COLREG Structure, Application and Definitions

The rulebook and where to read it

In short: Read this topic first in the COLREG area. The rules are the COLREG 1972 (IMO); UK ships get them through MSN 1781 (M+F); NP 100 is the general handbook that sits beside them.

• COLREG 1972 — the IMO Convention on the International Regulations for Preventing Collisions at Sea. Often called the “Rules of the Road”. It has been amended several times since 1972. The latest IMO amendments (MSN 1781 (M+F) Amd 3, Appendix 2):
  • A.910(22) — in force 29 November 2003: WIG craft (Rules 3, 18, 23, 31), Rules 8(a), 33(a) and 35(i), Annexes I and III.
  • A.1004(25) — in force 1 December 2009: Annex IV (distress signals).
  • A.1085(28) — in force 1 January 2016: new Part F, Rules 39–41.
  • Nothing later is listed in the MCA table. Older printed copies of the Rules may still show the pre-2009 Annex IV, with the radiotelegraph and radiotelephone alarm signals.
• MSN 1781 (M+F) — the MCA notice that carries the COLREG for UK purposes. It goes with the Merchant Shipping (Distress Signals and Prevention of Collisions) Regulations 1996 (SI 1996/75). These apply to UK ships wherever they are and to other ships in UK waters.
  • Current version: MSN 1781 (M+F) Amendment 3 (21 November 2023). It replaces Amendment 2 (2016), which itself replaced the original MSN 1781, its corrigendum and Amendment 1.
  • (M+F) = applies to merchant ships and fishing vessels; pleasure craft and yachts are covered too.
  • Amendment 3 follows the Merchant Shipping (Watercraft) Order 2023 (in force 31 March 2023): the Regulations now also reach powered watercraft that are not ships, e-foils included (MGN 684 (M)).
• NP 100 — The Mariner’s Handbook (UKHO, ADMIRALTY). General guidance on charts and their use, navigational information, sea and ice conditions, meteorology, restrictions to navigation and regulations. Not the COLREG text itself, but the standard companion on the bridge.

On the bridge: when the examiner asks “Where would you find the Collision Regulations on a UK ship?”, the answer is MSN 1781; for the international source, COLREG 1972 (IMO).

How COLREG is built: Parts A–F and the Annexes

In short: 41 rules in six Parts, plus four Annexes. Part B (steering and sailing) has three Sections: I = Rules 4–10, II = Rules 11–18, III = Rule 19.

Part B in three Sections

Some study guides print Section II as “Rules 5–18”. That is a misprint: the COLREG text (and the IMO) give Section II = Rules 11–18.

The four Annexes

• Annex I — positioning and technical details of lights and shapes.
• Annex II — additional signals for fishing vessels fishing in close proximity.
• Annex III — technical details of sound signal appliances.
• Annex IV — distress signals.

Rule 1 — where and to whom the rules apply

In short: COLREG applies to all vessels on the high seas and all connected waters navigable by seagoing vessels; local rules may add to it but should stay as close to it as possible.

• Rule 1(a) — all vessels upon the high seas and all waters connected to them that seagoing vessels can navigate.
• Rule 1(b) — local special rules for roadsteads, harbours, rivers, lakes and inland waterways are allowed, but must conform as closely as possible to COLREG. On the bridge: harbour byelaws or a river authority’s rules can override COLREG locally — check the port information before entry.
• Rule 1(c) — governments may allow extra station or signal lights, shapes or whistle signals for warships, vessels in convoy and fishing vessels fishing as a fleet.
• Rule 1(d) — traffic separation schemes may be adopted by the IMO for the purpose of these Rules.
• Rule 1(e) — vessels of special construction or purpose that cannot fully comply (lights, shapes, sound signals) comply as closely as possible, as their Government decides.

Rule 2 — responsibility

In short: Following the letter of the rules never excuses bad seamanship (Rule 2(a)), and you may depart from a rule only to avoid immediate danger (Rule 2(b)).

• Rule 2(a) — nothing in the Rules frees the vessel, owner, master or crew from the consequences of neglecting the Rules, or neglecting any precaution required by the ordinary practice of seamen or by the special circumstances of the case.
• Rule 2(b) — when applying the Rules, take into account all dangers of navigation and collision and any special circumstances, including the limitations of the vessels involved, which may make a departure from the Rules necessary to avoid immediate danger.
• On the bridge: a stand-on vessel that watches the give-way vessel do nothing cannot say “I kept my course, so it is not my fault” — good seamanship and Rule 17 expect action in time.
• Examiners ask about Rule 2 directly in the MCA oral. Say both halves: (a) seamanship, (b) departure only for immediate danger.

