COLREGS Guides

After dark you cannot see a boat, only its lights. The Navigation Rules turn those lights into a code: the color, the number, the arrangement and the angle from which you see them tell you what kind of vessel it is, which way it is pointing, and whether it is the vessel that has to keep clear. This guide walks through the code and gives you a step-by-step way to read it.

Start with the building blocks

Rule 21 defines the handful of lights everything else is made from. Each one shines over a fixed arc, which is what lets you work out a vessel’s heading.

The arcs are built to fit together. The masthead light’s forward arc matches the two sidelights, so a boat coming straight at you shows white, red and green together. Sidelights and sternlight cover the whole circle between them with no dark gap.

Seeing which way a boat is heading

Because red is always on a vessel’s port side and green on her starboard side, the color you see tells you her aspect:

• White, red and green together: she is heading almost straight at you. If you are both power-driven vessels this is the head-on situation, and each turns to starboard (Rule 14).
• Red sidelight (with her masthead light): you are looking at her port side, which means she is crossing in front of you from your starboard side. You have her on your starboard side, so in a crossing situation between power-driven vessels you are the give-way vessel and should avoid crossing ahead of her (Rule 15). The manual puts it the same way: another vessel’s red sidelight off your own starboard bow means she has the right of way.
• Green sidelight (with her masthead light): you are looking at her starboard side, so she is crossing from your port side and has you on her starboard side. She is the give-way vessel; you are the stand-on vessel and should hold your course and speed unless it becomes clear she is not giving way (Rule 17).
• Only a white sternlight: you are behind her, within the arc from which a vessel is considered to be overtaking. Rule 13 says a vessel coming up from more than 22.5 degrees abaft the other’s beam, where she could see only the sternlight and neither sidelight, is overtaking and must keep clear.

Reading the lights, vessel by vessel

Power-driven vessels underway (Rule 23)

A masthead light forward, sidelights and a sternlight. A second, higher masthead light abaft the first is required at 50 meters and above and optional below that, so a large ship shows two white masthead lights, the rear one higher. Watching how those two lights line up over time is how experienced lookouts judge a ship’s aspect at night. Small boats have options: under 12 meters, an all-round white light plus sidelights may replace the full set, and on the Great Lakes an all-round white light may replace the second masthead light and the sternlight.

Sailing vessels (Rule 25)

Under sail alone: sidelights and a sternlight, and no masthead light. That missing masthead light is what tells you she is not power-driven. Under 20 meters, the sidelights and sternlight may be combined in one lantern carried at or near the top of the mast. A sailing vessel may also show a red-over-green pair of all-round lights at the masthead, but not together with that combined lantern. The moment she starts her engine she becomes a power-driven vessel and must show a masthead light; by day, a sailboat of 12 meters or more that is motoring with her sails up shows a cone, point down, forward.

Vessels towing (Rule 24)

A towing vessel shows two masthead lights in a vertical line (three if the tow is longer than 200 meters), plus sidelights, a sternlight, and a yellow towing light above the sternlight. The tow itself shows sidelights and a sternlight. The real danger is between them: the towline can run for hundreds of feet under tension, nearly invisible, and can capsize or dismast a small boat that crosses it. Never cut between a tug’s lights and her tow’s lights.

Fishing vessels (Rule 26)

Two all-round lights in a vertical line. Green over white means trawling; red over white means fishing with other gear. If gear extends more than 150 meters out from the vessel, an extra all-round white light points toward it. Sidelights and a sternlight are added when she is making way through the water. Remember that the light marks only the end of the line: the net or line between the boat and the marker light is usually invisible at night.

Vessels that cannot get out of your way (Rule 27)

• Two red all-round lights, one above the other: a vessel not under command. Something has broken; she cannot maneuver as the rules require. Treat her like a fixed obstruction with a wide margin (Rule 27(a)).
• Red, white, red in a vertical line: a vessel restricted in her ability to maneuver because of her work: dredging, cable laying, diving, or an unwieldy tow (Rule 27(b), Rule 27(c)).
• Add two reds on one side and two greens on the other: a dredge whose work blocks part of the channel. Red marks the side that is obstructed; green marks the side that is clear (Rule 27(d)).
• Three all-round green lights: mine clearance. Stay at least 1,000 meters away (Rule 27(f)).
• A small dive boat may show red-white-red plus a rigid replica of the International Code flag “A,” at least 1 meter tall (Rule 27(e)). If you see it, divers are in the water nearby: slow down and keep well clear.

Vessels under 12 meters are not required to show these signals unless they are diving (Rule 27(g)), so the absence of red-white-red on a small boat does not prove she is not restricted. And none of these lights is a distress signal; distress has its own signals in Annex IV.

Other vessels you may meet

• White over red near the masthead: a pilot vessel on duty (Rule 29). She may be about to come alongside a moving ship; do not drift in for a closer look.
• Three red lights or a cylinder (International only): a deep-draft vessel constrained by her draft. This signal is optional, so her absence of red lights does not prove she is free to leave the channel (Rule 28).

Anchored and aground (Rule 30)

At anchor: an all-round white light forward and a lower one near the stern, or a single all-round white light if she is under 50 meters. Aground adds two all-round red lights in a vertical line, or three balls by day. An anchor light says a boat chose to be there; the two reds say the water chose for her, and whatever put her aground is probably still under your own hull if you close the distance. Under 7 meters, anchored away from channels, a boat needs no light at all, so watch for unlit small craft near anchorages.

A five-step method for a light you do not recognize

1. Count and color. How many lights, what colors, and are they stacked vertically? Vertical stacks of all-round lights (red-red, red-white-red, green-white, red-white, three greens, white-red) are the special-status signals above.
2. Find the sidelights. A red or a green tells you her aspect. Both at once means she is head-on to you.
3. Look for the masthead light. One or two white masthead lights together with sidelights means power-driven; sidelights with none means sail (or a tow being towed).
4. Watch the bearing. If the compass bearing to her is not changing while the range closes, risk of collision exists (Rule 7). Do not wait for the lights to look dangerous.
5. Decide what you owe. Give way early and obviously if you are the give-way vessel (Rule 16, Rule 8). If you are stand-on, hold course and speed until it is clear she is not giving way (Rule 17).

Lights, range and your own boat

Minimum visibility ranges scale with the size of the vessel (Rule 22): a vessel under 12 meters need only show a masthead light for 2 miles and sidelights for 1 mile, while a vessel of 50 meters or more must show a masthead light for 6 miles and sidelights for 3. So a fully compliant recreational boat can still go unnoticed from a ship’s bridge until surprisingly late. That is a reason to keep your own lights clean and unobstructed, and not to assume you have been seen.

Keep your own lighting honest, too. From sunset to sunrise, other lights on your vessel must not be confusable with the required ones or wash out their visibility (Rule 20). A spotlight left on or party lights strung around the cockpit are not just tacky; they can make another boat misjudge what you are and which way you are heading. On inland waters you may lower your navigation lights if that is necessary to pass under a bridge.

Common mistakes

• Reading the sidelights the wrong way around. Red is port, green is starboard, always. Say it out loud: red on my starboard bow means I give way.
• Assuming sidelights alone mean a sailboat. A sailboat under power must show a masthead light and is a power-driven vessel. Check for the masthead light, not just the sails.
• Assuming a small boat with plain lights is unrestricted. Vessels under 12 meters are exempt from the special signals unless diving.
• Steering for the gap between two lights. A tug and tow, or a boat and her fishing gear, may be joined by something you cannot see.
• Treating the absence of a light as proof of nothing there. Some signals are optional (draft-constrained), and some vessels are exempt by size.