COLREGS: International Regulations for Preventing Collisions at Sea

On the open ocean, there are no traffic lights, lane markings, or stop signs. Two vessels approaching each other in a busy shipping lane need a shared set of rules to prevent disaster. The Convention on the International Regulations for Preventing Collisions at Sea, known as COLREGs, provides that framework.

Adopted by IMO in 1972 and in force since 1977, COLREGs apply to every vessel on the high seas and in all connected navigable waters.

What Are COLREGs?

COLREGs are the internationally recognized rules of the road for vessels at sea. Published by the International Maritime Organization (IMO), COLREGs define how vessels should behave to avoid collisions, what lights and shapes they must display, what sound signals they must use, and how responsibility is assigned when two vessels meet.

The 1972 Convention replaced the earlier Collision Regulations of 1960 and introduced the formal recognition of traffic separation schemes (TSS) under Rule 10. COLREGs apply to all vessels, from kayaks to supertankers. Individual countries may adopt supplementary inland navigation rules , but domestic regulations should align as closely as possible with COLREGs.

How COLREGs Are Structured

COLREGs contain 41 rules organized into six parts. Part A (Rules 1 through 3) covers general provisions and definitions. Part B (Rules 4 through 19) is the operational core, covering steering and sailing rules. Part C (Rules 20 through 31) specifies navigation lights and day shapes . Part D (Rules 32 through 37) covers sound and light signals. Part E (Rule 38) addresses exemptions. Part F (Rules 39 through 41) supports the IMO Member State Audit Scheme . Four technical annexes cover light positioning, fishing vessel signals, sound signal equipment, and distress signals.

Professional mariners should maintain a current COLREG s publication aboard. American Nautical Services carries the latest IMO publications , including the COLREGs consolidated text.

Key Steering and Sailing Rules Every Mariner Must Know

Part B is the section that governs day-to-day collision avoidance decisions on the bridge. A few rules within Part B carry particular weight during watchkeeping.

Rule 5: Lookout

"Every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances." Rule 5 establishes that the lookout is continuous and uses every resource available, including radar, AIS, ECDIS, and visual observation. A proper lookout is the foundation of collision avoidance, and failure to maintain one is cited in a large percentage of maritime collision investigations.

Rule 6: Safe Speed

Every vessel must proceed at a safe speed at all times, accounting for visibility, traffic density, vessel maneuverability, sea state, and the limitations of radar equipment. Safe speed is not a fixed number. A speed that is safe in open water with clear visibility may be dangerously fast in a congested fairway or dense fog.

Rule 8: Action to Avoid Collision

When action is necessary, Rule 8 requires it to be positive, made in ample time, and large enough to be readily apparent to the other vessel. Small, gradual course changes are dangerous because they may not be detectable on radar. The mariner must check the effectiveness of any action until the other vessel is past and clear.

Rule 13: Overtaking

Any vessel overtaking another must keep clear of the vessel being overtaken. A vessel is considered overtaking when approaching from a direction more than 22.5 degrees abaft the beam of the other vessel. The overtaking obligation holds regardless of whether the overtaking vessel is power-driven or under sail.

Rule 14: Head-On Situations

When two power-driven vessels are meeting on reciprocal or nearly reciprocal courses to involve risk of collision, each must alter course to starboard so that each passes on the port side of the other. At night, this situation exists when a vessel sees both sidelights (red and green) of the approaching vessel. Rule 14 applies only to power-driven vessels. Encounters between sailing vessels are governed separately under Rule 12.

Rule 15: Crossing Situations

When two power-driven vessels are crossing to involve risk of collision, the vessel that has the other on her starboard side is the give-way vessel and must take action to avoid crossing ahead of the stand-on vessel. The stand-on vessel maintains course and speed under Rule 17 but must take action if the give-way vessel fails to act.

