IALACOLREG
View your progress
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.

Rule 20 applies the Part C rules in all weather.

b
Show prescribed lights from sunset to sunrise. Other lights are permitted only if they 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.
c
The prescribed lights, if carried, must also be shown from sunrise to sunset in restricted visibility and may be shown in all other circumstances when considered necessary.
d
Show the prescribed shapes by day.
e
The prescribed lights and shapes must meet Annex I.

IMO COLREG 1972Official text

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:

Recognition Sequence

Classify the vessel state first: underway, making way, stopped, at anchor, aground, towing, fishing, pilotage or special condition.

Read special lights vertically from top to bottom before using sidelights and sternlight to confirm aspect.

Use the complete display and any vessel length stated in the scenario.

Optional second masthead lights, two anchor lights or deck illumination do not by themselves prove a minimum vessel length.

Exam Focus

Avoid identifying a vessel from one colour alone.

Many mistakes come from spotting a red light and guessing before checking the full pattern.

If the question mentions 'making way', 'underway but stopped', 'at anchor' or 'aground', that wording usually determines which extra lights or shapes appear.

Key Takeaways

1

Show prescribed navigation lights from sunset to sunrise

2

In daytime restricted visibility, show the prescribed lights if carried

3

Other lights must not confuse or obscure navigation lights or interfere with a proper look-out

4

The prescribed shapes are exhibited by day

Common Mistakes

Treating daytime fog as if navigation lights are optional

Using decorative or work lights that obscure or confuse the statutory lights

Test Your Knowledge

Practise this topic and review the explanation for each answer.

More rules in this Part

Last reviewed:

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.