Tonkie / Wikimedia Commons
The question this guide answers
At night, a yacht becomes a small arrangement of coloured and white lights. Those lights can reveal her direction of travel, whether she is under power or sail, whether she is anchored, and whether her ability to manoeuvre is restricted. They do not, however, tell you everything. This guide explains what a watchkeeper can legitimately infer from the International Regulations for Preventing Collisions at Sea (COLREGs), what remains uncertain, and why a light pattern is the beginning of collision assessment rather than the conclusion.
Start with aspect, not a memorised rhyme
The most useful first question is not “What phrase do I remember?” but “Which sectors of the other vessel can I see?” Under Rule 21, the red port sidelight and green starboard sidelight each cover 112.5 degrees: from right ahead to 22.5 degrees abaft the beam on their respective sides. A white sternlight covers 135 degrees centred astern. A white masthead light covers 225 degrees, from right ahead to 22.5 degrees abaft either beam. Those arcs create a visual geometry. Green plus a white masthead light indicates that you are viewing the vessel’s starboard aspect; red plus masthead white indicates her port aspect; both sidelights with masthead light or lights suggests a near head-on aspect; a sternlight without sidelights suggests that you are looking from abaft her beam. Distance, glare, vessel trim and intervening structure can complicate the picture, so the observation must be checked over time.
The baseline power-driven yacht
Rule 23 requires a power-driven vessel underway to show a forward masthead light, sidelights and a sternlight. At 50 metres or more, a second masthead light is required abaft and higher than the forward one. A vessel below 50 metres is not obliged to carry that second masthead light, although it may do so. This is a useful size clue, not a measuring instrument: seeing two masthead lights can be consistent with a vessel of 50 metres or more, but configuration, perspective and permitted choices mean that a single glance should not be used to assign an exact length. For yachts below 12 metres, Rule 23 permits an all-round white light and sidelights instead of the standard arrangement. Very small, slow power-driven craft below 7 metres and not exceeding 7 knots have a further simplified option.
Power or sail? The machinery test matters
A sailing vessel underway normally shows sidelights and a sternlight, without the white masthead light of a power-driven vessel. A sailing yacht below 20 metres may combine the sidelights and sternlight in one lantern near the masthead. Some sailing vessels may additionally show red over green all-round lights at or near the masthead, but not together with that combined tricolour lantern. The decisive legal distinction is propulsion, not the presence of sails: Rule 3 defines a sailing vessel as one under sail while fitted machinery is not being used. A yacht motor-sailing is therefore treated as power-driven for the applicable lights. By day, Rule 25 requires a cone with its apex down where it can best be seen. At night, the power-driven light pattern is the practical clue.
What the colours encode
The colour sequence communicates status as well as direction. Two all-round red lights in a vertical line identify a vessel not under command: an exceptional circumstance prevents her from manoeuvring as required. Red-white-red identifies a vessel restricted in her ability to manoeuvre because of the nature of her work. Three all-round red lights in a vertical line may be shown by a power-driven vessel constrained by her draught, in addition to the ordinary power-driven lights. A pilot vessel on duty shows white over red near the masthead; the lead photograph on this article depicts that distinctive pilot signal. These patterns describe a legal status. They are not decorative codes and should not be inferred from deck lighting or isolated reflections.
A compact night-light reference
Observed pattern | COLREG meaning | What it does not prove Red sidelight + white masthead light | Power-driven vessel seen on her port aspect | Exact course, speed or closest point of approach Green sidelight + white masthead light | Power-driven vessel seen on her starboard aspect | That she has seen you or will take the expected action Both sidelights + masthead light(s) | Near head-on aspect may be developing | Collision risk without checking bearing and movement White sternlight only | You are likely viewing from abaft the beam | That overtaking is safe Sidelights + sternlight, no masthead white | Sailing vessel underway is a likely interpretation | That no engine is running unless the complete pattern is confirmed Red over red | Vessel not under command | The cause or duration of the problem Red-white-red | Restricted in ability to manoeuvre | Which side is safe to pass unless additional signals show it White over red | Pilot vessel engaged on pilotage duty | Priority over every other vessel One or two all-round whites while stationary | Vessel at anchor, depending on length and arrangement | That she cannot swing or that her anchor is holding
Towing patterns deserve extra space
A towing situation can extend far beyond the lights of the towing vessel. Rule 24 replaces the ordinary forward masthead-light arrangement with two masthead lights in a vertical line; when the tow exceeds 200 metres from the towing vessel’s stern to the after end of the tow, three are shown. The tug also shows sidelights, a sternlight and a yellow towing light vertically above the sternlight. The tow itself carries prescribed lights, and a tow exceeding 200 metres also carries a diamond day shape. At night, never treat the illuminated tug as the end of the hazard. Look for the relationship between tug and tow, use radar where fitted, and allow for an inconspicuous or partly submerged object.
