Collision regulations (COLREGs)

In short

The International Regulations for Preventing Collisions at Sea, 1972 (COLREG 72, as amended) are the IMO convention that tells every vessel at sea who gives way, who stands on, what lights and shapes to show, and what sound signals to make. They are divided into six Parts (A-F) with 41 Rules and four Annexes, and they apply to all vessels on the high seas and connected navigable waters. In India they are given force by the Merchant Shipping Act, 2025 (which replaced the 1958 Act from 15 March 2026) and the collision-prevention rules notified under it; the Directorate General of Maritime Administration (formerly DG Shipping) enforces them and examines them in the Second Mate (FG) and Mate/Master orals. For a deck cadet they are the single most examined subject - "Rule of the Road" is a full written paper plus a lights-and-shapes signals practical, and every interview with SCI, Great Eastern, Synergy, Anglo-Eastern, Fleet Management, MSC or Maersk will probe them.

Explained

Structure, application and the general definitions (Rules 1-3)

• Part A - General (Rules 1-3): application, responsibility, general definitions. Part B - Steering and Sailing (4-19), divided into Section I (any visibility, 4-10), Section II (in sight of one another, 11-18), Section III (restricted visibility, 19). Part C - Lights and Shapes (20-31), Part D - Sound and Light Signals (32-37), Part E - Exemptions (38), Part F - Verification of compliance with the provisions of the Convention (39-41, in force since 1 January 2016).
• Annexes: I - positioning and technical details of lights and shapes; II - additional signals for fishing vessels fishing in close proximity; III - technical details of sound signal appliances; IV - distress signals.
• Rule 1 (Application): applies to all vessels upon the high seas and in all waters connected to them navigable by seagoing vessels. Rule 1(b) lets a government make special rules for roadsteads, harbours, rivers and inland waters - these must conform as closely as possible to COLREGs (in India, port-specific rules under each port's authority).
• Rule 2 (Responsibility) - the "general prudential rule": 2(a) nothing exonerates any vessel from the consequences of neglect of the Rules or of the ordinary practice of seamen; 2(b) in special circumstances, including the limitations of the vessels involved, a departure from the Rules is allowed to avoid immediate danger. Rule 2(b) is a licence to depart, never a licence to ignore.
• Rule 3 definitions: *vessel* (every description of water craft used as a means of transport on water, including WIG craft and seaplanes); *power-driven vessel* (propelled by machinery); *sailing vessel* (under sail, machinery not being used); *vessel engaged in fishing* (fishing with nets, lines, trawls or other apparatus that restricts manoeuvrability - not trolling lines); *vessel not under command (NUC)* - through exceptional circumstance unable to manoeuvre as required; *vessel restricted in ability to manoeuvre (RAM)* - the nature of her work restricts her; *vessel constrained by her draught (CBD)* - a power-driven vessel severely restricted in her ability to deviate because of her draught in relation to the available depth and width; *underway* - not at anchor, not made fast to the shore, not aground; *restricted visibility*.
• Underway vs making way: a vessel stopped in the water but not anchored is underway but not making way - this is why Rule 35 has a separate "two prolonged blasts" signal.

Conduct in any condition of visibility (Rules 4-10)

