COLREG Rule 23 Explained — Lights for Power-Driven Vessels Underway

A plain-language guide to COLREG Rule 23: exactly which navigation lights a power-driven vessel underway must show, why there are two masthead lights, and the simplified options for small craft, hovercraft and WIG craft.

What does Rule 23 actually say?

COLREG Rule 23 — formally "Power-driven vessels underway" — defines the lights and (by reference) the night-time identity of the most common vessel you'll meet at sea: a vessel being propelled by machinery. It sits at the start of the "lights and shapes" block of the COLREGs (Rules 20–31), and almost every other lights rule is described as a variation on Rule 23.

Getting Rule 23 right matters for two reasons. First, you must show the correct lights yourself so others can identify and avoid you. Second — and this is where most collisions-in-the-making start — you must correctly read another vessel's lights to know what type of vessel she is, which then tells you who gives way under Rule 18.

The standard light arrangement

Rule 23(a): a power-driven vessel underway shall exhibit —

• (i) a masthead light forward — a white light shining ahead and to each side over an arc of 225°;
• (ii) a second masthead light abaft of and higher than the forward one — compulsory for vessels of 50 m or more; a vessel under 50 m may show it but is not obliged to;
• (iii) sidelights — a red light to port and a green light to starboard, each covering 112.5° from dead ahead to just abaft the beam;
• (iv) a sternlight — a white light shining astern over 135°.

Put together, the sidelights (2 × 112.5° = 225°) and the sternlight (135°) cover the full 360° around the vessel, while the masthead light(s) repeat the forward 225°. That overlap is deliberate: it lets an observer work out the vessel's aspect — whether she is end-on, crossing, or showing you her stern.

Why two masthead lights?

The second masthead light is not decoration. Because it is mounted aft of and higher than the forward one, the two lights give you a powerful cue about a vessel's heading at night and at distance:

• When the two white masthead lights are vertically in line (one directly above the other), the vessel is pointing straight at you — end-on.
• When they are well separated horizontally, you are seeing her broadside — a crossing aspect.
• The direction of the offset (which light is to the left or right of the other) tells you which way she is heading.

This is why large vessels are required to carry both lights: at the ranges supertankers and container ships are detected, the two-masthead cue is often clearer than the sidelights, which may not yet be distinguishable.

Small-craft options (under 12 m, under 7 m)

Rule 23(d) gives smaller power-driven vessels simplified options, recognising that a 6 m RIB cannot mount a proper two-tier masthead array:

Hovercraft and WIG craft

Two special power-driven vessels add a light to the Rule 23(a) set:

• Air-cushion vessel (hovercraft) in non-displacement mode — Rule 23(b) — adds an all-round flashing yellow light. A hovercraft "flying" on its cushion can move and turn unpredictably, so the flashing yellow warns other vessels to give it a wide berth.
• WIG (Wing-In-Ground) craft — Rule 23(c) — only when taking off, landing and in flight near the surface, adds a high-intensity all-round flashing red light. The rest of the time, operating on the water as a displacement craft, a WIG shows ordinary Rule 23(a) lights.

Power-driven vs sailing — reading the lights

The single most useful thing Rule 23 gives you is the ability to separate a power-driven vessel from a sailing vessel at night — because that decides the give-way order under Rule 18.

Remember: a sailing vessel that switches on her engine becomes a power-driven vessel under the COLREGs the instant the engine is propelling her — and she must then show masthead light(s) and conform to Rule 23, not Rule 25, even with sails still set. By day she should show the motoring cone (Rule 25(e)).

How Rule 23 connects to Rules 24–31

Nearly all the other lights rules build on the Rule 23 baseline:

• Rule 24 — Towing and pushing: a towing vessel replaces the single masthead light with two (or three) in a vertical line, and adds a yellow towing light.
• Rule 25 — Sailing vessels and vessels under oars: sidelights and sternlight, no masthead light; optional red-over-green at the masthead.
• Rule 26 — Fishing vessels: green-over-white (trawling) or red-over-white (other fishing), plus Rule 23-style lights when making way.
• Rule 27 — NUC and RAM: two red (NUC) or red-white-red (RAM) all-round lights, plus Rule 23 lights when making way.
• Rule 28 — Constrained by draught: three red all-round lights in addition to the Rule 23 lights.

