assists in cargo and safety operations.
• Boil-Off Gas (BOG)
  Vapour generated naturally as cryogenic cargo (LNG, LPG, ethylene, ammonia) absorbs heat through tank insulation and boils.
• Boil-Off Rate (BOR)
  Daily fraction of cryogenic cargo lost to boil-off, expressed as % of cargo mass per day.
• Bollard Pull (BP)
  Static towing force a tug can exert measured against a fixed bollard; primary spec for tugs (typically 30–100 t for harbour, 150–300 t for AHTS).
• Bosun
  Senior unlicensed deck rating responsible for the upkeep of deck equipment, mooring gear, and supervision of the deck crew.
• Bow Thruster
  A transverse propulsion unit fitted in a tunnel through the bow section to improve low-speed manoeuvrability in port.
• Break Bulk
  Non-containerised cargo loaded individually as bags, bales, crates, or drums rather than in standard containers.
• breakbulk vessel
  General cargo ship carrying non-containerised goods — bagged, crated, palletised, or bundled — loaded and discharged piece by piece.
• Bridge Alarm
  An alarm system on UMS vessels that channels all critical engine room and safety alarms to the navigating bridge when the engine room is unmanned.
• Bridge Log
  A watchkeeping record maintained on the bridge detailing courses steered, speed, weather conditions, and watch handovers.
• Bridge Resource Management (BRM)
  Non-technical-skills training in bridge teamwork, communication, decision-making, and human-factors error prevention.
• Broken Stowage
  The wasted space in a cargo hold that cannot be utilised due to the shape or packaging of the cargo being carried.
• Bulk Carrier
  A single-deck cargo vessel designed to carry unpackaged dry bulk cargoes such as grain, coal, iron ore, or fertiliser.
• Bulkhead
  Vertical partition dividing a ship into watertight compartments.
• Bunker
  Fuel oil carried aboard and consumed by a ship's main and auxiliary engines; also refers to the act of taking on fuel.
• Bunker Delivery Note (BDN)
  Document recording details of fuel oil delivered, including density, viscosity, sulphur content, and supplier ID.
• Bunkering
  The operation of taking fuel oil aboard a vessel, requiring careful monitoring of quantities, trim, and pollution prevention measures.
• Cadet
  A trainee officer serving aboard ship as part of an approved maritime academy programme to gain sea service for certification.
• Call Sign
  An ITU-assigned alphanumeric identifier used to identify a vessel during radio communications.
• Capesize
  Bulk carrier > 100,000 DWT — too large for Panama Canal, must round Cape of Good Hope or Cape Horn; iron ore, coal, bauxite trades.
• Capstan
  A vertical-axis powered drum on deck used to heave in mooring lines or warping lines.
• Carbon Intensity Indicator Rating (CII)
  Operational efficiency rating A–E assigned annually to ships >5,000 GT under MARPOL Annex VI.
• Cargo Control Room Operator (CCR Operator)
  An officer or rating on a tanker or bulk carrier who monitors and controls cargo loading, discharging, and ballasting operations from a centralised control room.
• Cargo Pump
  A large-capacity pump on a tanker used to discharge liquid cargo from cargo tanks to shore or to another vessel.
• Celestial Navigation
  The art and science of determining a vessel's position by measuring the altitudes of celestial bodies with a sextant.
• Certificate of Competency (CoC)
  Statutory licence issued by a flag state under STCW certifying a seafarer's qualification for a deck or engine rank.
• Certificate of Proficiency (CoP)
  STCW certificate for ratings and supporting-level functions (e.g., AB, RFPNW, fast-rescue boat).
• Certificate of Registry
  An official document issued by the flag state confirming the vessel's registration, nationality, and ownership details.
• Charter Party (C/P)
  A contract between a shipowner and a charterer for the hire of a vessel or cargo space, setting out terms and conditions of the voyage.
• Chemical Distribution Institute (CDI)
  Chemical-tanker equivalent of SIRE: independent ship inspection scheme used by chemical major vetting departments.
• Chemical Tanker
  A tanker built to carry noxious liquid substances in bulk, constructed to IBC Code standards for cargo containment.
• Chief Engineer (C/E)
  The senior engineer officer responsible for the operation, maintenance, and safety of all machinery and technical systems aboard.
• Chief Mate (C/O)
  The officer second in command to the master, responsible for cargo operations, stability, deck maintenance, and crew safety.
• Chronometer
  A precision timekeeping instrument used in celestial navigation to provide accurate GMT for the calculation of longitude.
• Citadel
  Hardened safe room with food, water, and communications where crew can shelter during a piracy boarding while awaiting military intervention.
• Classification Society
  A non-governmental organisation that establishes and verifies structural and mechanical standards for ship construction and maintenance.
• Clearing Line
  A bearing or line on the chart that, when maintained, keeps a vessel clear of a specific hazard such as a shoal or reef.
• Closed-Loop Scrubber (EGCS)
  Exhaust gas cleaning system that uses caustic soda (NaOH) in a closed water loop to remove SOx; sludge stored aboard for shore disposal.
• Cold Lay-up
  A long-term idling of a vessel with systems shut down and crew reduced to a minimum, used when reactivation is uncertain.
• Collision Bulkhead
  Forward-most watertight bulkhead designed to limit flooding from bow damage.
• COLREG Rule 13 — Overtaking
  Any vessel overtaking another shall keep out of the way of the vessel being overtaken — applies regardless of relative status under other rules.
• COLREG Rule 14 — Head-On
  When two power-driven vessels meet on reciprocal courses, each shall alter course to starboard.
• COLREG Rule 15 — Crossing
  When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way.
• COLREG Rule 18 — Responsibilities
  Hierarchy of stand-on / give-way: power-driven vessels give way to sailing, sailing to fishing, fishing to NUC and RAM.
• COLREG Rule 19 — Restricted Visibility
  Conduct of vessels in or near restricted visibility — early action, safe speed, and avoidance of port-side alterations to a vessel forward of the beam.
• COLREG Rule 5 — Lookout
  Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate.
• COLREG Rule 6 — Safe Speed
  Every vessel shall proceed at a safe speed so she can take proper and effective action to avoid collision.
• COLREG Rule 7 — Risk of Collision
  Every vessel shall use all available means to determine if risk of collision exists; if in doubt, such risk shall be deemed to exist.
• Combination Ladder
  A pilot boarding arrangement combining a pilot ladder below with an accommodation ladder above to accommodate height differences.
• Common-Rail Injection
  Fuel-injection system where pressurised fuel is held in a high-pressure rail and injected per-cylinder via electronically controlled injectors.
• Company Security Officer (CSO)
  The shore-based person designated by a company to be responsible for developing and implementing its Ship Security Plans under the ISPS Code.
• Compass
  Instrument indicating direction relative to magnetic north or true north, fundamental to ship navigation.
• Compass Bearing
  A bearing measured by a ship's compass, requiring correction for both variation and deviation to obtain a true bearing.
• Concentrated Inspection Campaign (CIC)
  Time-limited PSC focus area (e.g., crew familiarisation, ballast water management) coordinated across MoUs.
• Container Ship
  A cargo vessel designed to carry ISO standard intermodal containers stacked in cells below deck and in tiers on deck.
• Continuous Survey Machinery (CMS)
  Class arrangement allowing machinery items to be surveyed in rotation by the chief engineer rather than in one yard period.
• Continuous Synopsis Record (CSR)
  Statutory document recording the history of a ship's flag, name, owner, ISM company and class society.
• Contract of Affreightment (COA)
  Long-term contract obliging the carrier to lift a stated quantity of cargo over a defined period using nominated tonnage.
• Controllable Pitch Propeller (CPP)
  A propeller whose blade pitch can be varied in service, allowing speed and thrust control without changing engine rotation direction.
• Cool-Down
  Gradual cooling of LNG/LPG cargo tanks with cargo vapour before loading to prevent thermal shock to membrane / pressure vessel walls.
• Crankcase
  The lower enclosed section of an engine housing the crankshaft, connecting rods, and bearing lubrication system.
• Crude Oil Washing (COW)
  A tank cleaning method using crude oil cargo as the washing medium to dissolve and remove waxy residue from tank surfaces.
• Crude Oil Washing (COW)
  Cleaning crude oil tanks with their own cargo as the wash medium, replacing earlier seawater-flush methods.
• Cruise Ship
  A large passenger vessel offering multi-day voyages with onboard leisure facilities as the primary commercial product.
• Cumulonimbus (CB)
  A deep, vertically developed convective storm cloud extending to the tropopause, producing squalls, lightning, heavy rain, and microbursts.
• Custody Transfer (CT)
  Measurement and transfer of cargo from seller to buyer in a contractually-binding way, conducted by a third-party surveyor.
• Damage Stability
  Stability of a ship after one or more compartments are flooded; assessed deterministically (older rules) or probabilistically (SOLAS II-1 Reg. 6).
• Damaged Waterline
  Equilibrium waterline after damage flooding; must remain below margin line / bulkhead deck for SOLAS compliance.
• Danger Angle
  A horizontal or vertical sextant angle that, when not exceeded, ensures a vessel remains clear of a coastal hazard.
• Dangerous Goods (DG)
  Cargo classified under the IMDG Code as posing risks to health, safety, property, or the environment during sea transport.
• Data Collection System (DCS)
  IMO mandatory annual reporting of fuel oil consumption for ships >5,000 GT, used to support CII and EEXI.
• Dead Reckoning (DR)
  Calculating current position using a known past position, then advancing it using speed, course, and elapsed time.