Rule 3 — the vessel types

In short: Rule 3 defines each type of vessel; the ranking of who keeps out of whose way (Rule 18) and the lights (Part C) depend on getting the type right.

Vessel, power-driven vessel, sailing vessel

• Vessel — Rule 3(a): every kind of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used for transport on water.
• Power-driven vessel (PDV) — Rule 3(b): any vessel propelled by machinery. PDV is also the “default” type for lights and shapes.
• Sailing vessel — Rule 3(c): a vessel under sail, provided that propelling machinery, if fitted, is not being used. Sails up and engine running = a power-driven vessel (by day she shows a cone, point down — Rule 25(e)).

Vessel engaged in fishing

• Rule 3(d): a vessel fishing with nets, lines, trawls or other apparatus that restrict her manoeuvrability.
• It does not include fishing with trolling lines or other gear that does not restrict manoeuvrability.
• The status comes from the activity, not the hull: COLREG has no “fishing vessel” category, only a vessel engaged in fishing. A trawler on passage with gear stowed is just a power-driven vessel.

Seaplane and WIG craft

• Seaplane — Rule 3(e): any aircraft designed to manoeuvre on the water. It is still a “vessel” under Rule 3(a).
• WIG craft (wing-in-ground) — Rule 3(m): a multimodal craft that, in its main operating mode, flies close to the surface using surface-effect action.

Not under command (NUC)

• Rule 3(f): a vessel that, through some exceptional circumstance, is unable to manoeuvre as the Rules require and so cannot keep out of the way of another vessel.
• Examples: main engine or steering failure, fouled propeller.
• ❌ Not a planned stop: shutting down the engine on purpose (drifting, waiting for a berth) does not make you NUC.

Restricted in her ability to manoeuvre (RAM)

• Rule 3(g): a vessel restricted by the nature of her work, so unable to keep out of the way. The list “includes but is not limited to”:
  1. laying, servicing or picking up a navigation mark, submarine cable or pipeline;
  2. dredging, surveying or underwater operations;
  3. replenishment or transferring persons, provisions or cargo while underway;
  4. launching or recovering aircraft;
  5. mine clearance;
  6. a towing operation that severely restricts the tug and her tow in their ability to deviate from their course.
• NUC vs RAM: NUC = something went wrong (exceptional circumstance); RAM = the job she is doing.
• Watch the wording: Rule 3(g) mentions pipelines, but the Rule 10 exemption for cable work in a TSS speaks only of a submarine cable — the word “pipeline” is not there.

Constrained by her draught (CBD)

• 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.
• Only a PDV can be CBD — a sailing vessel cannot.
• For lights and shapes, a CBD vessel is a PDV first: she shows normal PDV lights plus her CBD signal (Rule 28: cylinder by day; the cylinder is 1.2 m high × 0.6 m diameter, Annex I).
• Short summaries often say “draught in relation to the available depth” only; the current Rule 3(h) also includes the width of navigable water.

Towing and pushing

• Rule 3 has no separate definition of a towing vessel: she is a power-driven vessel engaged in towing or pushing, with her own lights and shapes (Rule 24).
• She becomes RAM only when the tow severely restricts her and her tow in deviating from their course (Rule 3(g)(vi)). A routine tow is not RAM — and this changes the answer in Rule 18 questions.

Rule 3 — underway, making way and the other terms

In short: Underway is defined (not at anchor, not made fast to the shore, not aground); making way is not defined in Rule 3.

• Underway — Rule 3(i): the vessel is not at anchor, not made fast to the shore and not aground. A ship stopped and drifting is still underway.
• Making way — no definition in COLREG. It is used in the ordinary seaman’s sense: the vessel is moving through the water. The difference matters for lights and fog signals — for example, a PDV making way sounds one prolonged blast at intervals of not more than 2 min; underway but stopped and making no way, two prolonged blasts (Rule 35(a)–(b)).
• Length and breadth — Rule 3(j): length overall and greatest breadth.
• In sight of one another — Rule 3(k): only when one vessel can be observed visually from the other. Radar contact alone is not “in sight”.
• Restricted visibility — Rule 3(l): any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms or similar causes.

Definition traps examiners use

In short: Most definition questions in the oral are traps. Five to know cold.

1. “Give me the COLREG definition of making way.” → There is none. Rule 3 defines underway; making way means moving through the water.
2. “Is a fishing vessel a vessel engaged in fishing?” → Only while her gear restricts her manoeuvrability. Trolling does not count.
3. “The engineer stops the engine for 20 minutes of planned work. Are you NUC?” → No. NUC needs an exceptional circumstance.
4. “Can a sailing yacht be constrained by her draught?” → No. CBD is always a power-driven vessel.
5. “A tug with a tow: RAM or not?” → Only if the tow severely restricts her ability to deviate from her course.