Two ships close on each other on a clear night, each with a competent officer on the bridge, each watching the other’s lights, and each quietly assuming the other will turn. That is how most collisions happen. Not in thick fog or a crowded harbor, but in good visibility, between professionals, because one of them read the encounter wrong or waited too long to act on what they saw. The COLREGs exist to take that ambiguity out of the meeting. They are the one body of knowledge every deck officer is expected to know cold, because they decide, before two vessels are ever close, who gives way and who holds course.
Learn them properly and a converging contact stops being a question and becomes a procedure. You know your obligation, you know the other vessel’s, and you know what a safe, early, and obvious action looks like. That confidence is the whole point. The sea has no lanes painted on it and no signals hung over it, so the rules have to live in the heads of the people on watch.
One Ruleset, Every Sea
The COLREGs, formally the Convention on the International Regulations for Preventing Collisions at Sea, 1972, are published by the International Maritime Organization and apply to every vessel on the high seas and on all waters connected to them. They were adopted in 1972, entered into force in 1977, and replaced the older 1960 rules. British officers and exam papers often call the same text the IRPCS, and other nations have their own initials for it, but it is one IMO instrument and the substance is identical worldwide.
The convention is built from 41 rules grouped into six parts, A through F, supported by four technical annexes covering the positioning of lights and shapes, extra signals for fishing vessels, the construction of sound appliances, and the list of distress signals. That structure matters more than it first appears, because the rules are written to be read in order. The early rules set duties that apply to every vessel all the time, and the later rules only tell you who turns once those foundations are in place.
One caution before you rely on them anywhere near land. Coastal states may make local rules for harbors, rivers, and inland waters, such as the US Inland Rules or a port’s own by-laws, and inside the relevant demarcation line those local rules take precedence. The international rules are the backbone, but always know which set of rules is in force where you are.
Part A: Who the Rules Bind (Rules 1 to 3)
Before the convention tells anyone to alter course, it establishes who is bound and what the terms mean. Rule 1 fixes the application to all vessels on the high seas and connected navigable waters. Rule 3 defines the vocabulary the rest of the text depends on, including what counts as a vessel “not under command,” one “restricted in her ability to maneuver,” and one “constrained by her draft,” distinctions that decide priority later on.
The rule to take to heart here is Rule 2, Responsibility, and it is arguably the most important sentence in the whole convention. It does two things. First, it makes clear that following a rule to the letter never excuses you from the consequences of neglecting good seamanship: nothing in the rules relieves a vessel of the ordinary practice of seamen or of the precautions a particular situation demands. Second, it allows, and in fact requires, a departure from the rules where that is necessary to avoid immediate danger. The rules are a framework for normal encounters, not a script to be obeyed into a collision. An officer who alters course “because the rule said to stand on” while a casualty develops has misunderstood Rule 2 entirely.
Part B, Section I: How You Keep the Watch (Rules 4 to 10)
Part B is the working core of collision avoidance, and its first section, Rules 4 to 10, sets the standard of conduct that applies in any condition of visibility, day or night, clear or thick. These are the habits the rest of the rules assume you already have.
Rule 5 requires a proper lookout at all times, by sight and hearing and by all available means appropriate to the conditions. In modern practice that means radar and AIS backing up the human watch, never replacing it. A target on a screen tells you a great deal, but the officer who stops looking out of the window has already lost the plot. Rule 6 then demands a safe speed, one that lets you take proper and effective action and stop within a distance appropriate to the circumstances. Safe speed is not a fixed number. It bends with traffic density, visibility, the maneuverability of your ship, and the limitations of your radar, and the rule lists those factors precisely so that “we were doing our usual passage speed” is never a defense.
Rule 7 governs how you decide whether a risk of collision actually exists, and it sets a deliberately low bar: if there is any doubt, you must assume that risk exists. The classic test is the compass bearing of an approaching vessel. If that bearing does not appreciably change as the range closes, a risk of collision exists, the situation seamen shorthand as a constant bearing and decreasing range. Rule 8 then defines what your response must look like. Action to avoid collision must be positive, made in ample time, and large enough to be readily apparent to the other vessel, whether by eye or by radar. A succession of small alterations is exactly what Rule 8 warns against, because it leaves the other officer unsure of your intentions at the worst possible moment.