• Rule 5 - Look-out: a proper look-out by sight and hearing as well as by all available means appropriate to the prevailing circumstances - radar, ARPA, AIS, VHF listening. Most collision enquiries find a Rule 5 and Rule 7 failure.
• Rule 6 - Safe speed: every vessel at all times shall proceed at a safe speed so she can take proper and effective action to avoid collision and stop within a distance appropriate to the circumstances. Factors for all vessels: visibility, traffic density, manoeuvrability and stopping distance, background light at night, sea/wind/current state, draught in relation to depth. Additional factors for radar-fitted vessels: characteristics and limitations of the radar, scale in use, sea clutter, that small craft/ice may not be detected, number and location of targets, radar's ability to assess visibility.
• Rule 7 - Risk of collision: use all available means; proper use of radar if fitted, including long-range scanning and systematic observation of detected objects (plotting or equivalent); assumptions must not be made on scanty information, especially scanty radar information. Risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change, and may exist even with an appreciable bearing change when approaching a very large vessel, a tow, or at close range.
• Rule 8 - Action to avoid collision: positive, made in ample time, with due regard to good seamanship; alterations of course and/or speed shall be large enough to be readily apparent to another vessel visually or by radar - a succession of small alterations is to be avoided. Action shall result in passing at a safe distance and be checked until the other vessel is finally past and clear. If necessary, slacken speed, stop or reverse. Rule 8(f) - a vessel required not to impede must take early action to allow sufficient sea room; a vessel required to keep out of the way is not relieved of that duty merely because the other must not impede her.
• Rule 9 - Narrow channels: keep as near to the outer limit of the channel on your starboard side as is safe and practicable. Vessels of less than 20 m, sailing vessels and vessels engaged in fishing shall not impede a vessel that can safely navigate only within the channel; do not cross a narrow channel if the crossing impedes such a vessel (crossing vessel may use the Rule 34(d) doubt signal). Overtaking in a narrow channel needs the overtaken vessel's agreement: overtaker sounds two prolonged + one short (to pass on her starboard side) or two prolonged + two short (to pass on her port side); the vessel to be overtaken agrees with prolonged-short-prolonged-short (Morse "C"). Avoid anchoring in a narrow channel.
• Rule 10 - Traffic separation schemes (IMO-adopted): proceed in the appropriate lane in the general direction of traffic flow; keep clear of the separation line or zone; join and leave at the termination of the lane, or if entering/leaving at the side, do so at as small an angle to the general direction of flow as practicable. Cross a lane only on a heading as nearly as practicable at right angles to the flow. Vessels under 20 m, sailing vessels and vessels engaged in fishing shall not impede a power-driven vessel following a lane; use inshore zones only if under 20 m, a sailing vessel, fishing, or en route to/from a port, offshore installation, pilot station or place within the inshore zone, or to avoid immediate danger. Indian examples: the TSS off the Gulf of Khambhat approaches, and the Nine Degree Channel / Gulf of Mannar area routeing measures.

Vessels in sight of one another (Rules 11-18)

• Rule 11: Section II applies only to vessels in sight of one another - a radar/AIS contact in fog does not make a crossing situation.
• Rule 12 - Sailing vessels: (a) different tacks - the vessel on the port tack keeps out of the way; same tack - the windward vessel keeps out of the way; (b) a vessel on the port tack seeing a vessel to windward and unable to determine her tack keeps out of the way. Tack is determined by the side on which the mainsail (or, for a square-rigged vessel, the largest fore-and-aft sail) is carried - wind on the port side = port tack.
• Rule 13 - Overtaking: the overtaking vessel keeps out of the way of the vessel being overtaken. Overtaking exists when approaching from a direction more than 22.5° abaft the beam, i.e. at night you would see only the stern light. Rule 13 overrides Rules 14-17; subsequent bearing changes do not make the overtaker a crossing vessel, and she must keep clear until finally past and clear.
• Rule 14 - Head-on: two power-driven vessels meeting on reciprocal or nearly reciprocal courses so as to involve risk of collision - each alters course to starboard and passes port to port. Deemed to exist when you see the other vessel ahead or nearly ahead - by night both sidelights and masthead light(s) in line or nearly in line; by day the corresponding aspect. If in doubt whether such a situation exists, assume it does and act accordingly.
• Rule 15 - Crossing: when two power-driven vessels are crossing, the vessel which has the other on her own starboard side keeps out of the way and shall, if the circumstances admit, avoid crossing ahead of the other. The give-way sector matches the starboard sidelight arc: from right ahead to 22.5° abaft the starboard beam (112.5°).
• Rule 16 - Give-way vessel: take early and substantial action to keep well clear.
• Rule 17 - Stand-on vessel: (a)(i) keep course and speed; (a)(ii) she may take action by her manoeuvre alone as soon as it becomes apparent that the give-way vessel is not taking appropriate action; (b) when collision cannot be avoided by the give-way vessel's action alone, she shall take such action as will best aid to avoid collision; (c) a power-driven vessel taking action under (a)(ii) in a crossing situation shall, if the circumstances admit, not alter to port for a vessel on her own port side; (d) this Rule does not relieve the give-way vessel of her obligation.
• Rule 18 - Responsibilities between vessels (except where 9, 10 and 13 otherwise require). Pecking order from the most privileged: NUC → RAM → fishing → sailing → power-driven. A seaplane shall keep well clear of all vessels. A CBD vessel is not in the hierarchy as a privileged class - other vessels must only "avoid impeding her safe passage" (Rule 18(d)), and she must navigate with particular caution.

