COLREG in plain words: who gives way, lights, sound signals and interview questions

COLREG — the International Regulations for Preventing Collisions at Sea — are the rules of the road at sea: who gives way, which lights and shapes a vessel shows, and what a blast on the whistle means. Every deck officer is examined on them for their certificate, and a master interviewing a new officer will often draw lights on a sheet of paper and ask what vessel that is. This page gives the structure of the rules, the rules that decide most encounters, the lights and sound signals asked most often, the traps, and questions to test yourself.

What COLREG is

The rules were adopted at IMO in 1972 and came into force on 15 July 1977, replacing the 1960 rules. They made traffic separation schemes part of the rules and have been amended several times since; the latest part, on verifying compliance, was added in 2016. They apply to all vessels on the high seas and in all waters connected to them that seagoing vessels can navigate. A state may have special rules for its harbours, rivers and inland waters, but those must follow COLREG as closely as possible.

Rule 2 sets the tone for all the rest: nothing in the rules excuses neglect of any precaution required by the ordinary practice of seamen, and to avoid immediate danger a vessel may depart from the rules. The rules do not replace seamanship; they assume it.

How the rules are built

• Part A — General (Rules 1–3): application, responsibility and definitions — power-driven and sailing vessel, vessel engaged in fishing, not under command, restricted in her ability to manoeuvre, constrained by her draught, underway, restricted visibility.
• Part B — Steering and sailing (Rules 4–19): conduct in any visibility (4–10), vessels in sight of one another (11–18), restricted visibility (19).
• Part C — Lights and shapes (Rules 20–31).
• Part D — Sound and light signals (Rules 32–37).
• Part E — Exemptions (Rule 38).
• Part F — Verification of compliance (Rules 39–41).
• Annexes: I — position and technical details of lights and shapes; II — additional signals for fishing vessels fishing close together; III — sound signal appliances; IV — distress signals.

Key numbers

The rules that decide most encounters

• Rule 5 — Look-out: at all times, by sight and hearing and by all available means — radar, AIS, listening on VHF.
• Rule 6 — Safe speed: a speed at which you can take proper and effective action and stop within an appropriate distance. It depends on visibility, traffic density, your manoeuvrability, background lights at night, wind, sea and current, draught and the limits of your radar.
• Rule 7 — Risk of collision: if the compass bearing of an approaching vessel does not appreciably change, the risk exists. It may exist even when the bearing changes — with a very large vessel, a tow, or at close range. Do not draw conclusions from scanty information, especially scanty radar information.
• Rule 8 — Action to avoid collision: positive, made in ample time and large enough to be readily apparent to the other vessel, by eye or on radar. Avoid a succession of small alterations. Check that the action works until the other vessel is finally past and clear.
• Rule 9 — Narrow channels: keep as near to the outer limit on your starboard side as is safe and practicable. Vessels under 20 metres and sailing vessels must not impede a vessel that can navigate safely only within the channel.
• Rule 10 — Traffic separation schemes: go with the general direction of traffic in your lane, join and leave at the ends, and if you must cross, do it on a heading as nearly as practicable at right angles to the traffic flow.
• Rule 13 — Overtaking: the overtaking vessel keeps out of the way, whatever kind of vessel either one is. You are overtaking when you come up from more than 22.5° abaft the other vessel's beam — at night you would see only her sternlight. In doubt, assume you are overtaking.
• Rule 14 — Head-on: two power-driven vessels meeting on reciprocal or nearly reciprocal courses each alter course to starboard and pass port to port.
• Rule 15 — Crossing: of two power-driven vessels crossing, the one that has the other on her own starboard side keeps out of the way and, if circumstances admit, avoids crossing ahead of her.
• Rule 16 — Give-way vessel: takes early and substantial action to keep well clear.
• Rule 17 — Stand-on vessel: keeps her course and speed. She may act on her own as soon as it becomes apparent that the give-way vessel is not taking appropriate action, and she must act when collision can no longer be avoided by the give-way vessel alone. In a crossing situation a power-driven stand-on vessel should not alter course to port for a vessel on her own port side, if circumstances admit.
• Rule 18 — Responsibilities between vessels: unless Rules 9, 10 and 13 require otherwise, a power-driven vessel keeps out of the way of a vessel not under command, a vessel restricted in her ability to manoeuvre, a vessel engaged in fishing and a sailing vessel; a sailing vessel keeps out of the way of the first three; a vessel engaged in fishing, as far as possible, of the first two. Every vessel except those two avoids impeding a vessel constrained by her draught.
• Rule 19 — Restricted visibility: for vessels not in sight of one another. Proceed at a safe speed with the engines ready for immediate manoeuvre. There is no give-way or stand-on vessel here — everyone acts. If a vessel detected by radar alone is developing a close-quarters situation, act in ample time, but avoid altering course to port for a vessel forward of the beam (other than one being overtaken) and avoid altering towards a vessel abeam or abaft the beam. On hearing a fog signal apparently forward of the beam, reduce speed to the minimum at which you can keep your course, take all way off if necessary, and navigate with extreme caution until the danger is over.

