Rule 20: when lights and shapes are shown.

What does COLREGs Rule 20 say?

Rule 20(a): there is no weather in which Part C stops applying ('in all weathers').

Rule 20(b): the rules concerning lights apply 'from sunset to sunrise', and during those hours any other light you show must meet the tests the paragraph sets — covered below.

Rule 20(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 it is deemed necessary.

Rule 20(d): shapes are the daytime signal — the shapes rules apply 'by day'. Rule 20(e): the lights and shapes must comply with Annex I.

Which rules does Rule 20 apply to?

Part C — Rules 20 to 31, the lights and shapes rules. Rule 21 defines the lights, Rule 22 sets their minimum ranges, and Rules 23 to 31 say which lights and shapes each kind of vessel shows: power-driven vessels underway, towing and pushing, sailing vessels and vessels under oars, fishing vessels, vessels not under command or restricted in their ability to manoeuvre, vessels constrained by their draught, pilot vessels, anchored vessels and vessels aground, and seaplanes and WIG craft.

What does 'in all weathers' mean?

That there is no weather in which the lights and shapes rules stop applying. In our view the point is that the conditions in which lights are hardest to see — heavy rain, spray, a big sea — are the ones in which they matter most, so a lantern that has failed or been washed out is a defect to put right, not something the weather excuses.

When must navigation lights be shown?

From sunset to sunrise, under Rule 20(b) — not from whenever it gets dark or someone on the bridge decides it is night. And under Rule 20(c), also from sunrise to sunset in restricted visibility.

Rule 3(l) defines restricted visibility by its causes — fog and mist, but also falling snow, heavy rainstorms, sandstorms and anything similar. So a heavy rain squall at midday counts, not only fog.

Why does Rule 20(c) say 'if carried'?

Because not every vessel carries the lights. Some rules let a small vessel do without them: under Rule 25(d), for instance, a sailing vessel of less than seven metres and a vessel under oars that do not show the sailing lights must have ready at hand an electric torch or lighted lantern showing a white light, to be shown in sufficient time to prevent collision. Rule 20(c) cannot require a light that is not fitted; where it is fitted, it goes on in restricted visibility by day.

Can I show navigation lights by day in good visibility?

Yes. Rule 20(c) allows the prescribed lights to be shown in any other circumstances 'when it is deemed necessary'. In practice that covers poor light before sunset or after sunrise, or any time you judge that your lights will help another vessel see you and read what you are doing.

When are shapes shown?

By day, under Rule 20(d). Shapes are not switched off by restricted visibility: in fog by day, Rule 20(c) brings on the lights and Rule 20(d) still requires the shapes. A vessel at anchor in daytime fog, for example, shows her anchor light or lights and her ball.

Can I show other lights at night, such as deck lights?

Only within the limits in Rule 20(b). The lights allowed alongside the prescribed ones are those that 'cannot be mistaken for the lights specified in these Rules', or that do not impair their visibility or distinctive character, or 'interfere with the keeping of a proper look-out'.

As we read it, a light worth keeping on should pass all three. A floodlit deck that no one could mistake for a navigation light can still drown out your sidelights or ruin the night vision of the person keeping the look-out Rule 5 requires, and that is reason enough to switch it off. At anchor the rules go the other way: under Rule 30(c) a vessel may light her decks with her working or equivalent lights, and one of 100 metres or more must.

What about lights used to attract attention?

That is Rule 36. When it is necessary, a vessel may use a light or sound signal to get another vessel's attention, or shine her searchlight towards the danger, so long as the signal cannot be confused with one the rules provide for elsewhere, the light cannot be taken for an aid to navigation, and no vessel is embarrassed by it. Strobes and other high intensity intermittent or revolving lights are to be avoided.

What does Annex I add?

Rule 20(e) requires the lights and shapes to comply with Annex I, which sets out their positioning and technical details. For shapes, Annex I section 6 requires them to be black, sets their minimum sizes — a ball, for example, at least 0.6 metre in diameter — and a vertical distance between shapes of at least 1.5 metres. On a vessel of less than 20 metres, smaller shapes 'commensurate with the size of the vessel' may be used, with the spacing reduced to match.

How might Rule 20 come up in an MCA oral?

It may come up as the 'when' behind a lights or shapes question. Examples of the kind of question to be ready for:

“Between what times must navigation lights be shown?”

“It's midday and the fog comes down while you're at anchor. What do you show?”

“Can you have your deck lights on while underway at night?”

“A heavy rain squall closes the visibility to half a mile in the afternoon. Do your lights go on?”

“When are shapes shown, and do they come down in fog?”

Where do candidates go wrong on Rule 20?

The common slips: saying lights are shown 'at night' instead of from sunset to sunrise; forgetting that lights go on by day in restricted visibility; taking the shapes down when the lights go on in daytime fog; and treating any extra light as acceptable so long as it isn't red or green, when Rule 20(b) is also concerned with lights that impair the prescribed ones or interfere with the look-out.