Rule 21: the lights, defined.

What does COLREGs Rule 21 say?

Rule 21(a): a masthead light is a white light 'over the fore and aft centreline', showing an unbroken light over 225 degrees, from right ahead to 22.5 degrees abaft the beam on either side.

Rule 21(b): sidelights are a green light on the starboard side and a red light on the port side, each showing an unbroken light over 112.5 degrees, from right ahead to 22.5 degrees abaft the beam on its own side. In a vessel of less than 20 metres they may be combined in one lantern on the fore and aft centreline.

Rule 21(c): a sternlight is a white light 'as nearly as practicable at the stern', showing an unbroken light over 135 degrees — 67.5 degrees from right aft on each side.

Rule 21(d): a towing light is yellow but otherwise matches the sternlight. Rule 21(e): an all-round light is visible, unbroken, from every direction. Rule 21(f): a flashing light flashes regularly, at '120 flashes or more per minute'.

Why does a definitions rule matter?

Because Rules 23 to 31 use these words without repeating them. When Rule 23 says a power-driven vessel shows 'sidelights' and 'a sternlight', the colours, positions and arcs come from Rule 21. Get the definitions wrong and every lights answer built on them is wrong too.

What are the arcs of the masthead light, sidelights and sternlight?

Masthead light: 225 degrees, white, from right ahead to 22.5 degrees abaft the beam on both sides. Sidelights: 112.5 degrees each — green to starboard, red to port — from right ahead to 22.5 degrees abaft the beam on their own side. Sternlight: 135 degrees, white, 67.5 degrees either side of right aft.

How do the arcs fit together?

The arithmetic is the easiest way to remember them. Each sidelight covers 90 degrees from ahead to the beam plus 22.5 degrees abaft it: 112.5 degrees. The two together cover the same 225 degrees as the masthead light. What is left of the circle — 360 minus 225 — is 135 degrees, which is exactly the sternlight's arc: 67.5 degrees either side of right aft.

So from anywhere around a power-driven vessel showing the Rule 23(a) lights you see either her masthead light or lights and a sidelight (or both sidelights, from right ahead), or her sternlight — and the changeover is the same line, 22.5 degrees abaft her beam.

Why 22.5 degrees abaft the beam?

It is the line that matters for overtaking. Rule 13(b) draws the overtaking line in the same place: coming up from more than 22.5 degrees abaft her beam makes you the overtaking vessel, and the rule ties that position to her lights — her sternlight in view, neither sidelight. The light arcs and the overtaking rule draw the same boundary.

Do the lights cut off exactly at the edge of their arcs?

Not quite. Annex I section 9 lets a light fade out at the edges. In the forward direction a sidelight's intensity must decrease to reach practical cut-off between one and three degrees outside its sector, so from right ahead you may see both sidelights at once. At the 22.5-degrees-abaft-the-beam line — the after limits of the masthead light and sidelights, and the edges of the sternlight's arc — the intensity may drop by 50 per cent over the last five degrees inside the sector and must reach practical cut-off not more than five degrees outside it.

In practice that means the lights cannot settle a borderline case near 22.5 degrees abaft the beam. Rule 13(c) settles it instead: a vessel in any doubt as to whether she is overtaking must assume that she is.

What is a towing light?

Under Rule 21(d), a yellow light with the same characteristics as the sternlight: the same 135-degree arc, shining aft. Rule 24(a)(iv) puts it to use — a power-driven vessel when towing shows a towing light in a vertical line above her sternlight, so from astern you see yellow over white.

What is an all-round light?

One you can see, unbroken, from every direction — all 360 degrees of the horizon (Rule 21(e)). The rules use all-round lights in many colours: the white anchor lights of Rule 30(a), for example, or the two all-round reds of a vessel not under command in Rule 27(a).

How fast does a flashing light flash?

At regular intervals, at a frequency of 120 flashes or more per minute (Rule 21(f)) — at least two a second. Rule 23 uses it twice: an all-round flashing yellow light for an air cushion vessel in the non-displacement mode, and a high intensity all-round flashing red light for a WIG craft taking off, landing and in flight near the surface.

It is not the same thing as the manoeuvring light in Rule 34(b), which supplements whistle signals with flashes of about one second, about one second apart.

What is a combined lantern?

Rule 21(b) allows a vessel of less than 20 metres to combine her two sidelights in one lantern, carried on the fore and aft centreline — red one side, green the other, in a single fitting. Don't confuse it with the lantern in Rule 25(b), which lets a sailing vessel of less than 20 metres combine her sidelights and sternlight in one lantern at or near the top of the mast.

How might Rule 21 come up in an MCA oral?

It may come up as the arcs themselves, or as a question about what you would see from a given position. Examples of the kind of question to be ready for:

“What is the arc of a sidelight, and where does it show from and to?”

“Why is a sternlight 135 degrees?”

“You can see a vessel's sternlight but neither sidelight. Where are you in relation to her, and what does that make you if you're coming up on her?”

“What colour is a towing light, and where is it shown?”

“How often does a flashing light flash?”

Where do candidates go wrong on Rule 21?

The common slips: giving the sidelight arc as 112.5 degrees but not saying it runs from right ahead to 22.5 degrees abaft the beam; mixing up the 22.5 degrees (abaft the beam) and the 67.5 degrees (from right aft); calling the towing light white or forgetting that it shines aft like a sternlight; and confusing the combined sidelight lantern of Rule 21(b) with the combined lantern at the top of a sailing vessel's mast under Rule 25(b), which holds her sidelights and sternlight and is not a masthead light.