Maritime Glossary

Concise, source-referenced definitions of COLREG, IALA and GMDSS terminology used across this site.

46 terms

Vessel types & status

VesselCOLREG Rule 3(a): Every description of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water.
Power-driven vesselCOLREG Rule 3(b): Any vessel propelled by machinery. A sailing vessel using its engine — even if sails are also set — counts as power-driven for the COLREGs.
Sailing vesselCOLREG Rule 3(c): Any vessel under sail provided that propelling machinery, if fitted, is not being used.
Vessel engaged in fishingCOLREG Rule 3(d): A vessel fishing with nets, lines, trawls or other gear that restricts its manoeuvrability. Trolling lines and similar gear that do not restrict manoeuvrability are excluded.
Not under command (NUC)COLREG Rule 3(f): A vessel which through some exceptional circumstance is unable to manoeuvre as required by the COLREGs and is therefore unable to keep out of the way of another vessel. Shows two all-round red lights or two balls.
Restricted in ability to manoeuvre (RAM)COLREG Rule 3(g): A vessel which from the nature of her work is restricted in her ability to manoeuvre as required by the COLREGs (e.g. cable laying, dredging, mine clearance). Shows red-white-red all-round lights or ball-diamond-ball shapes.
Constrained by her draught (CBD)COLREG Rule 3(h): A power-driven vessel which, because of her draught in relation to the available depth and width of navigable water, is severely restricted in her ability to deviate from the course she is following. Shows three all-round red lights or a cylinder shape.
UnderwayCOLREG Rule 3(i): A vessel that is not at anchor, made fast to the shore, or aground. "Underway" does not require the vessel to be moving — a drifting vessel with engines stopped is still underway.
Making way through the water: A vessel underway and actually moving through the water under engine, sail or any propelling force. Distinct from "underway" — a drifting vessel is underway but not making way.
WIG craftCOLREG Rule 3(m): Wing-In-Ground craft: a multimodal craft which, in its main operational mode, flies in close proximity to the surface using surface-effect lift.

Steering & sailing

Give-way vesselCOLREG Rule 16: The vessel required by the COLREGs to keep out of the way of another. She must take early and substantial action to keep well clear.
Stand-on vesselCOLREG Rule 17: The vessel required to keep her course and speed when another vessel must give way. She may, and ultimately must, manoeuvre if collision cannot be avoided by the give-way vessel alone.
Head-on situationCOLREG Rule 14: Two power-driven vessels meeting on reciprocal or nearly reciprocal courses so as to involve risk of collision. Each must alter course to starboard so they pass port-to-port.
Crossing situationCOLREG Rule 15: Two power-driven vessels crossing so as to involve risk of collision: the vessel that has the other on her own starboard side must keep out of the way and avoid crossing ahead.
OvertakingCOLREG Rule 13: A vessel coming up on another from a direction more than 22.5° abaft her beam — i.e. seeing only her sternlight at night. The overtaking vessel must keep out of the way until finally past and clear.
Risk of collisionCOLREG Rule 7: The condition that exists when a compass bearing of an approaching vessel does not appreciably change. Risk shall be deemed to exist if there is any doubt.
Safe speedCOLREG Rule 6: A speed at which the vessel can take proper and effective action to avoid collision and be stopped within a distance appropriate to the prevailing circumstances and conditions.
Close-quarters situationCOLREG Rule 8(d): A situation where two vessels are so near that any further reduction in distance creates immediate risk of collision. Action to avoid collision must result in passing at a safe distance.
Action in extremisCOLREG Rule 17(b): The last-moment action by the stand-on vessel when collision can no longer be avoided by the give-way vessel alone. Beyond this point, both vessels share the duty to manoeuvre.
In sight of one anotherCOLREG Rule 11: Vessels are deemed to be in sight of one another only when one can be observed visually from the other. Section II of Part B applies in this condition; radar contact alone does not count.
Restricted visibilityCOLREG Rule 19: Any condition where visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms or any similar cause. Rule 19 governs vessel conduct, not Section II of Part B.
Narrow channelCOLREG Rule 9: A channel or fairway in which a vessel must keep as near to the outer limit on her starboard side as is safe and practicable. Vessels under 20 m, sailing vessels and fishing vessels must not impede vessels confined to the channel.
Traffic Separation Scheme (TSS)COLREG Rule 10: An IMO-adopted routeing measure that separates opposing streams of traffic by lanes and a separation zone. Vessels using a TSS must proceed in the appropriate lane in the general direction of traffic flow.