In other words: master Rule 23 first, and the rest of the lights rules become "Rule 23 plus a signal."

Exam tips and common confusions

Rule 23 appears in Day Skipper, Coastal Skipper, Yachtmaster and STCW deck examinations — often as a night light-recognition question. Where candidates lose marks:

• Forgetting the 50 m threshold. The second masthead light is compulsory only at 50 m and above. Below that it is optional — many vessels under 50 m carry it anyway.
• Confusing "all-round white" with a masthead light. On a small craft the single all-round white light replaces both the masthead light and the sternlight — it is not a masthead light, and the vessel is still power-driven.
• Assuming no masthead light means trouble. No masthead light usually means a sailing vessel (Rule 25), not a defective power-driven one.
• Missing the hovercraft yellow flash. All-round flashing yellow = hovercraft in non-displacement mode, not a special-purpose signal.
• Treating sail-plus-engine as a sailing vessel. Under power she is a power-driven vessel and shows Rule 23 lights, sails or no sails.

Practise light recognition on the simulator

Reading lights from a book is one thing; identifying them on a moving night bridge picture at range is another. SkipperCheck's AIS / radar / COLREG bridge simulator drills exactly this — including dedicated power-driven-vessel light scenarios:

• Power-driven vessel lights — the Rule 23(a) baseline
• Power-Driven vs Sailing Vessel — tell them apart by their lights
• Towing vessel lights — Rule 24 on top of Rule 23
• Anchor lights — Rule 30, for contrast

The simulator is part of the Skipper Refresher Course. Free demo scenarios let you try the bridge interface first.

Frequently asked questions

What is COLREG Rule 23?

Rule 23 — "Power-driven vessels underway" — sets out the lights a power-driven vessel must exhibit: a masthead light forward, a second higher masthead light aft (compulsory at 50 m+), sidelights (red port / green starboard) and a sternlight. Hovercraft and WIG craft add a flashing light; small craft have simplified options.

How many masthead lights does a power-driven vessel show?

At 50 metres or more, two — one forward and a second abaft and higher. Under 50 metres, at least one forward; the second is optional. The two-light arrangement helps you judge a vessel's heading and aspect at night.

What lights does a power-driven vessel under 12 metres show?

Under Rule 23(d), a power-driven vessel under 12 m may show an all-round white light plus sidelights instead of the full masthead-and-sternlight set. A vessel under 7 m with a maximum speed of 7 knots or less may show an all-round white light alone, and sidelights if practicable.

What extra light does a hovercraft show?

An air-cushion vessel in non-displacement mode shows an all-round flashing yellow light in addition to its normal Rule 23(a) lights. A WIG craft taking off, landing or flying near the surface shows a high-intensity all-round flashing red light.

How do I tell a power-driven vessel from a sailing vessel at night?

Look for the white masthead light. A power-driven vessel shows one or two masthead lights plus sidelights and a sternlight. A sailing vessel shows sidelights and a sternlight but no masthead light. A sailing yacht motoring (engine engaged) counts as power-driven and must show masthead lights.

Does Rule 23 apply to a yacht under engine?

Yes. The moment a sailing yacht's engine is propelling her, she is a power-driven vessel for COLREG purposes and must show Rule 23 lights — even with sails up. By day she shows the motoring cone, point down (Rule 25(e)).

Related reading

• COLREG Rule 24 Explained — Towing and Pushing Lights — the next rule up
• COLREG Rule 18 Explained — Responsibilities Between Vessels — who gives way to whom
• Short Range Certificate (SRC) — Complete Guide — the marine VHF qualification
• Maritime glossary — sailing, COLREG, VHF and navigation terms