Most collisions at sea don't happen because someone didn't know the rules. They happen because two skippers each assumed the other one did, and both waited too long. The COLREGs (International Regulations for Preventing Collisions at Sea, 1972) are the common language that stops that guessing game. They apply to every vessel on the high seas and on waters connected to them, from a 6-metre day sailer to a 300,000-tonne bulk carrier. Knowing them cold is not optional, and knowing them half-way is arguably worse than not knowing them at all.

What follows is a working refresher, not a legal citation. If you sit an exam, learn the rules verbatim. If you take a boat out this weekend, this is what you actually need in your head when a fishing vessel appears on a converging bearing at dusk.

The pecking order nobody tells you about clearly

The rules assume vessels have different levels of manoeuvrability, and they rank them. This ranking (Rule 18) is the single most important thing to internalise, because it overrides most of what follows. From most privileged to least:

  1. Vessels not under command (NUC).
  2. Vessels restricted in their ability to manoeuvre (RAM), for example laying a cable or servicing a nav mark.
  3. Vessels constrained by their draught (mostly in channels).
  4. Vessels engaged in fishing (nets or lines in the water, not just a rod over the transom).
  5. Sailing vessels.
  6. Power-driven vessels.
  7. Seaplanes and WIG craft.

A sailing yacht gives way to a trawler with gear in the water. A powerboat gives way to a yacht under sail. But a yacht under sail with the engine engaged is a power-driven vessel, cone or no cone. A lot of skippers forget that last point when they're motor-sailing to hold a schedule.

Two power-driven vessels meeting

Three geometries, three rules. Learn them as pictures, not paragraphs.

Head-on (Rule 14). Both alter course to starboard, so you pass port-to-port. The test for "head-on" is generous: if you see the other vessel's masthead lights in line, or both her sidelights, treat it as head-on. When in doubt, assume it is. A half-hearted 5-degree nudge is not a course change. Make it obvious : 20 degrees minimum, early, and hold it long enough for the other skipper to see your aspect change.

Crossing (Rule 15). The vessel that has the other on her starboard side gives way. The give-way vessel must take early and substantial action (Rule 16) and avoid crossing ahead of the stand-on vessel. The stand-on vessel keeps course and speed (Rule 17), but is required to act if it becomes clear the give-way vessel isn't doing her job.

Overtaking (Rule 13). Any vessel overtaking any other, sail or power, gives way. Overtaking is defined by the light sector: if at night you'd see only the sternlight of the vessel ahead, you're overtaking. This rule sticks with you until you are past and clear, even if the geometry starts to look like a crossing situation.

Two sailing vessels meeting

Rule 12 is short and often misremembered under pressure. Three cases:

  • Different tacks: the vessel on port tack gives way to the vessel on starboard tack.
  • Same tack: the windward vessel gives way to the leeward vessel.
  • Port-tack boat sees a windward vessel and can't tell what tack she's on: give way.

"Tack" is defined by which side the mainsail is on (or the largest fore-and-aft sail on a gaffer or square rig). Wing-on-wing downwind, it's the opposite side to the boom. Racing rules are not the COLREGs. On a start line packed with 40 boats, IRPCAS still applies to the ferry crossing the course.

Judging risk of collision

Rule 7 is the one experienced skippers actually live by. Risk of collision exists if the compass bearing of an approaching vessel does not appreciably change. That is the test. Not "she looks close", not "she's fast", but: take a bearing, wait a minute, take it again. If it's constant, you're on a collision course, full stop.

Practical points:

  • Use a hand-bearing compass, or line the target up with a stanchion from a fixed helm position. Eyeballing across a rolling deck is not a bearing.
  • At close range, even a changing bearing may not clear you. Big ships have long hulls.
  • Radar and AIS give you CPA and TCPA directly. Learn to read them. A well-tuned AIS display is the fastest way to identify risk of collision at range, especially at night or in fog, but it does not replace a visual watch (Rule 5) and it does not see the unlit fishing punt half a mile ahead.