• Deadweight (DWT)
  The total weight a vessel can carry including cargo, fuel, water, stores, and crew, measured in metric tonnes.
• Deck Cadet
  A trainee deck officer undertaking approved sea service under a maritime academy programme to qualify for an OOW certificate.
• Deck Cadet
  Officer-in-training in the deck department, completing 12-18 months of supervised sea time toward CoC OOW Deck.
• Deck Cargo
  Cargo stowed on the open weather deck and exposed to the sea environment, requiring additional securing and weatherproofing.
• Deck Log
  The official daily record of the ship's position, weather, incidents, crew changes, and navigational events kept by deck officers.
• Deep Water Route
  A route within defined limits, surveyed and confirmed to have sufficient depth, designated for use by vessels requiring deep draught.
• Deepwater Port
  Port with sufficient draft (typically >= 14 m) to accept VLCC, ULCC, ULCV, and Capesize vessels.
• Demurrage
  A penalty charge payable by the charterer when a vessel is kept at the loading or discharging berth beyond the agreed laytime.
• Designated Person Ashore (DPA)
  The ISM Code-required company contact point with direct access to the highest management level, responsible for safety and pollution prevention oversight.
• Deviation
  The error in a magnetic compass caused by the ship's own magnetic field, varying with the vessel's heading.
• Deviation Card
  A tabulated record of a magnetic compass's deviation on successive headings, prepared after compass adjustment or swing.
• Digital Selective Calling (DSC)
  A standard for transmitting pre-defined digital messages over VHF/MF/HF radio to initiate distress, urgency, or routine calls.
• Disbursement Account (DA)
  Itemised statement from the port agent listing all charges incurred during a port call.
• Dispatch
  A payment made by the shipowner to the charterer for completing loading or discharging operations before the agreed laytime expires.
• Displacement
  The total mass of water displaced by a floating vessel, equal to the combined weight of the hull, machinery, cargo, and stores.
• Diving Support Vessel (DSV)
  Vessel built around a saturation diving system, typically with DP-3, ROVs, and large moonpool.
• Docking Survey
  An in-water or drydock inspection of the hull, rudder, propeller, and sea valves required by classification societies every two to five years.
• Document of Compliance (DOC)
  A certificate issued to a shipping company confirming its Safety Management System meets ISM Code requirements.
• Doppler Weather Radar
  Ship-borne or shore-based weather radar that uses Doppler shift to estimate precipitation intensity and storm motion.
• Double Bottom
  Twin layer of shell plating with the inner-bottom forming the cargo hold or tank floor; required on most cargo vessels for damage stability and ballast.
• Draft
  The vertical distance between the keel and the waterline, determining the minimum water depth needed for safe navigation.
• Draught Marks (draught scale)
  Numeric scale painted on the bow, mid, and stern of the ship indicating the depth of submergence at that point.
• drillship
  Ship-shaped deepwater drilling unit with a central moonpool, drilling derrick, and DP-3 dynamic positioning for ultra-deepwater exploration and production drilling.
• Drillship
  Mobile drilling unit built on a ship-shaped hull, typically DP-3, capable of drilling in ultra-deep water.
• Drydock
  A scheduled period when a vessel is taken out of service and placed in a dry facility for hull inspection, maintenance, and repairs.
• Drydocking Frequency
  Standard 5-yearly cycle with one intermediate drydocking allowed up to 7.5 years for certain ship types under in-water survey schemes.
• DSC (Digital Selective Calling)
  Pre-formatted digital distress and safety calling technique transmitting identity, nature of distress, position, and time on dedicated watch-keeping frequencies.
• Dunnage
  Timber, mats, or other material placed under or between cargo to protect it from moisture, damage, and shifting.
• Dynamic Positioning (DP)
  A computer-controlled system that automatically maintains a vessel's position and heading using thrusters, without anchoring.
• Dynamic Positioning Operator (DPO)
  A certificated officer qualified to operate and monitor a vessel's dynamic positioning system during offshore operations.
• ECDIS Type-Specific Training
  Equipment-manufacturer training required in addition to generic ECDIS familiarisation under STCW.
• EGC (Enhanced Group Call)
  Inmarsat satellite broadcast system delivering Maritime Safety Information to all vessels within a defined geographical area simultaneously.
• Electro-Technical Officer (ETO)
  A certificated officer responsible for the maintenance and repair of electrical, electronic, and automation systems aboard.
• Electronic Chart Display and Information System (ECDIS)
  A navigation information system that displays ENCs and vessel position, accepted as a replacement for paper charts under SOLAS.
• Emergency Escape Breathing Device (EEBD)
  A self-contained breathing device providing approximately 10 minutes of breathable air to escape a smoke-filled space.
• Emergency Fire Pump
  A self-contained fire pump located outside the machinery space, capable of supplying fire mains if the main pumps are disabled.
• Emergency Generator
  A self-contained standby generator automatically supplying essential electrical power to safety systems if the main supply fails.
• Emergency Position Indicating Radio Beacon (EPIRB)
  A satellite beacon that transmits a distress signal with vessel identity and position to the Cospas-Sarsat system when activated.
• Emergency Shutdown (ESD)
  Automated/manual system that stops cargo transfer in seconds when a high-level alarm, leak, or other unsafe condition is detected.
• Emission Control Area (ECA)
  A sea area designated under MARPOL Annex VI where stricter limits on sulfur and NOx emissions from ships apply.
• EmS Guide
  IMO Emergency Response Procedures for Ships Carrying Dangerous Goods; gives Fire and Spillage schedules referenced from the DG list.
• Energy Efficiency Design Index (EEDI)
  Design-phase efficiency index (g CO2 per t·nm) required for newbuilds since 2013; tightens in successive phases.
• Energy Efficiency Existing Ship Index (EEXI)
  One-off retroactive efficiency index applied to ships built before EEDI; effective 2023.
• Engine Cadet
  A trainee engineer officer undertaking approved sea service as part of an engineering degree programme to qualify for a watchkeeping certificate.
• Engine Log
  The official daily record maintained by the engine department of machinery performance, fuel consumption, and maintenance events.
• Engine Room Resource Management (ERM)
  Engine-room equivalent of BRM — teamwork, communication, fatigue management, situational awareness for engineers.
• Enhanced Group Call (EGC)
  Inmarsat-C broadcast service for distributing safety, navigational, and security messages to ships in defined geographic areas.
• EPIRB Categories (EPIRB)
  Two categories of 406 MHz EPIRB: Cat I auto-deploy and float-free; Cat II manual deployment.
• Equasis
  A free public database providing safety-related information on commercial vessels including inspection history, class, and owner details.
• Escort Tug
  Powerful, manoeuvrable tug attached to a tanker via line during port approach to brake or steer if main engine/steering fails.
• Estimated Position (EP)
  A position obtained by advancing a known fix using course and speed while applying estimated leeway and current allowances.
• Exclusive Economic Zone (EEZ)
  A sea zone extending up to 200 nautical miles from a coastal state's baseline where it has sovereign rights over resources.
• Exhaust Gas Cleaning System (EGCS)
  A device fitted in the exhaust line that removes sulfur oxides from engine gases, allowing use of high-sulfur HFO under IMO 2020.
• Exhaust Gas Cleaning System (EGCS)
  Scrubber system reducing SOx from exhaust gas to comply with MARPOL Annex VI without using low-sulphur fuel.
• Exhaust Gas Economiser (EGE)
  Heat exchanger in the funnel that recovers waste heat from main-engine exhaust to generate steam.
• Exhaust Gas Recirculation (EGR)
  NOx-reduction technique that recirculates a portion of exhaust gas back into the engine intake, lowering combustion peak temperature.
• FAME / Biofuel
  Fatty Acid Methyl Ester biofuel blended into marine fuels; ISO 8217:2017 permits up to 7% (B7).
• Feeder Container Ship
  Small container ships (1,000-3,000 TEU) that ferry containers between regional ports and main hub-port hubs served by ULCVs.
• Fender
  A cushioning device of rubber, foam, or rope placed between a vessel and a berth or another vessel to prevent damage.
• Finnish-Swedish Ice Class
  Ice class IA Super, IA, IB, IC, II, III used in the Baltic; controls fairway dues and icebreaker assistance order.
• FIO / FIOST
  Free-In-and-Out (FIO) and Free-In-Out-Stowed-Trimmed (FIOST) clauses placing cargo handling cost / responsibility on the charterer.
• Fire Drill
  A mandatory shipboard exercise practising the crew's response to a fire emergency, required at least monthly under SOLAS.
• Fire Safety Systems Code (FSS Code)
  IMO code detailing international requirements for fire detection, suppression, and firefighting equipment on SOLAS ships.
• Fitter
  A skilled engine room rating who performs mechanical repairs and maintenance on machinery, pipework, and deck equipment.
• Fix
  A confirmed vessel position established by two or more simultaneous position lines from observed bearings, ranges, or electronic navaids.
• Fixed Pitch Propeller (FPP)
  A propeller with blades cast at a fixed angle to the hub; speed and direction are changed by varying engine rpm and rotation.
• Flag of Convenience (FOC)
  Registration of a vessel in a country offering lower taxes or less strict regulations than the owner's home nation.
• Flag State Inspection
  An official inspection of a vessel carried out by or on behalf of the flag administration to verify compliance with its regulations.
• Fleet Broadband (FBB)
  An Inmarsat satellite broadband service providing high-speed voice and data communications on mobile maritime terminals.
