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20

Application (Lights)

A 30 m vessel at anchor: one white all-round anchor light at night; a black ball by day; both in daytime restricted visibility. The single anchor-light arrangement is permitted for vessels under 50 m.

1The rule in plain words

In one sentence…

COLREG sets when lights and shapes are shown and requires extra lights not to confuse or hinder a proper look-out.

When do the light rules apply?

They apply from sunset to sunrise in all weathers.

What about daylight?

If carried, prescribed lights are also exhibited from sunrise to sunset in restricted visibility, and may be exhibited whenever necessary.

Can you add another light?

Only if it cannot be mistaken for a COLREG light, does not impair the visibility or distinctive character of prescribed lights, and does not interfere with a proper look-out.

What about shapes and the annex?

Rules concerning shapes are complied with by day; the lights and shapes specified must meet the specifications of Annex I to COLREG.

2Key points

  1. 1.

    All weathers · lights: sunset → sunrise.

  2. 2.

    Sunrise + restricted visibility → prescribed lights, if carried.

  3. 3.

    Other light: no mistaking · no impairment · no interference.

  4. 4.

    Shapes: by day · lights and shapes → Annex I specifications.

3Frequent mistakes

  • Wrong: "Bad weather suspends the light rules."

    Right: The Rules apply in all weathers.

  • Wrong: "Prescribed lights are forbidden in daytime."

    Right: If carried, they are also exhibited in restricted visibility by day.

  • Wrong: "Shapes follow the sunset-to-sunrise window."

    Right: Rules concerning shapes are complied with by day.

4Exam keys & interpretation

Exam keys

  • "Sunrise to sunset" needs the cue "restricted visibility"; do not treat daylight as a blanket rule.

  • "If carried" qualifies prescribed lights in daytime; it is not a ban.

  • Test an extra light against mistaken identity, visibility or distinctive character, and proper look-out.

How to read it

  • Read "may be exhibited" as permission when necessary, not an automatic daylight requirement.

  • Annex I supplies technical specifications; it does not replace compliance with the lights-and-shapes rules.

5Official text

IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)

Full wording of the rule, reproduced from the official source below.

(a) Rules in this part shall be complied with in all weathers. (b) The Rules concerning lights shall be complied with from sunset to sunrise, and during such times no other lights shall be exhibited, except such lights as cannot be mistaken for the lights specified in these Rules or do not impair their visibility or distinctive character, or interfere with the keeping of a proper look-out. (c) The lights prescribed by these Rules shall, if carried, also be exhibited from sunrise to sunset in restricted visibility and may be exhibited in all other circumstances when it is deemed necessary. (d) The Rules concerning shapes shall be complied with by day. (e) The lights and shapes specified in these Rules shall comply with the provisions of Annex I to these Regulations.
Source:

6Test your knowledge

Question 1/4

In what weather conditions must the Rules in this part be complied with?

More questions on this rule →

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Previous: Rule 19 · Conduct of Vessels in Restricted Visibility

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Frequently asked questions about COLREG Rule 20

When must navigation lights be displayed under COLREG Rule 20?

From sunset to sunrise. The prescribed lights, if carried, must also be shown in daytime restricted visibility and may be shown in other circumstances when considered necessary. The prescribed shapes are shown by day (Rule 20(c)–(d)).

Can a vessel show other lights at night besides her navigation lights?

Only lights that cannot be mistaken for the prescribed lights, do not impair their visibility or distinctive character, and do not interfere with keeping a proper look-out. Decorative or work lights that obscure or confuse the statutory lights are not acceptable.

Are navigation lights required in fog during daylight?

Yes. Rule 20 requires the prescribed lights during restricted visibility as well as from sunset to sunrise, so daytime fog is no exception. Treating daytime fog as if navigation lights were optional is a common mistake.