Restricted visibility - Rule 19

• Applies to vessels not in sight of one another when navigating in or near an area of restricted visibility. There is no stand-on vessel and no give-way vessel in Rule 19.
• Proceed at a safe speed adapted to the prevailing circumstances, with engines ready for immediate manoeuvre; a power-driven vessel shall have her engines ready.
• Comply with Section I (Rules 4-10) and, on detecting another vessel by radar alone, determine whether a close-quarters situation is developing or risk of collision exists - if so, take avoiding action in ample time.
• Rule 19(d) prohibitions: avoid (i) an alteration of course to port for a vessel forward of the beam, other than for a vessel being overtaken; (ii) an alteration of course towards a vessel abeam or abaft the beam.
• Rule 19(e): except where risk of collision has been determined not to exist, a vessel which hears apparently forward of her beam the fog signal of another vessel, or cannot avoid a close-quarters situation with a vessel forward of her beam, shall reduce speed to the minimum at which she can be kept on her course, and if necessary take all her way off, and in any event navigate with extreme caution until danger of collision is over.

Lights and shapes (Rules 20-31)

• Rule 20: lights from sunset to sunrise and in restricted visibility (and may be shown at other times if considered necessary); shapes by day.
• Rule 21 definitions and arcs: *masthead light* white, 225° (right ahead to 22.5° abaft the beam each side); *sidelights* green to starboard, red to port, 112.5° each; *sternlight* white 135°; *towing light* yellow, same characteristics as a sternlight; *all-round light* 360°; *flashing light* 120 or more flashes per minute.
• Rule 22 - visibility ranges: 50 m or more in length - masthead 6 M, sidelights 3 M, sternlight/towing/all-round 3 M; 12 m to under 50 m - masthead 5 M (3 M if under 20 m), sidelights 2 M, others 2 M; under 12 m - masthead 2 M, sidelights 1 M, others 2 M.
• Rule 23 - power-driven vessel underway: masthead light forward, a second masthead light aft of and higher than the forward one (compulsory for 50 m and over, optional under 50 m), sidelights and a sternlight. An air-cushion vessel in non-displacement mode adds an all-round flashing yellow; a WIG craft taking off/landing/in flight near the surface shows a high-intensity all-round flashing red.
• Rule 24 - towing and pushing: towing vessel shows two masthead lights in a vertical line (three if the tow's length exceeds 200 m), sidelights, sternlight and a towing light above the sternlight; by day, if the tow exceeds 200 m, a diamond shape. Vessel towed shows sidelights, sternlight and, if over 200 m, a diamond by day. A pushing/towing-alongside composite unit is lit as one power-driven vessel.
• Rule 25 - sailing and oars: sidelights plus sternlight; may combine in a tricolour at the masthead if under 20 m; may show optional red-over-green all-round lights at the masthead (not with a combined lantern). A sailing vessel also using machinery shows a cone, apex downwards, forward, and is then a power-driven vessel for the Rules.
• Rule 26 - fishing: *trawling* - green over white all-round plus (if making way) sidelights and sternlight; *other than trawling* - red over white, plus if the outlying gear extends more than 150 m horizontally, an all-round white light or a cone apex upward in that direction. Day shape for both: two cones apices together (or a basket if under 20 m).
• Rule 27 - NUC and RAM: NUC - two all-round red in a vertical line, plus sidelights and sternlight if making way; day, two balls. RAM - red-white-red all-round, day ball-diamond-ball. A vessel engaged in dredging or underwater operations with an obstruction adds two all-round red / two balls on the obstructed side and two all-round green / two diamonds on the side on which another vessel may pass. A vessel engaged in mine-clearance operations shows three all-round green (one at the masthead, one at each yardarm) / three balls - keep 1,000 m clear.
• Rule 28 - CBD: three all-round red in a vertical line; day, a cylinder.
• Rule 29 - pilot vessel on duty: white over red all-round ("white over red, pilot ahead") plus underway lights, or anchor lights when at anchor.
• Rule 30 - anchored and aground: anchored - an all-round white forward and a second lower one aft (a vessel under 50 m may show one all-round white); day, one ball forward. Aground - the anchor light(s) plus two all-round red in a vertical line; day, three balls in a vertical line. Vessels under 7 m not in or near a channel, fairway or anchorage need not show anchor lights.
• Rule 31 - seaplanes show lights as closely similar as practicable.