Anchored is not simply “a white light”
Rule 30 requires a vessel at anchor to show an all-round white light forward and another all-round white light near the stern at a lower level. Vessels below 50 metres may instead show one all-round white light where it can best be seen. A vessel of 100 metres or more must also illuminate her decks with working or equivalent lights; smaller vessels may do so. An aground vessel adds two all-round red lights in a vertical line. The practical trap is background lighting: a yacht’s decorative deck lights can mask an anchor light, and a shore full of lamps can make any isolated white light difficult to identify. Position, movement relative to the background and radar presentation all matter.
Visibility range is specified—but conditions still govern detection
Rule 22 sets minimum nominal ranges by vessel length and light type. For vessels of 50 metres or more, the masthead light is specified at 6 nautical miles and sidelights, sternlight and relevant all-round lights at 3 miles. For vessels from 12 metres to below 50 metres, the masthead requirement is 5 miles, except that vessels below 20 metres require 3 miles; sidelights and sternlights are specified at 2 miles. Below 12 metres, masthead and stern lights are specified at 2 miles and sidelights at 1 mile. These are regulatory minimum ranges under defined technical assumptions, not a promise that every observer will detect every light at that distance. Rain, haze, sea spray, background lights, eye adaptation, dirty lenses and installation faults can all reduce practical recognition.
Deck lighting can destroy the message
Rule 20 allows other lights at night only when they cannot be mistaken for prescribed lights, do not impair their visibility or distinctive character, and do not interfere with a proper lookout. This matters acutely on large yachts. Underwater lights, illuminated superstructures, beach-club lighting and floodlit decks may be visually impressive, but the navigation-light signature must remain unambiguous. From a bridge-management perspective, exterior hospitality lighting should be assessed from outside the yacht and from realistic encounter angles—not merely from the aft deck. If a red decorative lamp can be confused with a port sidelight, or white work lights wash out the sternlight, the aesthetic scheme has become an operational problem.
Lights do not allocate responsibility by themselves
Recognising a pattern is not the same as deciding who must manoeuvre. The steering and sailing rules still apply. A head-on situation between power-driven vessels requires both to alter course to starboard. In a crossing situation, the power-driven vessel that has the other on her starboard side keeps out of the way, if circumstances admit avoiding a crossing ahead. An overtaking vessel keeps clear until finally past and clear. Rule 18 also sets responsibilities between types of vessel, but narrow channels, traffic-separation schemes and overtaking rules can modify the simple hierarchy people often recite. “I saw green, so I stood on” is not a sufficient collision-avoidance analysis.
The three checks after identification
First, maintain a proper lookout. Rule 5 requires sight, hearing and all available means appropriate to the conditions. Second, establish whether risk of collision exists. Rule 7 warns against assumptions based on scanty information and says that doubt should be treated as risk. A steady compass bearing is a classic warning, although a changing bearing does not always remove risk at close range or around very large vessels and tows. Third, make any avoiding action early, substantial and easy to read. Rule 8 favours positive action in ample time and discourages a sequence of small alterations that another watchkeeper may not recognise.
AIS is identification support, not permission to stop looking
AIS can add a vessel name, reported course, speed and navigational status, while radar can provide range and relative movement. Neither changes the obligation to interpret the actual encounter. AIS data may be entered incorrectly, delayed or absent; small craft may not transmit; and a reported navigational status does not replace the lights and shapes required by COLREGs. Equally, a visually identified light pattern should be cross-checked against radar and AIS rather than forced to match them. When the sources disagree, the disagreement is operational information that demands caution.
A practical bridge routine
On first sighting, note bearing, colour, number and vertical relationship of the lights. Decide whether you are seeing a sidelight, masthead light, sternlight or an all-round status light. Observe the pattern long enough to see whether an intermittent obstruction or yaw changes it. Check radar range and relative trail if available, then compare AIS without allowing the name on the screen to replace the visual assessment. Recheck the bearing. Consider channel geometry, traffic separation, shallow water and nearby unlit hazards. If avoiding action is required, make it early and obvious, then continue monitoring until the vessel is finally past and clear.
What a guest should understand
Guests do not need to memorise every COLREG pattern, but they should understand why the bridge may become quiet and formal after dark. A cluster of lights that looks picturesque from the sundeck is a working language to the crew. Avoid shining torches towards the bridge, do not request that navigation lights be dimmed for a photograph, and keep exterior lighting changes under crew control. The elegance of a night passage depends on disciplined visibility as much as on atmosphere.
Source transparency and limits
This article was checked on 19 October 2026 against the IMO COLREG overview, the consolidated COLREG text published through the Netherlands Regulatory Framework and the U.S. Coast Guard Navigation Rules handbook. The comparison is an editorial explanation, not a substitute for certified training, the complete current rules, flag-State requirements, local inland rules, standing orders or professional judgement. Exact installation, approval and sector requirements are set out in COLREG Annex I. No performance measurement or onboard test was conducted by The Nautical Chapter for this article.