Lights and shapes of ships: how to identify them by night and by day

Why this topic decides the COLREGs block

Knowing which ship you have in front of you —and where it is heading— just from its lights at night or its shapes by day is one of the skills most often tested in the exam, within the COLREGs block, which is eliminating. The key: do not memorise each ship separately, understand what each light says and the rest falls into place by itself.

The basic lights and their sectors

Each light covers a specific arc of the horizon, and from that you work out the aspect of the vessel.

Masthead light: white, on the centreline, visible over 225° forward. It is carried by power-driven vessels (one if under 50 m, two if 50 m or more).

Sidelights: red to port and green to starboard, 112.5° each (from ahead to 22.5° abaft the beam).

Sternlight: white, visible 135° aft.

All-round light: visible over the full 360°. Towing light: yellow, like the sternlight, placed above it.

Note that the two sidelights together cover 225° (the same arc as the masthead light) and, with the sternlight (135°), complete the 360°.

How to read the aspect by night

By combining what you see, you know where the vessel is heading:

• Both sidelights (red and green) ahead → you have her bow-on, coming towards you.
• Only the red → you see her port side (she crosses to your left); only the green → you see her starboard (she crosses to your right).
• Only the white sternlight → you see her by the stern: you are overtaking her.

The lights of each type of vessel

• Power-driven vessel under way: masthead light(s) + sidelights + sternlight.
• Sailing vessel under way: sidelights + sternlight, no masthead light. Those under 20 m may combine the three in a tricolour lantern at the masthead; optionally, two all-round lights at the masthead, red over green (not together with the tricolour).
• Fishing: two all-round lights in a vertical line → green over white if trawling, red over white if other fishing.
• Not under command (NUC): two red lights all-round in a vertical line (plus sidelights and sternlight if making way; it does not carry a masthead light).
• Restricted in ability to manoeuvre (RAM): red-white-red all-round in a vertical line.
• Constrained by her draught: the power-driven lights plus three red lights all-round in a vertical line.
• Pilot vessel on duty: white over red all-round.
• Towing: the tug adds a yellow towing light above the sternlight (and a second or third masthead light depending on the length of the tow).
• At anchor: one white all-round light (two, fore and aft, if 50 m or more).
• Aground: the anchor lights plus two red lights all-round in a vertical line.

The day shapes

By day, black shapes of seven forms are used, which the IMO chose so they are not confused at a distance. Once you know the shapes, you can read any daytime signal.

One ball: vessel at anchor (forward). Two balls: not under command. Three balls in a vertical line: aground.

A cone, point down: sailing vessel also using the engine (for COLREG purposes, a power-driven vessel).

A cylinder: vessel constrained by her draught.

Ball-diamond-ball: restricted in ability to manoeuvre.

Two cones joined at their points (a bicone): vessel engaged in fishing.

A diamond: a tow longer than 200 m (shown by both tug and tow).

(And the ALFA flag —white and blue— for a vessel with a diver down.)

Tips to memorise them

• Keep the arcs in mind: masthead 225°, sidelights 112.5° each, stern 135° (together, 360°).
• Red = trouble: two red → not under command; red-white-red → restricted in ability to manoeuvre; three red → constrained by draught.
• Fishing: green over white = trawling; red over white = other fishing; by day, a bicone.
• Balls = stationary: one at anchor, two not under command, three aground.
• The sailing vessel carries no masthead light; if you see a masthead light, it is power-driven.

Frequently asked questions