Sound and light signals (Rules 32-37)

• Rule 32 definitions: short blast about 1 second; prolonged blast 4 to 6 seconds. Whistle required for 12 m and over; bell for 20 m and over; gong (or equivalent) for 100 m and over (Annex III).
• Rule 34 - manoeuvring and warning signals (vessels in sight of one another): one short - I am altering course to starboard; two short - altering to port; three short - I am operating astern propulsion. Matching light signals with the all-round white manoeuvring light (one/two/three flashes) are optional. Rule 34(d) - doubt signal: five or more short and rapid blasts (and may be supplemented by light flashes) when either vessel fails to understand the intentions of the other or doubts whether sufficient action is being taken. Rule 34(e) - a vessel nearing a bend or an area where other vessels may be obscured sounds one prolonged blast, answered by one prolonged.
• Rule 35 - sound signals in restricted visibility (whether by day or night), at intervals of not more than 2 minutes: power-driven making way - one prolonged; power-driven underway but stopped (not making way) - two prolonged (about 2 s apart); NUC, RAM, CBD, sailing, fishing, towing/pushing - one prolonged + two short; a vessel towed (last vessel if manned) - one prolonged + three short, sounded immediately after the towing vessel; pilot vessel may add four short as an identity signal. Anchored vessel - rapid ringing of the bell for about 5 s at intervals of not more than 1 minute (a vessel 100 m and over also sounds the gong aft after the bell), and may sound one short, one prolonged, one short to warn of her position. Aground - three separate and distinct strokes of the bell before and after the rapid ringing.
• Rule 36 - attention signals must not be mistaken for any other signal; Rule 37 and Annex IV - distress signals (gun fired at intervals of about a minute, continuous fog-apparatus sounding, rockets/shells throwing red stars, SOS by any method, "MAYDAY" by radiotelephone, NC flags, square flag with a ball, flames, red parachute flare, orange smoke, slowly raising and lowering arms, EPIRB, DSC distress alert, Inmarsat alert; the radiotelegraph and radiotelephone alarm signals appear only in older editions, while the orange canvas with a black square and circle and the dye marker are still listed in Annex IV).

Collision avoidance in practice and the Indian legal frame

• Relative-motion plotting: radar plot or ARPA gives CPA (closest point of approach) and TCPA (time to CPA). A target whose relative-motion line passes close to own ship's centre means risk; the rule of thumb on a merchant ship is to plan a passing CPA of at least 1 to 2 miles in open water (company standing orders set the actual figure; pick the larger CPA in poor visibility).
• Trial manoeuvre on ARPA tests an intended course/speed change before making it; it does not replace visual look-out or a compass-bearing check.
• VHF for collision avoidance is discouraged - the MAIB/IMO position and most company SMS manuals: identification may be wrong, language is a barrier, and time is lost. Act on the Rules, not on an agreement over the radio.
• Indian application: COLREG 72 as amended is applied through the Merchant Shipping Act, 2025 (which replaced the Merchant Shipping Act, 1958 from 15 March 2026) and the collision-prevention rules notified under it; administration and enforcement rest with the Directorate General of Maritime Administration (the renamed DG Shipping). Breaches are investigated under the Act's casualty-investigation provisions and can lead to a court of inquiry/formal investigation and suspension or cancellation of a certificate of competency.
• Related instruments: STCW 2010 (Manila amendments) Table A-II/1 requires competence in COLREGs for OOW certification; SOLAS Chapter V covers safety of navigation, voyage planning (Reg. 34), ships' routeing (Reg. 10) and VTS (Reg. 12); ships' routeing measures and TSSs are adopted by IMO and shown on charts and in Admiralty Sailing Directions. Each major Indian port's harbour/traffic regulations apply Rule 1(b) locally.
• Limitations: COLREGs allocate duties but prescribe no numerical CPA, no passing distance and no minimum speed; they assume both vessels are complying and are silent on how a VLCC's turning circle and stopping distance differ from a feeder's; they say nothing about autonomous vessels, and amendment is slow because it needs IMO Assembly resolution.