Lights & shapes

Masthead lightCOLREG Rule 21(a): A white light placed over the fore and aft centreline showing an unbroken arc of 225° — from right ahead to 22.5° abaft the beam on each side.
SidelightsCOLREG Rule 21(b): A green light on the starboard side and a red light on the port side, each showing an unbroken arc of 112.5° from right ahead to 22.5° abaft the beam.
SternlightCOLREG Rule 21(c): A white light placed as nearly as practicable at the stern showing an unbroken arc of 135° — 67.5° to each side from right astern.
Towing lightCOLREG Rule 21(d): A yellow light with the same characteristics as the sternlight (135° arc), shown by a vessel towing astern in addition to her sternlight.
All-round lightCOLREG Rule 21(e): A light showing an unbroken arc of 360°. Used for anchor lights, NUC, RAM, CBD and many fishing/restricted-status indications.
Flashing lightCOLREG Rule 21(f): A light flashing at regular intervals at a frequency of 120 flashes or more per minute. Required, for example, on air-cushion vessels in non-displacement mode.

Sound & light signals

Short blastCOLREG Rule 32(b): A blast of about 1 second's duration on the whistle, used in manoeuvring and warning signals (e.g. one short blast = "I am altering course to starboard").
Prolonged blastCOLREG Rule 32(c): A blast of from 4 to 6 seconds' duration on the whistle. Used for fog signals and the bend/blind-spot warning in narrow channels.
Danger / doubt signalCOLREG Rule 34(d): At least five short and rapid blasts on the whistle, sounded when one vessel doubts the other is taking sufficient action to avoid collision. May be supplemented by an equivalent light signal.
WhistleCOLREG Rule 32(a): Any sound-signalling appliance capable of producing the prescribed blasts and complying with the specifications in Annex III. The fundamental frequency depends on vessel length.

IALA buoyage

Lateral markIALA MBS: An IALA mark indicating the port or starboard side of a navigable channel in the conventional direction of buoyage. Colour assignment is reversed between Region A (red to port) and Region B (red to starboard).
Cardinal markIALA MBS: A mark whose name (N, E, S, W) indicates the side on which the navigable water lies relative to a danger. Identified by black-yellow colour patterns and double-cone topmarks pointing toward the black band(s).
Isolated danger markIALA MBS: A mark erected on, or moored above, an isolated danger of limited extent that has navigable water all around it. Black with one or more red horizontal bands and a topmark of two black spheres.
Safe water markIALA MBS: A mark indicating navigable water all around its position — typically a fairway or mid-channel mark. Red and white vertical stripes with a single red sphere as topmark.
Special markIALA MBS: A yellow mark indicating a special area or feature referred to in nautical documents (cable, pipeline, military exercise area, etc.). Not primarily a navigation hazard.
Emergency wreck marking buoyIALA MBS: A temporary buoy with vertical blue and yellow stripes and an alternating blue-yellow flashing light, used to mark a new wreck until permanent buoyage is in place.
IALA Region A & Region BIALA MBS: The two regional conventions for lateral marks. In Region A (Europe, Africa, most of Asia/Oceania) red is to port returning from sea; in Region B (Americas, Japan, Korea, Philippines) red is to starboard.

GMDSS & radio

GMDSSSOLAS Ch. IV: Global Maritime Distress and Safety System — the IMO/SOLAS framework that automates ship-to-shore distress alerting and the dissemination of maritime safety information by sea area (A1–A4).
Digital Selective Calling (DSC): A digital signalling layer over VHF/MF/HF radios used to send distress alerts and routine calls addressed to specific MMSIs. VHF DSC distress runs on channel 70.
MMSI: Maritime Mobile Service Identity — a unique 9-digit number that identifies a vessel, coast station or group on DSC, AIS and Inmarsat. The first three digits are the MID country code.
EPIRB: Emergency Position-Indicating Radio Beacon — a 406 MHz Cospas-Sarsat beacon that, once activated (manually or by water immersion), transmits the vessel's identity and GNSS position to satellites.
SART: Search and Rescue Transponder — a portable device that, when triggered by an X-band radar, returns a series of 12 dots on the SAR vessel's screen leading to the survivor's position. AIS-SART variants transmit AIS messages instead.
NAVTEX: International automatic system for broadcasting Maritime Safety Information (navigational warnings, weather, SAR alerts) on 518 kHz (English) and 490 kHz (national language).