The section closes with two rules about confined water. Rule 9 requires a vessel in a narrow channel or fairway to keep as near to the outer limit on her starboard side as is safe and practicable, and it restrains smaller and less-constrained vessels: a vessel under 20 meters, a sailing vessel, or a vessel engaged in fishing must not impede the passage of a vessel that can safely navigate only within the channel. Rule 10 covers traffic separation schemes, the marine equivalent of a divided highway, first established in the Dover Strait and now mandatory wherever they are charted. You join and leave at the ends where practicable, proceed in the established direction, and keep clear of the separation zone, and small craft stay out of the way of the through traffic the scheme is built to protect.
Part B, Section II: Who Gives Way (Rules 11 to 18)
When two vessels are in sight of one another, the second section of Part B decides the encounter. This is the part most people mean when they say “the rules of the road,” and it rewards precise understanding rather than rote memory.
Three geometries cover most meetings. In a head-on situation, Rule 14 has both power-driven vessels alter course to starboard so they pass port side to port side, the predictable, symmetric action that removes any guesswork. In a crossing situation, Rule 15 gives the duty to the vessel that has the other on her own starboard side: she is the give-way vessel and must keep out of the way, avoiding crossing ahead of the other. Overtaking, Rule 13, overrides everything else. Any vessel coming up on another from more than 22.5 degrees abaft her beam is overtaking, must keep clear until finally past and clear, and cannot shed that duty simply because the geometry later changes. If you are ever in doubt whether you are overtaking, the rule tells you to assume that you are and act accordingly.
The pair of rules officers most often misapply are Rules 16 and 17, the give-way and stand-on rules. The give-way vessel must take early and substantial action to keep well clear. The stand-on vessel keeps her course and speed, so that the give-way vessel has a stable picture to maneuver against, but “stand-on” is not a right of way to be defended to the last second. Rule 17 permits the stand-on vessel to take action as soon as it becomes apparent the other is not keeping clear, and it requires her to act once collision cannot be avoided by the give-way vessel alone. The officer who stands on rigidly while a give-way vessel does nothing has turned a safe rule into a dangerous one.
Rule 18 then ranks vessels by how freely they can maneuver, because not every ship can simply turn. In broad terms a power-driven vessel keeps out of the way of a sailing vessel, a sailing vessel keeps clear of one engaged in fishing, and all of them give way to a vessel constrained by her draft, one restricted in her ability to maneuver, and one not under command, in that ascending order of priority. The hierarchy is not about status or courtesy. It is a plain acknowledgment that a deep-draft tanker in a channel or a dredger on a wire cannot do what a maneuverable ship can, so the burden of avoiding falls on the vessel that is able to bear it.
Part B, Section III: When You Cannot See (Rule 19)
Restricted visibility changes the logic entirely, and Rule 19 is the rule candidates most often confuse with the in-sight rules. When vessels are not in sight of one another in or near an area of restricted visibility, the give-way and stand-on roles of Section II do not apply. There is no stand-on vessel in fog. Every vessel proceeds at a safe speed adapted to the conditions, with engines ready for immediate maneuver, and determines risk by radar, plotting contacts early rather than reacting to shapes that loom out of the murk.
The rule also constrains the avoiding action itself, and the constraints are worth memorizing because they run against instinct. For a vessel detected by radar alone, you should avoid an alteration of course to port for a vessel forward of the beam, other than one being overtaken, and avoid an alteration toward a vessel abeam or abaft the beam. If you hear the fog signal of another vessel apparently forward of your beam, or you cannot avoid a close-quarters situation with one forward of the beam, you must reduce to bare steerageway and if necessary take all way off. Fog rewards caution and punishes confidence, and modern radar has not changed that.
Part C: Reading a Ship by Her Lights and Shapes (Rules 20 to 31)
You cannot apply any of the steering rules until you know what you are looking at, and that is the job of lights and shapes. At night a vessel’s lights tell you her type, her aspect, and her status long before you can speak to her or read her on AIS. The geometry is fixed and worth knowing by heart: sidelights show a green light to starboard and a red to port over an arc of 112.5 degrees each, a sternlight covers the 135 degrees behind her, and a masthead light covers the 225-degree forward arc, so the combination you see resolves into a heading and an aspect.
