Lights and Shapes: Basic Rules, Arcs and Ranges

When lights and shapes are shown — Rule 20

In short: Read COLREG Structure, Application and Definitions first: it defines the vessel types that each light belongs to. Lights are for sunset to sunrise and for restricted visibility. Shapes are for daytime. Part C (Rules 20–31) applies in all weathers.

• COLREG Rule 20(a) — the rules of Part C apply in all weathers.
• COLREG Rule 20(b) — lights are shown from sunset to sunrise. In that time, show no other lights unless they cannot be mistaken for COLREG lights, do not impair their visibility or distinctive character, and do not interfere with a proper look-out. Bright deck lights that hide your sidelights break this rule.
• COLREG Rule 20(c) — the same lights are also shown from sunrise to sunset in restricted visibility, and in any other case where you judge it necessary. On the bridge: fog closes in at 1400 → navigation lights on (see Conduct in Restricted Visibility).
• COLREG Rule 20(d) — the rules on shapes apply by day.
• COLREG Rule 20(e) — lights and shapes must meet the specifications of Annex I (position, spacing, intensity, size).

Shapes: the basics (Annex I, section 6)

• Shapes are black.
• Ball: diameter at least 0.6 m.
• Cone: base diameter at least 0.6 m, height equal to the diameter.
• Cylinder: diameter at least 0.6 m, height twice the diameter.
• Diamond: two cones joined at the base.
• Vertical distance between shapes: at least 1.5 m.
• Vessels under 20 m may use smaller shapes, in proportion to their size, with the spacing reduced to match.
• Which vessel shows which shape is the subject of the next topic (special vessels).

The six lights: colour and arc — Rule 21

In short: COLREG Rule 21 defines six lights. Learn three numbers: 225° masthead, 112.5° each sidelight, 135° sternlight. The border is always 22.5° abaft the beam. Masthead (225°) + sternlight (135°) = 360°: the whole horizon is covered.

Masthead light — Rule 21(a)

• White, placed over the fore-and-aft centreline.
• Unbroken arc of 225°: from right ahead to 22.5° abaft the beam on each side.
• Required for power-driven vessels and for vessels towing (who show extra masthead lights).
• Range: 6 NM (50 m and over) · 5 NM (12 m to under 50 m; 3 NM if under 20 m) · 2 NM (under 12 m) — COLREG Rule 22.

Sidelights — Rule 21(b)

• Green on the starboard side, red on the port side, each on its own side.
• Each covers 112.5°: from right ahead to 22.5° abaft the beam on its side. Together they cover the same arc as the masthead light (2 × 112.5° = 225°).
• They tell you the other vessel’s aspect: green = you see her starboard side; red = her port side; both = she is heading straight at you.
• Vessels under 20 m may combine both in one lantern on the centreline.
• Range: 3 NM · 2 NM · 1 NM (COLREG Rule 22).

Sternlight — Rule 21(c)

• White, as nearly as practicable at the stern.
• Arc of 135°: 67.5° each side of right aft. It starts exactly where the sidelights end.
• It marks the stern. Link with Overtaking, Head-on and Crossing Situations: a vessel coming up from more than 22.5° abaft the beam is overtaking (COLREG Rule 13). At night she sees only the sternlight and neither sidelight.
• Range: 3 NM · 2 NM · 2 NM (COLREG Rule 22).

Towing light — Rule 21(d)

• Yellow, with the same characteristics as the sternlight: 135°, 67.5° each side of right aft.
• A vessel towing astern shows it in a vertical line above the sternlight (COLREG Rule 24(a)(iv)).
• It tells you the vessel is towing: don’t cross astern of her — there is a tow behind.
• Range: 3 NM · 2 NM · 2 NM (COLREG Rule 22).

All-round light — Rule 21(e)

• Unbroken arc of 360°. Position depends on the rule that uses it.
• Colour depends on its job: e.g. white for a vessel at anchor, red–white–red for a vessel restricted in her ability to manoeuvre.
• It shows a status or operation (anchored, NUC, RAM, fishing…), not the aspect.
• Range: 3 NM · 2 NM · 2 NM, for white, red, green or yellow (COLREG Rule 22).

Flashing light — Rule 21(f)

• A light flashing at regular intervals at 120 flashes or more per minute, usually all-round (360°).
• Rule 21(f) gives only the rate, not a colour or a range: these come from the rule that uses it. Some summaries call it a white light by default; the two uses in Rule 23 are coloured:
  • air-cushion vessel in non-displacement mode: all-round flashing yellow (COLREG Rule 23(b));
  • WIG craft taking off, landing or in flight near the surface: high-intensity all-round flashing red (COLREG Rule 23(c)).
• It flags a special condition or operation.

Minimum ranges of visibility — Rule 22

In short: COLREG Rule 22 sets the minimum range of each light by vessel length: 50 m and over, 12 m to under 50 m, under 12 m. Big ship: 6 – 3 – 3 – 3 – 3. Middle: 5 – 2 – 2 – 2 – 2. Small: 2 – 1 – 2 – 2 – 2 (masthead – side – stern – towing – all-round).

• The masthead light always reaches furthest: you see it before the sidelights. At 4 NM from a 100 m ship you may see her masthead lights (6 NM) but not yet her sidelights (3 NM).
• The only 1 NM figure in the table is the sidelight under 12 m. Every other light on a small vessel is 2 NM. ❌ A common slip is to give the all-round light under 12 m as 1 NM: it is 2 NM (COLREG Rule 22(c)).
• The 12 m to under 50 m band has one exception: masthead 5 NM, but 3 NM if under 20 m (COLREG Rule 22(b)).
• Inconspicuous, partly submerged vessels or objects being towed: white all-round light 3 NM (COLREG Rule 22(d)).
• These are minimum ranges on a dark night with clear air. Intensity specs are in Annex I. Haze, rain or background shore lights cut the real range.

What shows one white light?

In short: A single white light at night can be many things. Run through the list, then watch the bearing, use binoculars and the radar before you decide what she is.

• Sternlight of a power-driven vessel seen from astern (COLREG Rule 23).
• Sternlight of a sailing vessel seen from astern (COLREG Rule 25).
• Sternlight of a vessel being towed (COLREG Rule 24(e)).
• Power-driven vessel under 7 m whose maximum speed is not more than 7 kn: may show one all-round white light, with sidelights if practicable (COLREG Rule 23(d)(ii)). Both conditions must be met.
• Sailing vessel under 7 m: if she doesn’t show sidelights and sternlight, she keeps a torch or lantern with a white light ready and shows it in time to prevent collision (COLREG Rule 25(d)(i)).
• Vessel under oars: same torch or lantern (COLREG Rule 25(d)(ii)).
• Vessel at anchor under 50 m: one all-round white light where it can best be seen (COLREG Rule 30(b)).
• A masthead light alone: e.g. a power-driven vessel under 50 m seen from ahead, beyond the range of her sidelights.
• Dracone being towed: needs no light at the forward end, so you may see only the white light at its after end (COLREG Rule 24(g)).
• ❌ A liferaft light (it flashes) and a lighthouse are not on this list: they are not COLREG vessel lights.