Collision Regulations (COLREGs)
The rules of the road at sea — and the only module rated Complete depth alongside buoyage and lights: thorough knowledge, no hesitation. The IMO's International Regulations for Preventing Collisions at Sea cover every vessel on every connected water. Here: the structure and definitions, the foundation rules (lookout, safe speed, risk of collision), every give-way relationship with an interactive trainer, restricted-visibility conduct, an introduction to lights and day shapes, and the sound signals.
Structure of the Rules & the Definitions That Drive Them
The COLREGs are 38 rules in five parts: A — General, B — Steering & Sailing Rules, C — Lights & Shapes, D — Sound & Light Signals, E — Exemptions. They apply to all vessels, regardless of size, on the high seas and all connected navigable waters. Local authorities may run special rules inland (Europe's CEVNI; the US Inland Navigation Rules, taking over inside the charted COLREGS DEMARCATION LINE) — but those must stay as close to the COLREGs as possible, and US/Canadian drafters deliberately minimised differences on the Great Lakes.
The definitions matter because the type of vessel dictates the action:
| Term | Means |
|---|---|
| Vessel | Every watercraft capable of being used as water transportation — seaplanes included. |
| Power-driven vessel | Any vessel propelled by machinery. |
| Sailing vessel | Under sail and not using propelling machinery — engine on means power-driven, whatever the sails say. |
| Vessel engaged in fishing | Fishing with nets, lines, trawls or gear that restricts manoeuvrability — trolling lines don't count. |
| Not under command (NUC) | Exceptional circumstance (steering failure, etc.) — unable to manoeuvre as the rules require. |
| Restricted in ability to manoeuvre (RAM) | The nature of her work restricts her — dredging, surveying, cable-laying, towing. |
| Constrained by her draught (CBD) | A power vessel that can barely deviate from her track because of depth and channel width. |
| Underway | Not at anchor, not made fast to the shore, not aground. (No motion required — drifting is underway.) |
| Restricted visibility | Fog, mist, falling snow, heavy rain, sandstorms — any similar cause. |
The Foundation Rules — 5 to 10
Every vessel, at all times: a proper lookout by sight and hearing and every other available means, to fully appraise the situation and the risk of collision. Most collisions trace back here.
Always a speed from which you can take proper, effective avoiding action and stop in a safe distance — judged against visibility, traffic, manoeuvrability, background lights at night, wind, sea, current and nearby hazards.
Use every available means to decide. The classic test: an approaching vessel whose compass bearing doesn't appreciably change is on a collision course. Big ships and tows can hit you even with a changing bearing at close range. In doubt? The risk exists. Never judge on scanty information — especially scanty radar.
Positive, in ample time, good seamanship: course/speed changes large enough to be obvious to the other vessel by eye or radar. No nibbling successions of small alterations.
Keep as close as practical to the starboard side. Vessels under 20 m, sailing vessels and fishing vessels must not impede a vessel that can only navigate inside the channel.
The divided highways of shipping: use the correct lane with the flow, keep clear of the separation zone, join/leave at the ends or at a shallow angle, and cross — only if you must — heading as near 90° as practicable. Under-20 m, sailing and fishing vessels may use the inshore zone; don't anchor in or near a scheme; fishing vessels must not impede lane traffic.
Who Gives Way? — Scenario Trainer
Rules 11–18 sort out every meeting between vessels in sight of one another. In any meeting where there's a risk of collision, one vessel is told to keep out of the way (the give-way vessel) and the other to hold her course (the stand-on vessel). Which is which depends on the type of meeting. There are four to know:
Picture each boat's helmsman looking out. Whoever has the other vessel on their own starboard (right) side gives way — and does so by altering early, ideally passing behind the other, not across her bow. The vessel that has the other on her port side stands on.
Meeting nearly bow-to-bow, both vessels turn to starboard and pass port side to port side. This is the one meeting with no single stand-on vessel — each acts.
Any vessel catching up another from more than 22.5° abaft the beam (far enough back that at night she'd see only the other's white sternlight) is overtaking, and must keep clear — until fully past. This overrides everything else: even a power vessel overtaking a sailing vessel keeps clear. In doubt, assume you're overtaking.
