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4

Application

Rules 4–10 apply in any visibility. A vessel can still be in sight in mist; a radar contact alone is not visual observation. The visible vessel in restricted visibility also displays its prescribed navigation lights.

1The rule in plain words

In one sentence…

Section I of COLREG still applies in any condition of visibility, from a clear day to fog.

What does it establish?

The Rules in Section I apply in any condition of visibility; no condition is excluded.

What does it include?

Section I contains Rules 4 to 10: look-out, safe speed, risk of collision, action to avoid collision, narrow channels and traffic separation schemes.

Why does it help?

It reminds you that Section I has a general scope; Section II points to vessels in sight of one another, and III to restricted visibility.

2Key points

  1. 1.

    Application crosses visibility conditions → visibility does not switch it off.

  2. 2.

    Section I = Rules 4–10.

  3. 3.

    Topics: look-out · speed · collision · action · channels · separation.

  4. 4.

    Section II = vessels in sight · Section III = restricted visibility.

3Frequent mistakes

  • Wrong: "Section I applies only in clear visibility."

    Right: It also applies in restricted visibility.

  • Wrong: "In fog, only the restricted-visibility Rules count."

    Right: Rules 4 to 10 still apply; fog does not remove them.

  • Wrong: "Section I ends at Rule 9."

    Right: It includes Rules 4 to 10, inclusive.

4Exam keys & interpretation

Exam keys

  • If an option limits Rule 4 to daylight, clear visibility or vessels in sight, reject that limit.

  • For a range question, look for 4–10; do not confuse Section I with Sections II or III.

  • If the stem says fog, check whether it asks about application, not which section is being referenced.

How to read it

  • Read "any condition of visibility" literally: visibility does not switch Section I off.

  • Use 4–10 to locate Section I; references to II and III only guide how the question is framed.

5Official text

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

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

Rules in this Section apply to any condition of visibility.
Source:

6Test your knowledge

Question 1/4

What happens to Section I in fog?

More questions on this rule →

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Frequently asked questions about COLREG Rule 4

Which COLREG rules apply in any condition of visibility?

Rules 4 to 10, the Section I steering and sailing rules. Rule 4 states that they apply in any condition of visibility, whether clear or restricted.

Do different steering rules apply in fog under Section I?

No. The Section I rules must be followed regardless of weather or visibility. Thinking that different steering rules apply in fog and in clear weather is a common mistake.

How do Sections I, II and III of Part B differ?

Section I (Rules 4–10) applies in all visibility conditions. Section II applies only when vessels are in sight of one another, and Section III applies to vessels not in sight of one another when navigating in or near restricted visibility.