Key points

• COLREG 72 - IMO Convention, 41 Rules in Parts A-F plus 4 Annexes; in force in India through the Merchant Shipping Act, 2025 and rules under it, administered by the DG of Maritime Administration.
• Rule 2 - no Rule exonerates you from the ordinary practice of seamen; Rule 2(b) permits departure only to avoid immediate danger.
• Rule 5 - look-out by sight and hearing and all available means, at all times.
• Rule 6 - safe speed depends on visibility, traffic, manoeuvrability, background light, sea state, draught/depth, and (if fitted) radar limitations.
• Rule 7 - risk is deemed to exist if the compass bearing does not appreciably change; never assume on scanty radar information.
• Rule 8 - alterations must be large, early and readily apparent; avoid a succession of small alterations; check until finally past and clear.
• Rule 9 - keep to the starboard side of a narrow channel; overtake only by agreement (two prolonged + one or two short; answer Morse "C").
• Rule 10 - follow the lane's flow; cross a lane as nearly as practicable at right angles; join/leave at a small angle.
• Rule 13 - the overtaking vessel keeps clear and overrides Rules 14-17.
• Rule 14 - head-on: both alter to starboard, pass port to port; if in doubt, assume a head-on situation exists.
• Rule 15 - crossing: the vessel with the other on her starboard side gives way and avoids crossing ahead.
• Rule 17 - the stand-on vessel keeps course and speed, may act when the other clearly is not, shall act when collision cannot be avoided by the other alone, and should not alter to port for a vessel on her port side.
• Rule 18 pecking order - NUC > RAM > fishing > sailing > power-driven; seaplanes keep clear of all; CBD is only "not to be impeded".
• Rule 19 - in restricted visibility there is no stand-on vessel; do not alter to port for a vessel forward of the beam, nor towards a vessel abeam or abaft the beam.
• Light arcs - masthead 225°, sidelight 112.5° each, sternlight 135°; 225 + 135 = 360°.
• Rule 34(d) - five or more short and rapid blasts is the doubt/wake-up signal; Rule 34(e) - one prolonged for a blind bend.
• Rule 35 - fog signals at not more than 2-minute intervals; one prolonged (making way), two prolonged (stopped), one prolonged + two short (the privileged/constrained group).
• Day shapes - ball (anchored), three balls (aground), two balls (NUC), ball-diamond-ball (RAM), cylinder (CBD), two cones apices together (fishing), cone apex down (sail using engine), diamond (tow over 200 m).

Formulas and examples

Example 1 - Crossing situation: identify the duty. Own ship: power-driven, 280° T at 16 kt. Target: steady compass bearing 010° T, range 8 n.m., closing.

1. Bearing 010° relative to heading 280° = 010 − 280 = −270° ≡ +90°, i.e. 90° on own starboard bow, which lies within the starboard sidelight sector (0° to 112.5°).
2. Bearing is not appreciably changing → risk of collision exists (Rule 7(d)(i)).
3. Both vessels power-driven, in sight, crossing → own ship is the give-way vessel (Rule 15) and must avoid crossing ahead of the target.
4. Rule 16 action: a substantial alteration to starboard (typically 25-35°, or more) made early, bringing the target onto own port bow and passing astern of her; sound one short blast (Rule 34(a)); hold the new course until the target is finally past and clear, then resume.

Example 2 - CPA and TCPA from a radar plot. Target plotted at range 12.0 n.m., bearing steady 045°; after 12 minutes the range is 8.4 n.m. on the same bearing.