Lights and shapes

Arcs: masthead light — white, 225°, from right ahead to 22.5° abaft the beam on each side; sidelights — red to port and green to starboard, 112.5° each; sternlight — white, 135°; towing light — yellow, 135°; all-round lights — 360°.

Ranges for a vessel of 50 metres or more: masthead light 6 nautical miles, sidelights, sternlight, towing and all-round lights 3 miles. Between 12 and 50 metres: masthead light 5 miles (3 under 20 metres), the others 2 miles. A power-driven vessel of 50 metres or more underway shows two masthead lights, the after one higher.

The lights and shapes asked most (lights all-round unless stated):

• Not under command: two red lights in a vertical line; by day two balls. When making way, sidelights and a sternlight as well.
• Restricted in her ability to manoeuvre: red, white, red in a vertical line; by day ball, diamond, ball.
• Constrained by her draught: three red lights in a vertical line, in addition to her normal lights; by day a cylinder.
• Engaged in trawling: green over white. Engaged in fishing other than trawling: red over white. By day both show two cones with their points together. When making way, sidelights and a sternlight as well.
• Pilot vessel on duty: white over red.
• At anchor: a white light forward and a second, lower, white light aft (one white light under 50 metres); by day one ball forward.
• Aground: the anchor lights plus two red lights in a vertical line; by day three balls.
• Sailing vessel underway: sidelights and a sternlight, no masthead light; she may also show red over green at the masthead.
• Power-driven vessel towing: two masthead lights in a vertical line (three when the tow is longer than 200 metres), sidelights, sternlight and a yellow towing light above the sternlight; a diamond by day if the tow is longer than 200 metres.

Sound signals

A short blast lasts about one second, a prolonged blast four to six seconds.

Vessels in sight of one another:

• one short — "I am altering my course to starboard";
• two short — "I am altering my course to port";
• three short — "I am operating astern propulsion";
• five or more short and rapid — doubt about the other vessel's intentions or actions;
• one prolonged — approaching a bend or a stretch of channel where other vessels may be hidden;
• in a narrow channel, two prolonged and one short — "I intend to overtake you on your starboard side", two prolonged and two short — "on your port side"; the vessel being overtaken agrees with prolonged, short, prolonged, short.

In restricted visibility, at intervals of not more than two minutes:

• power-driven vessel making way — one prolonged blast;
• power-driven vessel underway but stopped — two prolonged blasts;
• not under command, restricted in her ability to manoeuvre, constrained by her draught, sailing, fishing, towing or pushing — one prolonged and two short;
• a manned vessel being towed — one prolonged and three short, right after the towing vessel's signal;
• at anchor — rapid ringing of the bell for about five seconds at intervals of not more than one minute (over 100 metres, a bell forward and a gong aft); she may add one short, one prolonged and one short to warn an approaching vessel;
• aground — the anchor signal, with three separate and distinct strokes on the bell before and after it.

What they ask at the interview

• Cadets and ABs on look-out: what a look-out is for and how to report a light or a vessel ("light one point on the starboard bow"); the lights of a power-driven vessel; what red over white means; fog signals.
• Third and second officers: Rules 7, 8 and 13–17; head-on and crossing situations recognised at night by lights alone; the difference between not under command, restricted in ability to manoeuvre and constrained by draught; rules for traffic separation schemes; CPA and TCPA from the radar; when to call the master.
• Chief officers and masters: Rule 2, the actions under Rule 17, several targets in restricted visibility, agreeing a passing on VHF and why it is risky, local rules.
• CES and similar tests: the COLREG section is a fixed part of every deck officer test.

Common mistakes

1. Altering to port in a head-on or close crossing situation "to pass starboard to starboard". Two vessels turning towards each other is the classic collision.
2. A succession of small alterations the other vessel cannot see on her radar. Rule 8 asks for one early, large and obvious action.
3. Believing the stand-on vessel "has the right of way" and must hold on regardless. She must act when the give-way vessel alone can no longer avoid the collision.
4. Applying the "in sight" rules in fog. In restricted visibility there is no stand-on vessel, and an alteration to port for a vessel forward of the beam is to be avoided.
5. Arranging a passing over VHF instead of following the rules. It costs time, and the voice that answers may belong to another ship.
6. Trusting AIS alone. Not every vessel transmits, and the data can be wrong; the look-out and the radar come first.
7. Treating a vessel constrained by her draught as if she were not under command. Others avoid impeding her, but she herself must navigate with particular caution.

Test yourself

Who on board needs this

Deck officers first: the master, the chief officer, the second and third officer are examined on COLREG for their certificate and again at the interview. Deck cadets learn it from their first voyage; an able seaman on look-out has to recognise lights and report them correctly.

Put your watchkeeping certificate and the vessel types you have kept watch on into your maritime CV: that is what a crewing manager reads first for a deck post.

Source: the International Regulations for Preventing Collisions at Sea, 1972, as amended. This page simplifies the wording of the rules; on the bridge, the full text of the rules and the master's standing orders prevail. It is a study aid, not a legal text.