Rule 4: the rules for any condition of visibility.

What does COLREGs Rule 4 say?

That the rules of its Section — Part B, Section I — apply 'in any condition of visibility'. That is the whole rule.

What does 'any condition of visibility' mean?

Exactly what it says: clear weather, night, and restricted visibility alike, and whether or not the vessels involved are in sight of one another. No Section I rule switches off because the visibility has changed — what changes is how you apply it.

Which rules are in Part B, Section I?

Section I, 'Conduct of vessels in any condition of visibility', runs from Rule 4 to Rule 10:

Rule 5, Look-out: every vessel keeps a proper look-out at all times, by sight and hearing and by all available means.

Rule 6, Safe speed: every vessel proceeds at all times at a safe speed, with factors to take into account for all vessels and more for vessels with operational radar.

Rule 7, Risk of collision: how to decide whether risk of collision exists — all available means, proper use of radar, no assumptions on scanty information, and the compass bearing that does not appreciably change.

Rule 8, Action to avoid collision: what avoiding action must be — positive, in ample time, large enough to be readily apparent, resulting in passing at a safe distance — and the duties of a vessel required not to impede another.

Rule 9, Narrow channels, and Rule 10, Traffic separation schemes: how to navigate in each, and who must not impede whom.

Why does it matter that these rules apply in any visibility?

Because the look-out, safe speed, risk-of-collision and avoiding-action rules are needed in every encounter. On a clear day they work alongside the in-sight rules of Section II; in fog they work alongside Rule 19. Rule 6 is not only a fog rule, and Rule 9 and Rule 10 do not lapse at night.

How is Part B organised?

In three sections, each with its own application:

Section I (Rules 4 to 10): any condition of visibility — Rule 4.

Section II (Rules 11 to 18), 'Conduct of vessels in sight of one another': applies to vessels in sight of one another — Rule 11.

Section III (Rule 19), 'Conduct of vessels in restricted visibility': applies to vessels not in sight of one another when navigating in or near an area of restricted visibility — Rule 19(a).

Which rules apply when two vessels are in sight of each other?

Section I, because it applies in any visibility, and Section II, because the vessels are in sight. Vessels are in sight of one another only when one can be observed visually from the other (Rule 3(k)).

Rule 8(a) ties the two together: any action to avoid collision is taken in accordance with the rules of Part B — so a give-way vessel under Section II takes action that also meets Rule 8.

Do other parts of the COLREGs have an application rule like Rule 4?

Yes — the other two sections of Part B, and Part C, each open with one, and the wording is worth knowing side by side. Rule 11 applies Section II to vessels in sight of one another. Rule 19(a) applies Rule 19 to vessels not in sight of one another when navigating in or near an area of restricted visibility. In Part C, Rule 20(a) says the rules on lights and shapes are to be complied with in all weathers, and Rule 20(b) and (d) set when: lights from sunset to sunrise, shapes by day.

Rule 4 is the widest of them. Section I has no condition attached at all.

Which rules apply in fog with a radar contact?

Section I and Rule 19 — not Section II, because a radar contact is not in sight. Rule 19(c) requires every vessel to have due regard to the prevailing circumstances and conditions of restricted visibility when complying with the Section I rules. That is where Rule 19 leans on Section I: look-out, safe speed, risk of collision and avoiding action, all adapted to the visibility.

As the vessels come into sight of one another, Section II applies from that moment, alongside Section I.

How might Rule 4 come up in an MCA oral?

It may come up as the question behind the question — which rules apply here, and why? Examples of the kind of question to be ready for:

“Which rules apply in any condition of visibility?”

“Does Rule 6 apply on a clear day?”

“You're in fog with a radar contact closing. Which sections of Part B apply?”

“The fog lifts and you can see the other vessel. What changes?”

“What is the difference between Rule 4 and Rule 11?”

Where do candidates go wrong on Rule 4?

The common slips: treating safe speed and look-out as fog-only rules; applying the Section II crossing or stand-on rules to a vessel seen only on radar; and not knowing which rules sit in which section. Being able to say which section applies, and why, is the point of the rule.