• Fleet Manager
  A senior shore-side executive responsible for the commercial, operational, and crewing management of a group of vessels.
• Fleet Xpress
  Inmarsat managed broadband service combining Ka-band Global Xpress with L-band FleetBroadband as backup.
• Floating Dock
  A pontoon structure that can be submerged to receive a vessel then raised to lift it clear of the water for repair work.
• Floating Production Storage and Offloading (FPSO)
  A floating vessel used by the offshore oil and gas industry for production, processing, storage, and offloading of hydrocarbons.
• Floating Production Storage and Offloading (FPSO)
  Vessel that processes hydrocarbons from a subsea well, stores the production in cargo tanks, and offloads to shuttle tankers.
• Floating Production Storage and Offloading Unit (FPSO)
  Ship-shaped floating facility receiving, processing, storing, and offloading hydrocarbons from subsea wells.
• Floating Storage and Offloading (FSO)
  A stationary floating vessel used to store crude oil or petroleum products at offshore fields pending transfer to shuttle tankers.
• Forty-foot Equivalent Unit (FEU)
  Standard unit of cargo capacity equivalent to one 40-foot ISO shipping container, equal to two TEU.
• Four-Stroke Engine
  An internal combustion engine requiring four piston strokes per cycle, widely used as medium-speed auxiliary and propulsion engines.
• Fourth Engineer (4/E)
  The most junior watchkeeping engineer officer, gaining experience under supervision of senior engineers.
• Free Pratique
  Permission granted by port health authorities for a vessel to interact with shore after confirming no quarantinable disease aboard.
• Free Surface Correction (FSC)
  Reduction in effective GM caused by liquid moving in a slack tank; subtracted from solid GM to give fluid GM.
• Free Surface Effect (FSE)
  The virtual reduction in GM caused by a liquid surface in a partially filled tank that moves as the ship heels, reducing stability.
• Freeboard
  The vertical distance from the waterline to the main deck edge, indicating reserve buoyancy and load line compliance.
• Freezing Spray
  Sea spray that freezes on contact with vessel surfaces in sub-zero air temperatures, causing rapid topside ice accretion and raising capsize risk.
• Freight Forwarder
  Agent that arranges transportation of goods on behalf of the shipper; may issue house bills of lading.
• Fresh Water Generator (FWG)
  A distillation unit using main engine jacket water waste heat to evaporate and condense seawater into fresh water at sea.
• Gangway
  A portable access ladder or bridge rigged over the ship's side to allow persons to embark and disembark safely.
• Garbage Management Plan
  MARPOL Annex V required document setting procedures for collection, segregation, storage, processing, and disposal of garbage.
• Garbage Record Book (GRB)
  A mandatory log recording all garbage disposal and incineration operations on board, required under MARPOL Annex V.
• Gassing Up
  Replacing inert gas in LNG/LPG cargo tanks with cargo vapour before cool-down, eliminating CO2 and water that would freeze at cryogenic temperatures.
• GENCON
  BIMCO general voyage charter party (1994 / 2022 revisions); industry-standard for dry-cargo voyage employment.
• General Average (GA)
  Maritime law principle: voluntary sacrifice or expense made for the common safety of vessel and cargo is shared proportionally by all interests.
• General Cargo Ship
  A multipurpose vessel with cargo holds, tween decks, and derricks or cranes, designed to carry a variety of break-bulk and unitised cargoes.
• Generator Set (genset)
  A diesel-electric power unit installed in a ship's hold or on deck to supply electrical power to reefer containers.
• Global Maritime Distress and Safety System (GMDSS)
  An internationally agreed set of safety procedures and communications equipment required on SOLAS ships for distress and safety.
• Global Positioning System (GPS)
  US satellite-based radionavigation system providing position, velocity, and time data worldwide to marine and other users.
• GMDSS General Operator's Certificate (GOC)
  Highest GMDSS radio operator qualification; required for service in Sea Areas A2/A3/A4.
• GMDSS Restricted Operator's Certificate (ROC)
  GMDSS qualification limited to Sea Area A1 (VHF DSC range only).
• GMDSS Sea Area A1 (Area A1)
  An area within range of at least one VHF DSC coast station providing continuous alerting coverage, typically within 20–50 miles of shore.
• GMDSS Sea Area A2 (Area A2)
  An area beyond A1 but within MF DSC continuous coverage, typically up to 400 nautical miles from shore.
• GMDSS Sea Area A3 (Area A3)
  An area beyond A1 and A2 within coverage of an Inmarsat geostationary satellite, covering most ocean areas between latitudes 70°N and 70°S.
• GMDSS Sea Area A4 (Area A4)
  The polar sea areas beyond GMDSS Area A3, not covered by geostationary satellites, where HF radio is used for distress alerting.
• Gob Rope
  Restraining line led from the towing point on a tug to a deck winch, controlling the angle of the towline.
• Graving Dock
  A watertight basin into which a vessel is floated, then dewatered, allowing access to the entire underwater hull for inspection and repair.
• Graving Dock
  Permanent excavated dock with a gate; fills with water to receive the ship, then is pumped dry for hull work.
• Great Circle
  The shortest route between two points on a sphere, formed by the intersection of the sphere and a plane through its center.
• Growler
  Small piece of glacier ice (<1 m above water, ~5 m across) — particularly dangerous because it is hard to detect on radar.
• Gyrocompass
  A compass using a gyroscope to find geographic north, unaffected by magnetic fields; mandatory on most large vessels.
• GZ Curve (GZ)
  Plot of the righting arm (GZ) against angle of heel — the cornerstone of intact-stability assessment.
• H2S Awareness Training
  Hydrogen-sulphide hazard awareness required for crew on tankers and OSVs handling sour cargoes.
• Hague-Visby Rules
  The amended 1924 Hague Rules governing carrier liability for cargo under bills of lading, widely incorporated into national law.
• Hamburg Rules
  The 1978 UN convention on the carriage of goods by sea, placing greater liability on carriers than the Hague-Visby Rules.
• Handysize
  Smallest of the modern bulk carrier classes: 10,000-40,000 DWT — flexible, port-restriction-tolerant.
• Harbour Tug
  Compact, high-bollard-pull tug used for berthing, unberthing, and shifting ships within a port.
• Hawse Pipe
  Tube through which the anchor chain runs from windlass to the side of the bow.
• Hawser
  A large rope or cable used for mooring or towing a vessel, typically greater than 25mm in diameter.
• Heavy Fuel Oil (HFO)
  Residual fuel oil with high viscosity and sulfur content, historically the primary bunker fuel for ocean-going vessels.
• heavy-lift vessel
  Specialised ship equipped with one or more large cranes (100–2 000 t SWL) for loading, transporting, and discharging oversized project cargo.
• Helicopter Underwater Escape Training (HUET)
  Mandatory survival training for personnel transferring offshore by helicopter; OPITO-accredited.
• HMPE Mooring Line (HMPE)
  High-Modulus Polyethylene synthetic mooring line — strength similar to steel wire at a fraction of the weight.
• Hogging
  Longitudinal hull bending where the middle is supported by a wave crest while the ends sag.
• Hopper Tank
  Sloped wing tank along the hold bottom corners on a bulker; aids cargo flow during discharge.
• Hot Lay-up
  A temporary idling of a vessel with full crew and systems maintained, enabling rapid return to service when market conditions improve.
• Hours of Rest
  Minimum 10 h rest in any 24 h and 77 h in any 7 days, as required by STCW VIII/1 and MLC 2006.
• Hull & Machinery Insurance (H&M)
  Marine insurance covering the vessel itself against perils of the sea, fire, collision, etc.
• Hull Cleaning
  Removal of fouling organisms and paint buildup from the underwater hull to restore vessel speed and fuel efficiency.
• Hull Stress Monitoring System (HSMS)
  Strain gauges + accelerometers + decision-support software to warn when hull-girder loads approach class limits.
• Hurricane
  A tropical cyclone in the Atlantic or eastern Pacific with sustained winds of 64 knots or more on the Beaufort scale.
• Hybrid Scrubber (EGCS)
  Scrubber that can switch between open- and closed-loop modes depending on local discharge regulations.
• IACS Polar Class
  Seven-tier ice class (PC1–PC7) under IACS UR I assigning ice-going capability from year-round in all polar waters (PC1) to summer-only thin first-year ice (PC7).
• Iceberg
  Mass of glacier ice >5 m above the water, classified small/medium/large/very large by IIP.
• Icebreaker
  Vessel with reinforced bow, high-power propulsion, and ice-friendly hull form designed to break level ice and clear channels.
• IGF Code
  International Code of Safety for Ships using Gases or other Low-flashpoint Fuels.
• IMDG Class 1 (Explosives)
  IMDG dangerous-goods classification for explosive substances and articles.
• IMDG Class 2 — Gases
  Compressed, liquefied, or dissolved gases; subdivided 2.1 flammable, 2.2 non-flammable non-toxic, 2.3 toxic.
• IMDG Class 3 — Flammable Liquids
  Liquids with flash point ≤ 60 °C; classified by packing group I/II/III based on flash point and boiling point.
• IMDG Class 4
  Flammable solids (4.1), substances liable to spontaneous combustion (4.2), substances which emit flammable gases when wet (4.3).
• IMDG Class 5
  Oxidising substances (5.1) and organic peroxides (5.2); pose fire risk by yielding oxygen.
• IMDG Class 6
  Toxic substances (6.1) and infectious substances (6.2).
• IMDG Class 7 (Radioactive)
  IMDG dangerous-goods classification for radioactive material.