Action that actually avoids collision

Rule 8 governs how you manoeuvre. The three words that matter: early, substantial, obvious.

  • Early: the sooner you act, the smaller the course change you need, and the more time the other vessel has to interpret it. Waiting until 0.5 NM to alter for a container ship doing 18 knots is not "early".
  • Substantial: a course change should be large enough to be seen from the other vessel's bridge, ideally on radar. Rule of thumb : 30 degrees or more.
  • Obvious: alter course rather than speed where you can. A change in aspect is visually unambiguous; a slight throttle back is not.

Avoid a sequence of small alterations. And if you have sea room, a bold turn to pass astern of a crossing vessel is almost always better than trying to squeeze ahead.

Lights, shapes and sound signals you can't afford to forget

Rules 20 to 37 cover lights, shapes and sound. You don't need to recite them, but a handful must be instant recognition:

  • Power-driven vessel underway: masthead light(s), sidelights, sternlight. Over 50 metres, a second masthead light abaft and higher.
  • Sailing vessel underway: sidelights and sternlight. Optional red-over-green at the masthead. The moment the engine goes on in gear, switch to steaming light.
  • Vessel at anchor: all-round white forward, and (over 50 m) a second aft and lower. Black ball by day.
  • Trawler: green over white, all-round. Other fishing: red over white ("red over white, fishing at night").
  • NUC: two all-round reds vertically. RAM: red-white-red vertically. These are the ones that outrank you.
  • Pilot vessel: white over red.

Sound signals in restricted visibility (Rule 35) matter more than most yachtsmen practice. A power-driven vessel underway: one prolonged blast every two minutes. Sailing vessel, or any vessel restricted, fishing, towing or NUC: one prolonged followed by two short. Get a proper foghorn, know the pattern, and use it. Also learn the manoeuvring signals under Rule 34: one short = "I am altering course to starboard", two short = "to port", five or more short = "I do not understand your intentions" or "I doubt you are taking sufficient action". That last one, the five-blast, is the one you sound at the ferry that's about to run you down.

Narrow channels, TSS and the real world

Rule 9 in narrow channels: keep to the starboard side. Vessels under 20 metres, sailing vessels and fishing vessels must not impede a vessel that can only navigate within the channel. In practice, on the Solent, the Elbe, the Gironde or the approaches to Marseille, that means yachts stay outside the buoyed channel where depth allows, and cross channels at a right angle, briskly.

Rule 10 in a Traffic Separation Scheme: join and leave at the ends where possible, otherwise cross on a heading as near as practicable to right angles to the traffic flow. Not course over ground, heading. In a strong cross-tide you may be crabbing sideways at 30 degrees to your track, and that's fine. The point is to present the shortest crossing profile to the ships in the lane. Route planning around TSS zones is worth doing at the chart table before you leave, along with the rest of your passage planning and a proper look at the chart for hazards along your track.

When it still goes wrong

Rule 2, the "rule of good seamanship", is the one that catches you if you followed rules 3 to 37 to the letter and still hit something. If departing from the rules is necessary to avoid immediate danger, do it, and be prepared to justify why. If a collision does happen, your priorities are lives first, the other vessel second, evidence third. That means a clean post-collision procedure : accounted crew, damage assessment, distress call if needed on the correctly configured VHF, and a factual log of times, positions, courses and speeds.

That last item, the factual log, is where most skippers come up short. Memory after an incident is unreliable, and paperwork is the first thing to get wet. A continuous, time-stamped record of position, heading, SOG and engine state, held automatically in the background, turns a "he said, she said" into a defensible account. The Oria Box logs your NMEA 2000 data continuously and replays voyages on the Oria platform, which is one of the quieter reasons owners fit one. The bigger reason is that most of your COLREGs decisions get better when you review them afterwards with the data in front of you. What was your bearing on that trawler when you finally altered? How much sea room did you actually have? The rules are the theory. The replay is where you learn.