Beyond the basic underway lights, the rules prescribe distinctive signals for vessels with special duties or limitations, and these are the ones that change your obligations under Rule 18. By day the same information is carried by shapes. A single black ball forward marks a vessel at anchor; two balls in a vertical line mean not under command; a ball, a diamond, and a ball mark a vessel restricted in her ability to maneuver; a cone with its apex downward marks a sailing vessel also being propelled by machinery; and a cylinder marks a vessel constrained by her draft. Recognizing them early, while a contact is still hull-down on the horizon, is what lets you decide who must give way before the encounter develops, and it ties this part of the convention directly back to the work of the officer of the watch.
Part D: Sound and Light Signals (Rules 32 to 37)
When sight is not enough, vessels speak to each other in blasts, and precision of language matters here. The convention defines two durations exactly: a short blast lasts about one second, and a prolonged blast lasts from four to six seconds. The difference is not pedantry, because the two carry completely different meanings, and the loose habit of calling everything a “long blast” hides that.
The maneuvering and warning signals of Rule 34 use short blasts to announce action in sight of another vessel: one short blast means “I am altering my course to starboard,” two short blasts mean “I am altering my course to port,” and three short blasts mean “I am operating astern propulsion.” The signal every officer should have on reflex is the doubt or danger signal, at least five short and rapid blasts, used when you cannot understand the intentions or actions of another vessel, or doubt whether she is taking enough action to avoid collision. The fog signals of Rule 35, by contrast, are built from prolonged blasts and identify a vessel and her state in restricted visibility, while Rules 36 and 37 cover signals to attract attention and the distress signals listed in Annex IV.
Parts E and F: Exemptions and Compliance (Rules 38 to 41)
The last two parts are short and rarely tested on the bridge, but they round out the picture. Part E, Rule 38, grants older vessels built to the 1960 regulations limited exemptions from certain light and sound requirements for specified periods, a transitional provision now largely of historical interest. Part F, made up of Rules 39 to 41 and brought in around 2016, ties the convention into the IMO Member State Audit Scheme, providing definitions and the framework for verifying that flag states actually implement what they have signed up to. Neither part changes how you handle a meeting, but they explain why the modern rule count is 41 rather than the original 38.
Rule 2 Again: The Spirit Behind the Letter
Run back through the convention and a pattern emerges. The rules tell you to keep a lookout, judge risk early, act in good time, and make your actions obvious, and only then do they tell you who turns. That ordering is deliberate, because the steering rules only work when the seamanship beneath them is sound. Most collisions are not caused by an officer who did not know Rule 15. They are caused by a poor lookout, a bearing that was never properly checked, a small alteration that the other vessel never saw, or a stand-on vessel that held its course long past the point of safety.
This is why Rule 2 bookends everything. The COLREGs are a framework for judgment, not a substitute for it, and the officer they reward is the one who is predictable to others and never lets a rule talk them out of an obvious safe action. Know the rules well enough that applying them is automatic, and you free up the attention to do the part no rule can do for you: watch, think, and act before the situation makes the decision for you. You can go deeper into individual encounters and signals across the Rules and Regulations section, but it all rests on this foundation.
Frequently Asked Questions
How many rules are in the COLREGs? There are 41 rules, arranged in six parts (A to F) and supported by four annexes. The convention originally had 38 rules; Part F and Rules 39 to 41 were added later to support the IMO Member State Audit Scheme, which is why you will still see older references to 38 rules.
Who has to follow the COLREGs? Every vessel on the high seas and on all waters connected to them and navigable by seagoing vessels. Coastal states can issue local rules for harbors, rivers, and inland waters, and inside the relevant demarcation line those local rules apply, so always know which ruleset governs where you are.
What is the difference between the give-way vessel and the stand-on vessel? The give-way vessel must take early and substantial action to keep clear. The stand-on vessel keeps her course and speed so the other has a stable picture to maneuver against. Stand-on is not an unconditional right of way: the stand-on vessel may act when it becomes clear the give-way vessel is not keeping clear, and must act once collision cannot otherwise be avoided.
What is the danger signal under the COLREGs? At least five short and rapid blasts on the whistle, under Rule 34. It signals that you do not understand another vessel’s intentions, or doubt whether she is taking enough action to avoid a collision.
Do the give-way and stand-on rules apply in fog? No. When vessels are not in sight of one another in restricted visibility, Rule 19 applies instead, and there is no stand-on vessel. Every vessel proceeds at a safe speed and determines risk by radar, and the rule places specific limits on which way you may alter for a contact detected by radar alone.