• IMDG Class 8 — Corrosives
  Substances causing severe damage when in contact with living tissue or other goods, including most acids and bases.
• IMDG Class 9 (Miscellaneous Dangerous Goods)
  Catch-all for substances dangerous in transport but not fitting Classes 1-8 (e.g., asbestos, dry ice, lithium batteries, env. hazardous).
• IMDG Code
  International Maritime Dangerous Goods Code — mandatory packaging, marking, segregation, and stowage rules for DG in packaged form.
• Immersion Suit
  A watertight thermal protective suit worn in cold water abandonment that significantly extends survival time.
• IMO 2020 Sulphur Cap
  Global cap on marine fuel sulphur content reduced to 0.50% mass on 1 January 2020 (0.10% in ECAs).
• IMO GHG Strategy
  IMO 2023 strategy targeting net-zero GHG emissions from international shipping 'by or around' 2050, with checkpoints for 2030 and 2040.
• IMO Number
  A unique seven-digit identifier assigned to a ship for life, used to track its history regardless of flag, owner, or name changes.
• Impressed Current Cathodic Protection (ICCP)
  Active corrosion protection injecting low-voltage DC into the hull via inert anodes; used on most large vessels.
• Inert Gas System (IGS)
  A system supplying inert gas (flue gas or nitrogen) to tanker cargo tanks to maintain an oxygen-deficient atmosphere and prevent explosion.
• Inmarsat-C
  Geostationary satellite system providing low-data-rate store-and-forward messaging and EGC SafetyNET broadcasts, recognised under GMDSS for Sea Area A3.
• Innocent Passage
  The right of foreign ships to navigate through a coastal state's territorial sea continuously and without prejudice to its peace or security.
• Inshore Traffic Zone (ITZ)
  The water between a traffic separation scheme and the adjacent coast, normally reserved for local and small craft traffic.
• Intact Stability Code (IS Code)
  IMO mandatory code for intact stability of all ships >24 m, including weather criterion and severe wind & rolling.
• Inter-Tropical Convergence Zone (ITCZ)
  A belt of converging trade winds near the thermal equator characterised by squalls, doldrums, and heavy rain, migrating seasonally with solar heating.
• Intercept
  In celestial navigation, the difference in minutes of arc between the observed altitude and the calculated altitude, used to plot a position line.
• Intermediate Fuel Oil (IFO)
  A blend of heavy residual and distillate fuel oil with viscosity typically 180 or 380 cSt, used as main engine bunker.
• Intermediate Survey
  A classification survey conducted midway through the five-year special survey cycle, typically around the second or third year.
• International Association of Classification Societies (IACS)
  A body of twelve major classification societies that collectively establish and promote uniform technical standards for ship construction and maintenance.
• International Convention for the Prevention of Pollution from Ships (MARPOL)
  The main international convention covering prevention of pollution of the sea by ships from operational and accidental causes.
• International Convention for the Safety of Life at Sea (SOLAS)
  The primary IMO convention setting minimum safety standards for ship construction, equipment, and operation.
• International Group of P&I Clubs (IGP&I)
  Association of 12 mutual P&I clubs sharing claims via a pooling/reinsurance arrangement covering ~90% of world tonnage.
• International Maritime Dangerous Goods Code (IMDG)
  IMO code governing the safe carriage of hazardous materials by sea, covering classification, packing, marking, and stowage.
• International Regulations for Preventing Collisions at Sea (COLREGs)
  IMO rules prescribing conduct of vessels to prevent collisions, including lights, shapes, sound signals, and steering rules.
• International Safety Guide for Oil Tankers and Terminals (ISGOTT)
  Industry standard guidance jointly published by OCIMF, ICS, and IAPH for oil-tanker / terminal cargo operations.
• International Safety Management Code (ISM Code)
  IMO code requiring shipping companies to establish a Safety Management System to ensure safe ship operation and pollution prevention.
• International Ship and Port Facility Security Code (ISPS Code)
  IMO code mandating security plans, officers, and assessments for ships and port facilities to prevent terrorism and unlawful acts.
• Internationally Recommended Transit Corridor (IRTC)
  Naval-monitored corridor through the Gulf of Aden with eastbound/westbound lanes for grouping and transit of merchant vessels.
• Intertropical Convergence Zone (ITCZ)
  A band of low pressure near the equator where trade winds converge, producing persistent convective weather and squalls.
• Iridium GMDSS
  IMO-recognised GMDSS satellite service using Iridium's low Earth orbit constellation, providing global coverage including polar Sea Area A4.
• Iridium GMDSS Service
  Iridium satellite service recognised by IMO since 2020 for GMDSS Sea Areas A1–A4, including polar regions.
• ISM Audit
  Internal and external audit of the Safety Management System under the ISM Code; underpins the DOC and SMC.
• ISO 8217
  International standard specifying marine fuel grades — distillates (DMA, DMB, DMZ) and residuals (RMA, RMB, RMD, RME, RMG, RMK).
• ISPS Audit
  Verification audit of ship/port-facility security plans under ISPS Code — initial, intermediate, renewal.
• ISPS Security Levels
  Three-tier security alert: Level 1 normal, Level 2 heightened, Level 3 specific threat — set by Contracting Government.
• jack-up rig
  Mobile offshore drilling or installation unit with retractable legs that are lowered to the seabed, lifting the hull clear of the water surface for operations.
• Jacking Gear
  A reduction-gear device coupled to the engine flywheel used to slowly turn the engine by hand or motor for inspection.
• Job Safety Analysis (JSA)
  Pre-task hazard identification process: list each step, identify hazards, define controls, and document.
• Junior Engineer
  Engine-room officer below 4/E rank, typically holding a partial CoC or completing on-board training; assists the watchkeeping engineer.
• Kort Nozzle
  Cylindrical duct around a propeller that increases bollard pull at low speed; common on tugs, fishing vessels, and OSVs.
• Laid Up
  The status of a vessel taken out of active trading and moored or anchored in a sheltered location while awaiting employment.
• Lapse Rate
  The rate at which atmospheric temperature decreases with increasing altitude, averaging approximately 6.5°C per kilometre in the standard atmosphere.
• Lashing
  Chains, wire, or webbing straps used to secure cargo on deck or in holds against movement during sea passage.
• Lashing Rod
  A threaded steel rod used in combination with turnbuckles to secure containers and deck cargo against movement at sea.
• Laytime
  The agreed time allowed in a charter party for loading or discharging cargo; excess time results in demurrage charges.
• Lead
  Linear crack or fracture in sea ice large enough for navigation.
• Leading Line
  Two charted objects in line that define a safe course into or within a port, keeping the vessel clear of hazards.
• Letter of Indemnity (LOI)
  Written undertaking — typically by charterer to owner — to indemnify against losses arising from delivering cargo without original bill of lading.
• Life-Saving Appliances Code (LSA Code)
  IMO code specifying performance standards and testing for all life-saving appliances required by SOLAS Chapter III.
• Lifeboat
  A survival craft capable of sustaining life at sea, required on all SOLAS vessels and capable of being launched under adverse conditions.
• Liferaft
  An inflatable or rigid survival craft stowed on deck and launched by hand or hydrostatic release if the vessel sinks.
• Light Ship
  A vessel's weight when fully equipped with permanent fixtures but without cargo, fuel, ballast, stores, or crew on board.
• Lighterage
  Use of barges or smaller vessels to discharge cargo from a deep-draft ship at anchor where the destination port cannot accommodate her draft.
• Lightweight Tonnage (LWT)
  Ship's mass when launched, empty: hull, machinery, equipment, no fuel, water, ballast, stores, or cargo.
• List
  A permanent transverse inclination of a vessel caused by an uneven distribution of weights across the ship.
• List vs Loll
  List = heel from asymmetric weight distribution (positive GM). Loll = heel from negative GM with the ship oscillating about an angle either side of upright.
• Lloyd's Open Form (LOF)
  Standard salvage agreement on 'no cure, no pay' basis administered by Lloyd's Salvage Arbitration Branch.
• LNG Bunkering
  Transfer of LNG fuel ship-to-ship, truck-to-ship, or terminal-to-ship; covered by IGF Code and ISO/TS 18683.
• LNG carrier (LNGC)
  Cryogenic tanker transporting liquefied natural gas at −163 °C in insulated containment tanks, governed by the IGC Code.
• LNG Carrier
  A purpose-built vessel carrying liquefied natural gas in insulated cargo tanks at approximately −163°C.
• Load Line Certificate
  A certificate assigned by the flag state or classification society specifying the minimum freeboard and maximum loading depth for each zone.
• Load on Top (LOT)
  Pre-COW practice of letting tank washings settle, decanting the water, and loading new cargo on top of the oil residue.
• Loading Master
  A terminal representative who supervises the safe conduct of cargo transfer operations between shore installation and vessel.
• Lockout-Tagout (LOTO)
  Procedure for isolating dangerous energy sources (electrical, mechanical, hydraulic, steam) before maintenance, with physical lock and identifying tag.
• Long Range Identification and Tracking (LRIT)
  A mandatory satellite-based tracking system requiring SOLAS ships to transmit identity and position to flag and port state authorities.
• Long-Range Identification and Tracking (LRIT)
  Mandatory worldwide ship-position-reporting system at 6-hour intervals via Inmarsat-C (or Iridium), separate from AIS.
• Longitudinal Stiffener
  Long, thin reinforcing member running fore-and-aft along plating to resist bending and buckling.