Rule 18: Vessel Hierarchy

Rule 18 establishes priority between vessel types. A power-driven vessel underway must keep clear of a vessel not under command, a vessel restricted in ability to maneuver, a vessel engaged in fishing, and a sailing vessel. A sailing vessel keeps clear of not-under-command vessels, restricted vessels, and fishing vessels. Rule 18 does not override the overtaking rule (Rule 13), narrow channel rules (Rule 9), or TSS rules (Rule 10).

Lights, Shapes, and Sound Signals

Part C requires vessels to display specific navigation lights between sunset and sunrise: a masthead light forward, sidelights (green to starboard, red to port), and a sternlight. Vessels over 50 meters carry a second masthead light aft and higher. Day shapes communicate status: a ball for a vessel at anchor, two balls in a vertical line for not under command, two cones apex-together for a vessel fishing.

Part D governs sound signals. One short blast means "altering course to starboard." Two short blasts mean "altering course to port." Three short blasts mean "operating astern propulsion." Five or more short and rapid blasts signal doubt about another vessel's intentions. In restricted visibility, a power-driven vessel making way sounds one prolonged blast at intervals of not more than two minutes.

Proper navigation equipment , including compliant lights, sound signaling devices, and radar, directly supports COLREGs compliance.

COLREGs in Restricted Visibility

Rule 19 governs conduct when vessels cannot see each other visually. Every vessel must proceed at a safe speed adapted to conditions. A vessel detecting another by radar alone must determine whether a close-quarters situation is developing and take avoiding action in ample time if the risk of collision exists.

Rule 19 specifically prohibits altering course to port for a vessel forward of the beam, except when overtaking. Radar, AIS, ECDIS with proper chart data , and signal equipment all support the bridge team's ability to assess risk in reduced visibility.

COLREGs Compliance and Port State Control

Port state control inspectors verify that vessels carry required navigation and safety publications , functioning navigation lights, sound signaling apparatus, and that bridge teams demonstrate COLREGs knowledge. Deficiencies can result in detention. COLREG violations may also lead to prosecution even when no collision has occurred. Following an actual collision, the first question any investigation board asks is whether each vessel complied with the applicable rules.

Know the Rules Before You Need Them

COLREGs exist because the sea does not forgive hesitation or confusion. A deck officer who knows Rule 14 by reflex does not need to think when two masthead lights and both sidelights appear ahead in the dark. Regular review of the rules, combined with practical bridge simulation and real-world watchkeeping , builds the kind of automatic response that prevents collisions. For the latest COLREGs publication, ECDIS systems with integrated AIS and radar, or any navigation compliance resource, contact American Nautical Services at +1 (954) 522-3321 or sales@amnautical.com.

Frequently Asked Questions

Q. What are COLREGs?

COLREGs are the Convention on the International Regulations for Preventing Collisions at Sea, adopted by IMO in 1972. COLREGs contain 41 rules covering steering, sailing, navigation lights, day shapes, and sound signals that apply to all vessels on the high seas and connected navigable waters.

Q. Do COLREGs apply to recreational boats?

Yes. COLREGs apply to all vessels, including recreational boats, on the high seas and in connected waters. Some countries have supplementary inland rules for rivers and lakes, but the international rules serve as the baseline for all navigation.

Q. What is the vessel hierarchy under COLREGs?

Under Rule 18, power-driven vessels give way to sailing vessels, fishing vessels, vessels restricted in ability to maneuver, and vessels not under command. Sailing vessels give way to fishing vessels and restricted or not-under-command vessels.

Q. What should a vessel do in a head-on situation?

Under Rule 14, when two power-driven vessels meet on reciprocal courses with risk of collision, both must alter course to starboard so each passes on the port side of the other.

Q. What sound signal means a vessel is altering course to starboard?

One short blast on the whistle or horn signals that a vessel is altering course to starboard. Two short blasts indicate an alteration to port. Three short blasts indicate the vessel is operating astern propulsion.

Q. How many rules are in COLREGs?

COLREGs contain 41 rules divided into six parts: Part A (General), Part B (Steering and Sailing), Part C (Lights and Shapes), Part D (Sound and Light Signals), Part E (Exemptions), and Part F (Verification of Compliance).