COLREGs lights: a method for recognising vessels at night

The building blocks

Rule 21 defines the lights. Learn these first and the rest follows.

• Masthead light: white, shown over the fore and aft centreline, covering an arc of 225 degrees, from right ahead to 22.5 degrees abaft the beam on each side.
• Sidelights: a green light on the starboard side and a red light on the port side, each covering 112.5 degrees, from right ahead to 22.5 degrees abaft the beam on its own side.
• Sternlight: white, covering 135 degrees, 67.5 degrees from right aft on each side.
• Towing light: yellow, with the same characteristics as the sternlight.
• All-round light: a light showing over an arc of 360 degrees.
• Flashing light: a light flashing at regular intervals at a frequency of 120 flashes or more per minute.

Reading aspect from the lights

The arcs mean that the combination you see changes with the vessel’s heading. A power-driven vessel seen from ahead shows both sidelights and a masthead light, with two masthead lights in line if she is large. Seen from the red side only the red light and masthead light are visible, and from astern you see only the white sternlight. That is how you work out which way a vessel is heading and whether you hold her on your starboard or port side.

Practise this with a small model or a sketch: put a ship on a sheet of paper, mark the arcs, and rotate it to see which lights an observer in different positions would see.

Power-driven vessels underway

Under Rule 23, a power-driven vessel underway shows a masthead light forward, a second masthead light abaft of and higher than the forward one if she is 50 metres or more in length, sidelights and a sternlight. A vessel of less than 50 metres is not obliged to show the second masthead light but may do so.

Towing and pushing

Under Rule 24, a power-driven vessel towing shows, in place of the forward masthead light, two masthead lights in a vertical line, or three when the length of the tow exceeds 200 metres, together with sidelights, a sternlight and a yellow towing light in a vertical line above the sternlight. When the tow exceeds 200 metres, a diamond shape is displayed where it can best be seen. Learn the rule for the vessel being towed and for pushing as separate points.

Sailing vessels and vessels engaged in fishing

Under Rule 25, a sailing vessel underway shows sidelights and a sternlight and may add, at or near the top of the mast, two all-round lights in a vertical line, the upper red and the lower green. A vessel under sail and under power at the same time shows a cone, apex downwards, in the day.

Under Rule 26, a vessel engaged in fishing shows two all-round lights in a vertical line: the upper green and the lower white for trawling, and the upper red and the lower white for fishing other than trawling. Sidelights and a sternlight are shown only when making way through the water.

Status lights: the vertical combinations

Vertical combinations of all-round lights tell you the vessel’s status, and they are among the most important to know.

• Vessel not under command (Rule 27): two all-round red lights in a vertical line, and two balls by day. Sidelights and a sternlight are added when making way.
• Vessel restricted in her ability to manoeuvre (Rule 27): three all-round lights in a vertical line, red at top and bottom and white in the middle, with a ball, diamond and ball by day.
• Vessel constrained by her draught (Rule 28): three all-round red lights in a vertical line, and a cylinder by day.
• Pilot vessel on duty (Rule 29): a white all-round light over a red all-round light at the masthead.

Vessels at anchor

Under Rule 30, a vessel at anchor shows an all-round white light in the forward part and, if she is 50 metres or more in length, a second all-round white light at or near the stern and lower than the forward one. By day she shows one ball in the forward part. Vessels aground show the anchor lights and two all-round red lights in a vertical line, and three balls by day.

A method for any arrangement

1. Look for all-round lights in a vertical line. Their colours give the vessel’s status. Say what they mean.
2. Look for masthead lights. One means a power-driven vessel under 50 metres or a smaller vessel. Two in a line, with the aft one higher, means a larger power-driven vessel. Two or three in a vertical line may mean towing.
3. Look for sidelights and sternlights. Which of the two sidelights you can see tells you her aspect and heading.
4. Look for a yellow light above the sternlight, which indicates towing.
5. Finish by stating what it means for you: which rule applies, whether you give way or stand on, and what you would do.

Check the current text

This guide summarises the principal arrangements. The regulations also deal with vessels of different sizes, the exceptions for small vessels, additional signals for special cases and shapes for daytime. Always check the exact requirements in the current official text and in Marine Order 30.

Official sources

This guide was written from the following sources. Requirements change, so confirm the current position with the source before relying on it.

• IMO: COLREG, Convention on the International Regulations for Preventing Collisions at Sea
• AMSA: Marine Order 30 (Prevention of collisions)

Related certificate guides

• AMSA OOW (Watchkeeper Deck)
• Master Unlimited
• Chief Mate Unlimited
• Watchkeeper Deck <500 GT
• AMSA oral exam preparation guide

Keep reading

• COLREGs: a method for reasoning through any situation
• OOW and Watchkeeper Deck oral exam: the themes behind the questions
• Maritime glossary for plain-language definitions of the terms used here.
• More in COLREGs, or browse the knowledge centre.