• Loss of Hire Insurance (LOH)
  Insurance covering loss of charter hire when the vessel is off-hire due to a covered H&M peril.
• Loss Prevention Circular
  Bulletin issued by P&I Clubs to highlight emerging claim trends and recommend operational improvements.
• Loxodrome
  A line on the earth's surface crossing all meridians at the same angle, synonymous with a rhumb line course.
• LPG carrier (LPGC)
  Gas tanker carrying liquefied petroleum gas (propane, butane) or petrochemical gases in pressurised, semi-refrigerated, or fully refrigerated tanks.
• LPG Carrier
  A gas tanker designed to carry liquefied petroleum gases such as propane or butane under pressure or refrigeration.
• Magnetic Bearing
  A bearing measured clockwise from magnetic north, requiring correction for variation to obtain a true bearing.
• Magnetic Compass
  A compass that aligns with Earth's magnetic field to indicate magnetic north; required as backup on all SOLAS vessels.
• Main Engine (ME)
  The primary propulsion engine driving the ship's propeller shaft, typically a large slow-speed two-stroke diesel on cargo ships.
• Mandatory Pilotage
  A legal requirement in certain ports or waterways that a licensed pilot must be employed to navigate a vessel through that area.
• Marine Diesel Oil (MDO)
  A distillate fuel oil with low sulfur content, used in auxiliary engines and during port operations where ECA rules apply.
• Marine Pollutant (MP)
  Substance identified in IMDG Code as harmful to the marine environment; carries the 'Marine Pollutant' (dead-fish-and-tree) mark.
• Marine Superintendent
  A senior shore-side manager responsible for the nautical operations, safety, and compliance of vessels in a company fleet.
• Maritime Labour Convention 2006 (MLC)
  ILO convention establishing minimum working, living, and social security standards for seafarers worldwide.
• Maritime Rescue Coordination Centre (MRCC)
  National coordinating centre that receives distress alerts and orchestrates SAR operations within a defined Search and Rescue Region (SRR).
• Maritime Security Centre — Horn of Africa (MSCHOA)
  EU NAVFOR coordination centre supporting BMP transits and convoy escort tasking.
• MARPOL Annex I
  The annex of MARPOL regulating prevention of pollution by oil, covering operational discharges and accidental spills from ships.
• MARPOL Annex II
  The annex of MARPOL controlling discharge of noxious liquid substances carried in bulk, applicable to chemical tankers.
• MARPOL Annex III
  Regulations for the prevention of pollution by harmful substances carried at sea in packaged form.
• MARPOL Annex V
  The annex of MARPOL governing disposal of garbage from ships, prohibiting most plastics discharge into the sea.
• MARPOL Annex VI
  The annex of MARPOL setting limits on sulphur oxides, nitrogen oxides, and other air pollutant emissions from ships.
• MARPOL Bunker Sample
  Representative sample drawn at the bunker manifold during delivery, sealed and retained for 12 months.
• Mast Riser
  A large-diameter high-velocity vent pipe fitted to a tanker's mast to discharge cargo vapours safely above ignition sources.
• Master
  The certificated officer in overall command of a vessel, legally responsible for the safety of the ship, crew, cargo, and environment.
• Material Safety Data Sheet (MSDS)
  A document providing information on the hazards, handling precautions, and emergency procedures for a chemical or dangerous cargo.
• Medical First Aid Guide (MFAG)
  IMO/WHO/ILO guide for treating injuries arising from incidents with dangerous goods aboard ship.
• Mercator Sailing
  A method of solving rhumb line problems using the properties of a Mercator chart to find course and distance between positions.
• Metacentric Height (GM)
  The vertical distance between the centre of gravity (G) and the metacentre (M), the primary indicator of a vessel's initial stability.
• Metacentric Height from Keel (KM)
  Vertical distance from keel to the metacentre M; sum of KB (keel to centre of buoyancy) and BM (metacentric radius).
• METAREA
  One of twenty-one geographic areas for which meteorological services co-ordinate the broadcast of marine weather forecasts.
• Methanol as Marine Fuel
  Liquid alcohol fuel adopted by some newbuild container and tanker fleets; bunkered at ambient temperature, simpler infrastructure than LNG.
• MF/HF SSB
  Single-side-band radio in MF (1.6–4 MHz) and HF (4–27 MHz) bands used for medium- and long-range maritime voice and DSC.
• Minimum Safe Manning
  Flag-state-issued document specifying the minimum manning by rank/qualification for safe operation of the ship.
• MLC Financial Security — Abandonment
  MLC 2014 amendment requiring shipowners to maintain insurance/financial security for repatriation, wages, and essential needs of abandoned seafarers.
• MMSI (Maritime Mobile Service Identity)
  Nine-digit numerical address used to identify ships, coast stations, and devices in DSC, AIS, and satellite distress alerting.
• MMSI Number (MMSI)
  A nine-digit Maritime Mobile Service Identity used to identify a ship's radio and AIS transmissions uniquely.
• MOB AIS (Man Overboard AIS)
  Personal AIS beacon worn by crew that activates on water immersion, broadcasting AIS position reports with an MMSI beginning 972 to identify a man-overboard casualty.
• Moment to Change Trim 1 cm (MCT1cm)
  The trimming moment in tonne-metres needed to alter a vessel's trim by one centimetre, used in stability calculations.
• Monsoon
  A seasonal reversal of prevailing winds affecting the Indian Ocean and western Pacific, bringing heavy rainfall on its wet phase.
• Moonpool
  Vertical opening through the hull of an offshore vessel allowing equipment to be lowered from a sheltered indoor location.
• Mooring
  Securing a vessel to a berth, buoy, or anchor using lines, chains, or anchors to keep it stationary.
• Mooring Equipment Guidelines 4 (MEG4)
  OCIMF guideline (2018) for mooring system design, operation, and inspection of tankers and gas carriers.
• MoorMaster
  Cavotec vacuum mooring system replacing conventional lines in some ferry, container, and bulk berths.
• Motorman
  An engine room rating who assists engineers with machinery operation, maintenance, and watchkeeping duties.
• MSI (Maritime Safety Information)
  Umbrella term for navigational warnings, meteorological forecasts, distress relays, SAR information, and piracy reports broadcast to ships under the GMDSS framework.
• Multipurpose Support Vessel (MPSV)
  OSV combining DP, large deck, crane, ROV, and accommodation for varied subsea/inspection/maintenance work.
• multipurpose vessel (MPP)
  Versatile geared cargo ship carrying containers, breakbulk, project cargo, and heavy lifts in a single voyage, typically 3 000–15 000 DWT.
• Muster Station
  A designated assembly point where crew and passengers gather when an emergency alarm is sounded prior to abandoning ship.
• Nairobi Wreck Removal Convention
  International convention (entered into force 2015) that gives coastal states the right and obligation to remove hazardous wrecks in their EEZ at the registered owner's expense.
• NAVAREA
  One of sixteen geographic sea areas defined by IMO/IHO for the co-ordination of worldwide navigational warning broadcasts.
• NAVAREA (NAVAREA)
  One of 21 IMO/IHO-designated sea areas within which a co-ordinating authority issues numbered navigational warnings to mariners.
• Navigational Telex (NAVTEX)
  An automated medium frequency broadcast system transmitting navigational warnings, weather forecasts, and search and rescue notices.
• NAVTEX (Navigational Telex)
  Automated narrow-band direct-printing system broadcasting Maritime Safety Information on 518 kHz (international) and 490 kHz (national) within 200–400 NM of coastal transmitters.
• NAVTEX Frequencies
  International NAVTEX uses 518 kHz (English); national NAVTEX 490 kHz; tropical 4209.5 kHz.
• Near-Miss Reporting
  Documented submission of an incident that did not cause harm but had the potential to.
• Neo-Panamax / New-Panamax
  Maximum vessel size for the post-2016 Panama Canal expansion: 366 m LOA × 49 m beam × 15.2 m draft tropical fresh.
• Newcastlemax
  Bulker class sized for the Port of Newcastle, Australia (50 m beam max): ~205,000 DWT, 300 m LOA, 18.5 m draft.
• Northern Sea Route (NSR)
  Russian Arctic shipping route along Siberia from Kara Gate to Bering Strait; transits regulated by Glavsevmorput (NSR Administration).
• Not Under Command (NUC)
  Vessel which through exceptional circumstance cannot manoeuvre as required by the rules; shows two black balls / two red lights vertically.
• Notice of Readiness (NOR)
  A formal written notice given by the master to the charterer or shipper that the vessel has arrived and is ready to load or discharge.
• NOx Tier III
  Strictest IMO NOx limit (~3.4 g/kWh at 130 rpm); applies to engines >130 kW installed on ships built ≥2016 trading in NOx ECAs.
• NYPE Charter
  New York Produce Exchange standard time charter form, widely used for dry-cargo time charters.
• Ocean-Going Tug
  Long-endurance tug used for ocean towage of barges, rigs, or disabled ships.
• Offshore Support Vessel (OSV)
  General class of vessels supporting offshore oil/gas/wind operations: PSVs, AHTS, MPSVs, ROV vessels, etc.
• Oil Discharge Monitoring and Control System (ODMCS)
  Equipment on oil tankers that continuously monitors the oil content and flow rate of overboard discharges to ensure MARPOL compliance.
• Oil Mist Detector (OMD)
  A crankcase monitoring device that samples air from each crankcase compartment and alarms if oil mist concentration reaches explosive levels.