Decided by the tack (the side the wind is coming from). On different tacks, the boat with the wind on her port side keeps clear. On the same tack, the windward (upwind) boat keeps clear of the leeward (downwind) one. The windward side is the side opposite the mainsail.
Now put it into practice. You are the blue boat — read each situation and make the call:
Give-Way & Stand-On Duties, and the Rule 18 Hierarchy
When risk of collision exists, the rules assign roles — and both vessels get duties, not just one:
Take early and substantial action to keep well clear. Early means the other bridge can see you doing it; substantial means no ambiguity.
1. Hold course and speed. 2. May act the moment it's apparent the give-way vessel isn't doing her job (avoid turning port for a vessel on your port side). 3. Must act when collision can no longer be avoided by the give-way vessel alone.
Rule 18 — the order of priority. A vessel higher in the list stands on; a vessel lower in the list keeps clear:
Power keeps clear of sail, fishing, RAM and NUC. Sail keeps clear of fishing, RAM and NUC. Fishing keeps clear (so far as possible) of RAM and NUC. And everyone who can, avoids impeding a vessel constrained by her draught. Two loopholes students forget: an overtaking vessel always keeps clear regardless of rank, and a sailing vessel motor-sailing is just a power vessel.
Rule 19 — Conduct in Restricted Visibility
Rule 19 applies to vessels not in sight of one another in or near restricted visibility — and it rewrites the game: there is no stand-on vessel in fog. Every vessel keeps clear of every other. Safe speed adapted to the conditions; power vessels with engines ready for immediate manoeuvre.
Determine early whether a close-quarters situation is developing and act in ample time — avoiding, so far as possible:
- an alteration to port for a vessel forward of the beam (unless overtaking her)
- an alteration toward a vessel abeam or abaft the beam
Unless you've positively established there's no risk:
- reduce speed to the minimum that holds steerage
- if necessary, take all way off
- navigate with extreme caution until the danger is past
Lights & Day Shapes
Night lights (white, red, green, yellow) encode a vessel's direction, propulsion and size — plus special states: towing, fishing, NUC, RAM, constrained by draught, aground, at anchor. Reading them is a sequence, not a guess: 1) aspect (ahead / astern / port / starboard), 2) propulsion, 3) length, 4) anything special.
- Sailing vessel underway: green starboard sidelight, red port sidelight, white sternlight. Under 20 m she may combine all three in one masthead tricolour — never lit while motoring
- Power <50 m: the same, plus a white masthead light (covering the sidelights' combined arc)
- Power ≥50 m: a second masthead light, aft and higher
- Small boats: often one all-round white on a stern pole, mounted above the structure
- <50 m: one all-round white light where best seen
- ≥50 m: forward all-round white plus a second, lower, near the stern
- >100 m: deck illumination as well
Sound Signals & Required Equipment
Manoeuvring signals (vessels in sight, power-driven, manoeuvring as authorised or required by the rules) — short blast ≈ 1 second, prolonged ≈ 4–6 seconds:
| Signal | Meaning |
|---|---|
| ● one short | I am altering course to starboard |
| ● ● two short | I am altering course to port |
| ● ● ● three short | My engines are going astern |
| ▬ ▬ ● two prolonged + one short | (Narrow channel) I intend to overtake on your starboard side |
| ▬ ▬ ● ● two prolonged + two short | (Narrow channel) I intend to overtake on your port side |
| ● ● ● ● ● five or more short, rapid | I do not understand your intentions, or I doubt you are taking enough action to avoid collision |
- ≥12 m: a whistle
- ≥20 m: a bell as well
- ≥100 m: plus a gong, distinct from the bell in tone
- <12 m: not obliged to carry the above — but must have some means of making an efficient sound signal
A white-and-blue swallowtail: diver down — keep well clear at slow speed. It's the day signal every charter crew in the islands needs on sight recognition.
Collision Regulations — Knowledge Check
12 questions at IYT exam standard (75% to pass), drawn from the IYT Module 22 knowledge review. DOK Complete — this one you should be able to run without hesitation.