1. Closing in 12 min = 12.0 − 8.4 = 3.6 n.m. → relative speed = 3.6 × (60/12) = 18 kt.
2. Bearing constant → the relative-motion line passes through own ship → CPA = 0 n.m. (collision course).
3. TCPA = 8.4/18 h = 0.4667 h = 28.0 minutes from the second observation.
4. Rule 8: at 8.4 n.m. and 28 min there is ample time - act now, boldly, rather than waiting for the situation to close.

Example 3 - CPA where the bearing does change. Target: range 10.0 n.m.; the relative-motion line makes an angle of 12° with the line of sight to own ship; relative speed 20 kt.

1. CPA = 10.0 × sin 12° = 10.0 × 0.2079 = 2.08 n.m.
2. Distance along the relative line to CPA = 10.0 × cos 12° = 10.0 × 0.9781 = 9.78 n.m.
3. TCPA = 9.78/20 h = 0.489 h = 29.3 minutes.
4. Judgement: 2.1 n.m. is acceptable in clear weather open sea, but in restricted visibility or heavy traffic open it out - and remember Rule 7(d)(ii): risk may still exist at a changing bearing when the target is a VLCC or a long tow.

Example 4 - Restricted visibility, fog signal forward of the beam. Visibility 0.4 n.m.; own ship power-driven, 14 kt; a fog signal of one prolonged blast is heard about 20° on the port bow; nothing on radar.

1. Rule 19 applies - vessels are not in sight; there is no stand-on vessel and Rule 15 does not apply.
2. Rule 19(e): the signal is apparently forward of the beam and risk has not been determined not to exist → reduce to the minimum speed at which she can be kept on her course, and take all way off if necessary.
3. Rule 19(d)(i): do not alter to port for a vessel forward of the beam - any alteration would be to starboard, after assessing the radar picture.
4. Also: engines on stand-by and manual steering, sound Rule 35 signals every 2 minutes or less, post an extra look-out forward, inform the master, switch on navigation lights, and plot/monitor systematically.

Example 5 - Light identification at night. Observed dead ahead: two all-round red lights in a vertical line, with a red sidelight visible; no masthead light.

1. Two all-round red in a vertical line = vessel not under command (Rule 27(a)).
2. Sidelight visible → she is making way through the water; a red sidelight means her port side is towards own ship.
3. Rule 18(a)(i): a power-driven vessel shall keep out of the way of a vessel not under command - own ship gives way regardless of the geometry.
4. Had the lights been red-white-red, she would be RAM; three reds would be constrained by her draught, which own ship must merely not impede; white over red would be a pilot vessel.

Where it is used

• Merchant navy deck ranks - the officer of the watch applies Part B every watch; COLREGs form a full written paper and the signals/orals examination for Second Mate, Chief Mate and Master (FG) under the DG of Maritime Administration.
• Indian shipping companies and managers - SCI, Great Eastern Shipping, Essar/Seaways, Synergy Marine, Anglo-Eastern, Fleet Management, Wilhelmsen, Maersk and MSC test Rule of the Road in cadet and officer interviews and in pre-joining assessments.
• Ports, VTS and pilotage - VTS operators and pilots at JNPA, Mumbai, Chennai, Paradip, Kandla/Deendayal and Kochi work the Rules together with each port's Rule 1(b) harbour regulations and SOLAS V VTS requirements.
• Indian Navy and Coast Guard - bridge watchkeeping certification and navigation boards use COLREGs; warships comply, with naval additional signals where applicable.
• Casualty investigation and marine insurance - the Directorate's casualty investigation branch, surveyors, P&I clubs and Admiralty lawyers apportion liability by reference to specific Rules (Rules 2, 5, 7, 8, 15, 17, 19 recur in findings).
• Simulators and training - full-mission bridge simulators at DGS-approved institutes (TSR, AMET, IMU campuses) and ARPA/radar observer courses assess COLREG decision-making with measured CPA/TCPA outcomes.