• Oil Record Book (ORB)
  A mandatory log in which all oil-related operations including bilge pumping and bunkering must be recorded under MARPOL Annex I.
• Oiler
  An engine department rating who lubricates machinery, maintains oil records, and stands engine room watches.
• Oily Water Separator (OWS)
  Equipment that separates oil from bilge water to reduce oil content to 15 ppm before overboard discharge under MARPOL.
• Open Moor
  Two anchors set with cables forming a wide V to limit swing in restricted anchorage.
• Open-Loop Scrubber (EGCS)
  Exhaust gas cleaning system that uses seawater as the alkali source to remove SOx, then discharges treated wash water overboard.
• Ordinary Seaman (OS)
  Entry-level unlicensed deck rating assigned general duties under the supervision of ABs and the bosun.
• P&I Club
  Mutual insurance association covering shipowners' third-party liabilities (cargo claims, pollution, crew, collision liability not covered by H&M).
• P&I Correspondent
  Local agent appointed by a P&I Club to assist members with on-the-spot incidents (claims, surveys, casualty response) in a specific port.
• Panama Canal Pilotage
  Mandatory transit pilotage by Panama Canal Authority (ACP) pilots, who take con of the vessel from sea buoy to sea buoy.
• Panamax (bulker)
  Bulk carrier sized to fit the original Panama Canal locks: max 32.31 m beam, 294 m LOA, ~12.04 m draft fresh; ~65,000-80,000 DWT.
• Parallel Index (PI)
  A radar technique using lines parallel to the course line to monitor closest approach to a fixed radar target.
• Parallel Mid-body
  Length of hull where cross-sections are constant (the 'box' between the entry and run sections).
• Paris Memorandum of Understanding (Paris MOU)
  A regional port state control arrangement covering European and North Atlantic waters with 27 member maritime authorities.
• Particular Average
  Loss falling on a single interest (ship or cargo) without contribution from others; opposite of general average.
• Passenger Ship Training
  STCW V/2 mandatory training for crew on ro-ro passenger and other passenger ships: crowd management, safety, crisis management.
• Performance Standard for Protective Coatings (PSPC)
  IMO standard for coatings in ballast tanks and double-side skin spaces — surface prep Sa 2.5, target life 15 yr.
• Permit-to-Work (PTW)
  Documented authorisation system controlling specific high-risk tasks (hot work, enclosed-space entry, electrical isolation, working aloft).
• Pilot
  A licensed navigational specialist employed to guide a vessel through a specific port, harbour, or restricted waterway.
• Pilot Boarding Point (PBP)
  A designated position at sea where a pilot boards or disembarks a vessel before proceeding to or after leaving a port.
• Pilot Ladder
  A rope side ladder rigged overside to enable a pilot to board or disembark from a ship at sea.
• Pilotage
  The navigational service provided by a licensed pilot who boards a vessel to guide it through a port or restricted waterway.
• Places of Refuge
  Sheltered locations where a ship in distress can be moved to stabilise, transfer cargo, or undergo emergency repair, granted by coastal-state authorities.
• Platform Supply Vessel (PSV)
  An offshore supply vessel designed to carry deck cargo, dry bulk, liquid mud, and fuel to offshore platforms and rigs.
• Platform Supply Vessel (PSV)
  Offshore supply vessel dedicated to ferrying personnel, deck cargo, and bulk products (mud, brine, water, fuel) between shore base and offshore installation.
• Polar Code
  IMO mandatory code (2017) governing ship design, equipment, and operations in Arctic and Antarctic waters.
• Polar Code Basic Training
  STCW V/4 basic-level training for officers on ships in polar waters; advanced course required for masters and chief mates.
• Polar Ship Certificate
  Statutory certificate confirming the ship meets Polar Code Category A, B, or C requirements.
• Polar Water Operational Manual (PWOM)
  Document carried aboard polar-going ships describing routing, equipment, crew capabilities, and emergency response in polar waters.
• Polynya
  Persistent area of open water surrounded by sea ice, shaped by wind/current or upwelling.
• Port Agent
  Local representative appointed to handle pre-arrival, in-port, and pre-departure formalities for the vessel.
• Port Captain
  A company representative ashore, usually a former master, who assists vessels in port with local knowledge, clearances, and operational support.
• Port State Control (PSC)
  Inspection regime where a coastal nation's surveyors inspect foreign vessels in its ports for compliance with international conventions.
• Port State Control Inspection (PSC inspection)
  An examination of a foreign-flagged vessel in port by officers of the port state to verify compliance with international conventions.
• Pre-departure Drug & Stowaway Search
  Documented search of all accessible spaces before departure from a high-risk port to detect contraband, stowaways, and security threats.
• Pre-Loading Hold Inspection
  Independent surveyor's check on bulker holds before loading sensitive cargoes (grain, sulphur, wood pellets) for cleanliness, dryness, and structural condition.
• Precautionary Area
  A routing measure where vessels must navigate with caution due to converging traffic lanes or hazardous conditions.
• Pressure/Vacuum Valve (P/V valve)
  A safety valve on a cargo tank vent that opens at set over- or under-pressure limits to protect the tank structure.
• Prismatic Coefficient (Cp)
  Ratio of underwater volume to volume of a prism with cross-section equal to the maximum hull section, length = LBP.
• Privately Contracted Armed Security Personnel (PCASP)
  Armed maritime security teams embarked on merchant vessels in high-risk waters under the IMO BMP framework.
• Product Tanker
  A tanker designed to carry refined petroleum products such as gasoline, jet fuel, and diesel in coated or stainless steel tanks.
• Propeller Cavitation
  Formation and collapse of vapour bubbles on propeller blades due to local pressure dropping below water vapour pressure.
• PSC Deficiency Codes
  Standardised numeric/alpha codes (e.g., 01101 'cargo securing manual') used in PSC reports and aggregated MoU statistics.
• PSC Detention
  Outcome of a port-state-control inspection where deficiencies prevent the vessel from sailing until rectified.
• Pumpman
  A specialised deck or engine rating on tankers responsible for operating cargo and ballast pumps and maintaining pump rooms.
• Purging (cargo tank)
  Replacing one gas atmosphere in a cargo tank with another (typically replacing flammable hydrocarbon with inert gas before tank entry, or vice versa before loading).
• Purifier
  A centrifugal separator that removes water and solid contaminants from fuel or lubricating oil before engine use.
• pusher tug
  River or coastal tug designed to push a barge tow from astern using a notch system, commonly employed on major river systems worldwide.
• Radar SART (SART)
  X-band search and rescue transponder that responds to a 9 GHz radar pulse by generating a distinctive 12-dot line on the rescuing vessel's radar display.
• Radio Officer (R/O)
  An officer certified to operate GMDSS equipment and manage all radio communications on board; a role increasingly merged with other duties.
• Recommended Track
  A route of undefined width for the convenience of vessels in transit, indicated on charts but not mandatory.
• Reefer
  A refrigerated container or cargo space maintaining a controlled temperature for perishable goods during transport.
• Reefer Container
  A refrigerated ISO container with an integral refrigeration unit maintaining controlled temperatures for perishable cargo.
• Reefer Plug
  An electrical socket in a ship's hold or on deck providing power to a reefer container's refrigeration unit.
• Remotely Operated Vehicle (ROV)
  Tethered underwater robot used for inspection, repair, and intervention work, typically operated from an MPSV or DSV.
• Renewal Survey
  A statutory survey conducted to renew a SOLAS or MARPOL certificate for a further period, typically five years.
• Rescue Coordination Centre (RCC)
  Generic term for a coordination centre handling SAR; MRCC is the maritime variant, ARCC the aviation variant.
• Restricted in Ability to Manoeuvre (RAM)
  Vessel which from the nature of her work is restricted in her ability to manoeuvre; shows ball-diamond-ball / R-W-R lights.
• Rhumb Line
  A course that crosses all meridians at the same angle, appearing as a straight line on a Mercator chart.
• RightShip
  A third-party maritime risk-assessment platform providing GHG ratings and vetting inspections used by cargo owners to screen vessels.
• RightShip Star Rating
  Risk rating (1–5 stars) used by dry-bulk charterers; combines technical, environmental, and inspection data.
• Ro-Ro Vessel
  A ship with built-in ramps allowing wheeled cargo such as cars, trucks, and trailers to be driven directly on and off.
• Rogue Wave (extreme wave)
  Wave more than ~2× the significant wave height; rare but documented and dangerous.
• Roll-On/Roll-Off (RoRo)
  Cargo handling method where wheeled vehicles or trailers drive on and off the vessel under their own power via ramps.
• Rotterdam Rules
  The 2008 UN convention on contracts for international carriage of goods wholly or partly by sea, modernising earlier cargo liability regimes.
• Running Fix
  A position obtained from two or more bearings of the same object taken at different times and advanced along the course line.
• Running Moor
  Anchoring with two anchors by laying the first anchor while making way ahead, then dropping the second after veering chain.
• Sacrificial Anode
  Block of zinc or aluminium attached to the hull that corrodes preferentially, protecting steel; used in addition to or instead of ICCP.
• Safe Working Load (SWL)
  The maximum load that lifting gear, mooring lines, or deck equipment is certified to bear without risk of failure in normal use.
• Safety Management Certificate (SMC)
  A certificate issued to a ship confirming its Safety Management System is operating in accordance with the ISM Code.
• SafetyNET
  The Inmarsat-C broadcast service transmitting maritime safety information including navigational warnings and weather forecasts in GMDSS Areas A3/A4.
• SafetyNET
  Inmarsat-C EGC broadcast service delivering Maritime Safety Information to vessels in ocean areas A3, used as the satellite component of the GMDSS MSI distribution system.
• SafetyNET
  Inmarsat Enhanced Group Call service used to broadcast Maritime Safety Information to GMDSS Sea Areas A3 and A4.
• Sagging
  Longitudinal hull bending where the middle is unsupported between two wave crests, putting the deck in compression and the bottom in tension.
• Salvage Award
  Sum payable to salvors fixed by tribunal/court, considering value salved, danger, skill, and prevention of environmental damage.
• Scantlings
  Dimensions (thickness, depth, spacing) of structural members — plating, frames, stiffeners, girders — set by class rules.
• Scavenge Air
  Pressurised air supplied to the cylinder of a two-stroke diesel engine to expel exhaust gases and supply fresh charge for combustion.
• Scavenge Fire
  Fire in the scavenge air space of a two-stroke crosshead engine, ignited by leaking fuel + carbon deposits + scavenge air.
• Scope of Cable
  Length of anchor chain veered, expressed as a multiple of water depth (e.g., 6× depth).
• SCOPIC Clause
  Special Compensation P&I Club clause attached to Lloyd's Open Form (LOF) salvage agreements, replacing the unworkable Article 14 of the Salvage Convention 1989.
• Scrubber Discharge Bans
  Local prohibitions (Singapore, China, UAE Fujairah, several EU/US ports) on washwater discharge from open-loop EGCS.
• Sea Fog
  Advection fog formed when warm, moist air moves over a cooler sea surface, reducing visibility to less than 1,000 metres.
• Sea State
  A description of wave conditions at a given location, typically expressed by significant wave height and period.
• Sea Waybill
  Non-negotiable transport document acting as cargo receipt and contract of carriage, delivered to the named consignee on identification.
• Seafarer Employment Agreement (SEA)
  Written contract between seafarer and shipowner required by MLC 2006; sets terms of employment, repatriation, leave, and complaint procedures.
• Search and Rescue Transponder (SART)
  A radar transponder that responds to X-band radar pulses to indicate the position of a survival craft to SAR aircraft and vessels.
• Second Engineer (2/E)
  The engineer officer second in command, directly overseeing day-to-day operation and maintenance of the main propulsion plant.
• Second Mate (2/O)
  A certificated deck officer responsible for navigation equipment, charts, publications, and often designated ship's safety officer.
• SEEMP
  Ship Energy Efficiency Management Plan: mandatory document detailing measures to monitor and improve ship energy efficiency.
• Segregation
  The physical separation of incompatible dangerous goods in stowage to prevent hazardous reactions if packaging fails.
• Selective Catalytic Reduction (SCR)
  Exhaust after-treatment that injects urea into hot exhaust gas, converting NOx to N2 and water on a vanadium-titanium catalyst.
• Self-Contained Breathing Apparatus (SCBA)
  A compressed-air breathing set worn by firefighting team members to allow entry into oxygen-deficient or toxic atmospheres.
• semi-submersible (semi-sub)
  Column-stabilised offshore unit with submerged pontoons providing stability for drilling, accommodation, or production in deepwater and harsh environments.
• Semi-Submersible Rig
  Floating MODU with submerged pontoons supporting the operating deck on columns, station-kept by anchors or DP.
• service operations vessel (SOV)
  Offshore wind-farm support vessel with motion-compensated walk-to-work gangway, long-stay accommodation, and DP-2 positioning for technician transfers.
• Sewage Treatment Plant (STP)
  A biological or chemical system treating shipboard sewage to standards permitting overboard discharge under MARPOL Annex IV.
• Sextant
  A precision optical instrument used to measure the angle between a celestial body and the horizon for position fixing.
• Shaft Alley
  The enclosed tunnel running through the stern of the ship enclosing the propeller shaft from the engine room to the stern tube.
• Shell Plating
  Outer steel skin of the hull from keel to deck edge.
• Shelltime
  Shell-issued tanker time charter party form, currently Shelltime 4 (1984, amended 2003).
• Shifting (berth)
  Movement of a vessel from one berth to another within the same port, typically without loading/discharging operations underway.
• Ship Agent
  Local representative arranging port services (pilots, tugs, stores, crew changes, customs) on behalf of the owner or charterer.
• Ship Security Officer (SSO)
  The officer on board responsible for implementing and maintaining the Ship Security Plan and liaising with the CSO and port facility security.
• Ship Security Plan (SSP)
  Confidential, ship-specific plan implementing the ISPS Code; approved by the Administration or RSO on its behalf.
• Ship-to-Ship Transfer (STS)
  The transfer of cargo between two vessels moored alongside each other at sea or in an anchorage.
• Ship's Cook
  A certificated crew member responsible for preparing all meals and maintaining hygiene standards in the ship's galley.
• Ship/Shore Link (SSL)
  Hardwired signalling cable between ship and terminal for ESD and safety status during cargo transfer.
• Ship/Shore Safety Checklist (SSSCL)
  Mandatory pre-cargo-transfer checklist verified jointly by ship and terminal representatives.
• Shipboard Marine Pollution Emergency Plan (SMPEP)
  Combined SOPEP + Annex II NLS pollution plan required for ships carrying noxious liquid substances.
• Shipboard Oil Pollution Emergency Plan (SOPEP)
  MARPOL Annex I plan giving guidance to the master and officers in the event of an oil pollution incident.
• Sight Reduction
  The mathematical process of converting a celestial observation into a position line using tables or a calculator.
• Significant Wave Height (Hs)
  Statistical measure of sea state: average height of the highest one-third of waves observed.
• SIRE 2.0
  OCIMF Ship Inspection Report Programme version 2.0 — risk- and behaviour-based tanker inspection regime, fully active 2024.
• Slamming
  Impact of the bow re-entering the water after pitching out, causing localised pressure spikes on the forefoot and forward bottom.
• Slop Tank
  A dedicated tank on tankers used to collect cargo residues and tank washing water, allowing oil-water separation before discharge.
• Snap-Back Zone
  Area on deck within which a parted mooring line can whip back; standing in this zone has killed many seafarers.
• Snap-Back Zone Marking
  Painted/marked deck areas indicating recoil danger from a parted line; revised guidance in MEG4 (2018).
• SOLAS Chapter III
  The chapter of SOLAS covering life-saving appliance requirements including lifeboats, liferafts, immersion suits, and drills.
• SOLAS Chapter IX
  The chapter of SOLAS making the ISM Code mandatory for ships, requiring companies to hold a Document of Compliance.
• SOLAS Chapter V
  The chapter of SOLAS dealing with safety of navigation, requiring ECDIS, AIS, VDR, pilot ladders, and bridge watchkeeping standards.
• SOx Emission Control Area
  Designated sea area where 0.10% sulphur cap applies — Baltic, North Sea, North America, US Caribbean, Mediterranean (from 2025).
• Special Survey
  A comprehensive five-yearly classification survey requiring drydocking and full structural inspection to renew class for a further five years.
• Spring Line
  A mooring line leading forward or aft at a shallow angle to prevent the ship from moving ahead or astern.
• Springing
  Steady-state hull-girder resonance excited by encounter waves at or near the natural frequency.
• Squall
  A sudden, brief and intense increase in wind speed often accompanied by rain or hail, posing an immediate hazard to vessels.
• Standards of Training, Certification and Watchkeeping (STCW)
  International convention setting minimum training, certification, and watchkeeping standards for merchant ship officers and crew.
• Starlink Maritime
  SpaceX low-earth-orbit broadband; offers high bandwidth at sea but is not yet GMDSS-recognised.
• Starting Air Receiver
  A pressure vessel storing compressed air at 30 bar used to crank and start the main engine and diesel generators.
• STCW Flag Endorsement
  Document issued by a flag state recognising another state's STCW CoC for service on its flag — often called a 'flag endorsement' or 'recognition endorsement'.
• STCW Medical Fitness Certificate
  Medical certificate confirming a seafarer meets STCW Reg. I/9 and ILO MLC standards for vision, hearing, and general health.
• Stern Tube
  The watertight bearing assembly through which the propeller shaft passes through the hull from the engine room to the propeller.
• Stockless Anchor
  Most common modern bower anchor type — flukes pivot independently to dig in regardless of orientation.
• Stowage Factor (SF)
  The volume in cubic metres or cubic feet that one metric tonne of a commodity occupies when stowed in a ship's hold.
• Stowage Plan
  A diagram showing the planned or actual position of all cargo units aboard a vessel for a given voyage.
• Stowaway
  Person who hides aboard a ship to gain free passage or escape circumstances ashore, without consent of the operator.
• Subrogation
  Insurer's right, after paying a claim, to step into the insured's shoes and pursue recovery from the wrongdoer.
• Suez Canal Transit
  Transit through the Suez Canal between Port Said and Suez, 193 km, no locks, governed by the Suez Canal Authority.
• Suezmax
  Crude tanker sized to fully transit the Suez Canal in laden condition — typically 120,000-200,000 DWT, ~250 m LOA, ~17 m draft.
• Summer Draft
  The maximum permissible draft when loading in a Summer load line zone, shown on the ship's side as the Summer load line mark.
• Supercargo
  A cargo owner's representative sailing aboard a vessel to oversee the handling and care of the cargo throughout the voyage.
• Supramax / Ultramax
  Mid-size geared bulkers ~50,000-65,000 DWT; Ultramax is the slightly-larger evolution at ~63,000-65,000 DWT.