Interview questions

• Q: When is risk of collision deemed to exist? A: When the compass bearing of an approaching vessel does not appreciably change (Rule 7(d)(i)); it may also exist with a changing bearing when approaching a very large vessel, a tow or at close range.
• Q: Who is the give-way vessel in a crossing situation and what must she avoid? A: The vessel that has the other on her own starboard side; she must take early and substantial action and, if circumstances admit, avoid crossing ahead of the other vessel.
• Q: Is there a stand-on vessel in restricted visibility? A: No. Rule 19 applies to vessels not in sight of one another, so Rules 15-17 do not apply; both vessels must take avoiding action in ample time.
• Q: What alterations are forbidden under Rule 19(d)? A: Altering to port for a vessel forward of the beam (other than one being overtaken), and altering towards a vessel abeam or abaft the beam.
• Q: Which Rule overrides the crossing and head-on rules? A: Rule 13 - overtaking. The overtaking vessel keeps clear until finally past and clear, whatever the later bearing change.
• Q: What is the Rule 18 order of responsibility? A: Not under command, then restricted in ability to manoeuvre, then engaged in fishing, then sailing, then power-driven; a vessel constrained by her draught is not in that order - others must only avoid impeding her safe passage.
• Q: What does a vessel showing red-white-red all-round lights indicate, and what must you do? A: Restricted in ability to manoeuvre; a power-driven vessel and a sailing vessel must keep out of her way (subject to Rules 9, 10 and 13).
• Q: What is the difference between "not to impede" and "keep out of the way"? A: Not to impede means operating early so as to leave the other vessel ample sea room, with no stand-on/give-way relationship created; keep out of the way is a positive duty on an identified give-way vessel under Rules 16 and 18.
• Q: Five or more short blasts - what does it mean and when do you use it? A: The Rule 34(d) doubt signal, used when you cannot understand the other vessel's intentions or doubt she is taking sufficient action to avoid collision; it may be supplemented by light flashes.
• Q: When may you depart from the Rules? A: Only under Rule 2(b), in special circumstances including the vessels' limitations, when a departure is necessary to avoid immediate danger - and the ordinary practice of seamen still binds you.
• Q: As the stand-on vessel, when do you act? A: You may act by your manoeuvre alone as soon as it is apparent the give-way vessel is not taking appropriate action (Rule 17(a)(ii)), and you must act when collision cannot be avoided by her action alone (17(b)); in a crossing situation do not alter to port for a vessel on your port side.
• Q: Why is VHF discouraged for collision avoidance? A: Target identification is often wrong, language and intention can be misunderstood, and the time lost in negotiating replaces the bold, early, Rule-compliant action the other vessel expects to see.
• Q: How does a vessel aground indicate herself? A: Anchor light(s) plus two all-round red lights in a vertical line at night, three balls in a vertical line by day, and in fog three distinct bell strokes before and after the rapid ringing of the bell.

Common mistakes

• Applying Rules 15-17 to a radar or AIS contact in fog - Section II needs the vessels to be in sight of one another; otherwise it is Rule 19.
• Making a series of small alterations or a 5-10° nudge: Rule 8 demands an alteration readily apparent visually and by radar, and small changes are missed on another ship's plot.
• Treating the stand-on vessel's duty as absolute: Rule 17 both permits and, later, requires her to act - "I was stand-on" is no defence in a formal investigation.
• Confusing arcs and light pairs - masthead 225° vs sternlight 135°, trawling green-over-white vs other fishing red-over-white, NUC two reds vs CBD three reds.
• Forgetting that a sailing vessel using her engine is a power-driven vessel and must show a cone apex downwards.
• Mixing up Rule 10 crossing (as nearly as practicable at right angles) with joining or leaving a lane (as small an angle as practicable).
• Quoting a fixed CPA as a COLREG requirement - the Rules say "safe distance"; the number comes from company standing orders and the master's night orders.
• Giving Rule 34 manoeuvring signals to a vessel not in sight, or omitting Rule 35 fog signals because it is daytime - Rule 35 applies by day and night.

Related topics

Bridge watchkeeping and bridge resource management · Radar, ARPA and navigational electronics · Terrestrial and coastal navigation · Seamanship: anchoring, mooring and ship handling · GMDSS and maritime communication · SOLAS, STCW and maritime law.