• Swell
  Long-period regular waves that have travelled out of the generating area, independent of current local wind conditions.
• SWL vs MBL
  Safe Working Load is the in-service rating; Minimum Breaking Load is the failure load. SWL = MBL / safety factor (typically 5).
• Syncrolift
  Mechanical ship-lift platform raising vessels onto land for transfer to repair berths; used by smaller yards.
• Tail Shaft
  Aftermost length of the propeller shaft, running through the stern tube to the propeller; subject to bearing wear and torsional fatigue.
• Tailshaft Survey
  A classification survey of the propeller shaft withdrawn from the stern tube to inspect for corrosion, cracking, and bearing wear.
• Tank Cleaning
  The process of removing cargo residues and gas from cargo tanks using water, chemicals, or crude oil washing prior to loading a new cargo.
• Tanker Familiarisation / Advanced Training
  STCW Reg. V/1-1 (oil/chemical) and V/1-2 (gas) basic and advanced courses required for tanker crews.
• Tanker Management & Self Assessment (TMSA)
  OCIMF programme requiring tanker operators to self-assess and continually improve their safety management against 13 elements.
• Technical Superintendent
  A shore-side engineer responsible for overseeing planned maintenance, drydockings, and technical compliance of assigned vessels.
• Territorial Waters
  The belt of coastal waters extending up to 12 nautical miles from the baseline over which a coastal state has full sovereignty.
• Third Engineer (3/E)
  A junior engineer officer responsible for a watchkeeping duties and maintenance of assigned machinery and systems.
• Third Mate (3/O)
  The most junior certificated deck officer, typically responsible for maintaining fire-fighting and life-saving appliances.
• Three-Point Bearing
  A position fix obtained by simultaneously measuring horizontal angles between three charted objects using a sextant or compass.
• Time Charter
  Charter party hiring the vessel for a fixed period; charterer directs employment, owner provides crew/insurance/maintenance.
• Tokyo Memorandum of Understanding (Tokyo MOU)
  A regional port state control arrangement covering the Asia-Pacific region, established in 1993 with 21 member authorities.
• Tonnage Certificate
  A statutory certificate stating a ship's gross and net tonnage calculated under the International Tonnage Convention 1969.
• Tonnes Per Centimetre (TPC)
  The mass required to change a vessel's mean draft by one centimetre, used to calculate draft changes during loading or discharging.
• Topside Tank
  Triangular tank in upper corner of a bulk carrier hold; carries ballast and reduces free surface of grain cargo.
• Towline
  Wire, fibre, or composite rope connecting tug to tow.
• Tractor Tug
  Tug with propulsion forward of midship — either Voith-Schneider or azimuth — towing from the stern.
• Traffic Separation Scheme (TSS)
  An IMO-adopted routing measure separating opposing traffic into lanes to reduce collision risk in congested waters.
• Transit
  Two or more objects in line with each other, used as a position line or to verify vessel alignment in a channel.
• Trim
  The difference between forward and aft draft; a vessel trims by the head if forward draft exceeds aft draft.
• Trim and Stability Booklet
  Approved booklet on board summarising loading conditions, hydrostatic data, and stability criteria compliance.
• Trim by the Head
  A trim condition where the forward draft exceeds the aft draft, reducing steering effectiveness and increasing wave-making resistance.
• Trim by the Stern
  A trim condition where the aft draft exceeds the forward draft, generally preferred for sea passage to improve propulsive efficiency.
• True Bearing
  A bearing measured clockwise from true north, used for chart plotting and unaffected by magnetic variation or compass deviation.
• Turbocharger (TC)
  A device driven by exhaust gas that compresses intake air to increase diesel engine power output and fuel efficiency.
• Turbocharger Surge
  An abnormal flow reversal in a turbocharger compressor, evidenced by a loud banging or barking noise, caused by sudden load changes.
• Turnbuckle
  A threaded tensioning device fitted in a lashing rod or wire to adjust and tighten cargo securing arrangements.
• Turning Gear
  An electric motor-driven gear unit that slowly rotates the main engine crankshaft for maintenance, cooling-down, or warming-through.
• Twenty-foot Equivalent Unit (TEU)
  Standard unit of cargo capacity equivalent to one 20-foot ISO shipping container, used to measure container ship capacity.
• Twistlock
  A securing device that fits into container corner castings and locks by a 90° twist to stack and secure containers.
• Two-Stroke Engine
  A type of internal combustion engine completing the power cycle in a single up-and-down piston stroke, common in large slow-speed marine main engines.
• Typhoon
  A tropical cyclone of the western North Pacific with sustained winds exceeding 64 knots, the regional name for a hurricane.
• Ultra Large Container Vessel (ULCV)
  Container ships >= 14,500 TEU — too large for the new Panama locks; restricted to Asia-Europe and Asia-East-coast-US-via-Suez routes.
• Ultra Low Sulphur Fuel Oil (ULSFO)
  Residual marine fuel with ≤ 0.10% sulphur, used to meet Emission Control Area limits.
• Ultrasonic Thickness Measurement (UTM)
  Non-destructive ultrasonic gauging of plate thickness to monitor wastage; required at intermediate / renewal class surveys.
• Unattended Machinery Space (UMS)
  A classification notation permitting the engine room to operate without a watchkeeper at night, subject to enhanced alarms and automation.
• United Kingdom Maritime Trade Operations (UKMTO)
  Royal Navy office in Dubai serving as the primary point of contact for merchant vessels in the Indian Ocean / Gulf high-risk areas.
• United Nations Convention on the Law of the Sea (UNCLOS)
  The international treaty defining rights and responsibilities of nations in their use of the world's oceans, including maritime zones.
• Unmanned Machinery Space (UMS)
  Engine room operated without engineer watch present, relying on automation, alarms, and a duty engineer on call.
• Valemax
  Vale-built 380,000-400,000 DWT very large ore carriers (VLOC) for the Brazil-China iron ore trade.
• Vapour Recovery System (VRS)
  Closed-loop return of cargo vapour from ship to shore (or shore to ship) during loading to prevent atmospheric venting.
• Variation
  The angular difference between true north and magnetic north at a given location, shown on navigational charts.
• Veering
  A clockwise shift in wind direction in the Northern Hemisphere, typically associated with the passage of a warm front.
• Vertical Centre of Gravity (KG)
  Vertical distance from the keel to the ship's centre of gravity. Lower KG = more stable.
• Very High Frequency Radio (VHF)
  Marine radio operating in the 156–174 MHz band used for short-range ship-to-ship and ship-to-shore communications.
• Very Large Crude Carrier (VLCC)
  A crude oil tanker with a deadweight of 200,000 to 320,000 DWT, among the largest vessels operating at sea.
• Very Low Sulfur Fuel Oil (VLSFO)
  Fuel oil with a sulfur content of 0.50% m/m or less, required globally under IMO 2020 sulfur cap regulations.
• Vessel Traffic Service (VTS)
  A shore-based radar and communications service that monitors and manages vessel movements in a port or busy waterway.
• Vessel Traffic Service (VTS)
  Shore-based service that monitors and assists vessel movements in defined waters.
• Vetting Inspection
  Pre-fixture or pre-charter inspection by an oil/chemical major or charterer; results determine acceptability for their cargo.
• VHF Channel 13
  Bridge-to-bridge channel for navigational coordination, especially in US, Mexican, and some Asian waters.
• VHF Channel 16 (Ch 16)
  The international distress, safety, and calling frequency at 156.8 MHz; all SOLAS ships must maintain a continuous watch on this channel.
• VHF Channel 70 / DSC
  Dedicated VHF DSC channel (156.525 MHz) used to alert and call other stations before voice contact on ch.16.
• VLCC (VLCC)
  Very Large Crude Carrier — single-hull-era 200,000-320,000 DWT crude tanker; today all double-hull post-OPA-90.
• Voith-Schneider Propeller (VSP)
  Cycloidal propeller producing thrust in any direction by varying blade pitch as the disc rotates; common on tractor tugs.
• Voyage Charter
  Charter party where the shipowner places the vessel at the charterer's disposal for one specified voyage at an agreed freight rate.
• Voyage Data Recorder (VDR)
  A tamper-proof device recording bridge audio, radar images, and navigational data onto a protected capsule for post-incident investigation.
• VSAT
  Very-Small-Aperture-Terminal satellite communications providing always-on broadband at sea (Ku/Ka band).
• Weather Criterion
  Stability criterion checking that a ship can withstand a steady beam wind plus a sudden gust while rolling — Part A 2.3 of the IS Code.
• Weather Routing
  A service providing vessel operators with recommended routes based on forecast weather and sea conditions to optimise safety and economy.
• Web Frame
  Heavy transverse frame, larger than ordinary frames, providing structural support typically every 3–5 frame spaces.
• Well Intervention Vessel
  Specialised OSV equipped to perform downhole work on subsea wells without a full drilling rig.
• Whipping
  Transient hull-girder vibration triggered by a slam, decaying over several oscillations.
• WIBON
  'Whether In Berth Or Not' — clause allowing NOR to be tendered before the vessel reaches the berth.
• Windlass
  A powered deck machine used to heave in or pay out anchor chain and sometimes mooring lines.
• Wing Tank
  Tank running along the side of a vessel, often a ballast tank on bulk carriers or a side cargo tank on tankers.
• Wiper
  The most junior engine room rating who cleans machinery spaces and assists with basic maintenance tasks.
• York-Antwerp Rules
  International rules governing the calculation and apportionment of general average; current versions YAR 1994, 2016.