Rule 23: a power-driven vessel underway.

What does COLREGs Rule 23 say?

Rule 23(a): a power-driven vessel underway shows (i) a masthead light forward; (ii) a second masthead light 'abaft of and higher than the forward one' — though a vessel of less than 50 metres is not obliged to show it, but may; (iii) sidelights; and (iv) a sternlight.

Rule 23(b): an air cushion vessel operating in the non-displacement mode adds an all-round flashing yellow light. Rule 23(c): a WIG craft, only when taking off, landing and in flight near the surface, adds a high intensity all-round flashing red light.

Rule 23(d) gives alternatives for small power-driven vessels: under 12 metres, an all-round white light and sidelights; under 7 metres with a maximum speed not exceeding 7 knots, an all-round white light, with sidelights if practicable; and, under 12 metres, leave on conditions to fit the white light off the centreline if centreline fitting is not practicable.

What is a 'power-driven vessel underway'?

Definitions from Rule 3. A power-driven vessel is one propelled by machinery (Rule 3(b)); a sailing vessel is one under sail with no propelling machinery in use (Rule 3(c)). So a sailing yacht motoring, or motor-sailing, is a power-driven vessel and shows Rule 23 lights. Underway means not at anchor, made fast to the shore or aground (Rule 3(i)).

Underway is not the same as making way. A power-driven vessel stopped in the water but not anchored is still underway and still shows her Rule 23 lights. Compare Rule 27(a): a vessel not under command shows sidelights and a sternlight only when making way through the water.

What lights does a sailing yacht show under engine?

The Rule 23 lights: a masthead light, sidelights and a sternlight — or, under 12 metres, the Rule 23(d) alternatives. The combined lantern at the top of the mast that Rule 25(b) allows a sailing vessel of less than 20 metres combines only her sidelights and sternlight; it has no masthead light, and it is a sailing vessel's light. By day, if she is motor-sailing, Rule 25(e) requires a cone, apex downwards, shown forward where it can best be seen.

When must a power-driven vessel show two masthead lights?

At 50 metres in length or more. Rule 23(a)(ii) requires the second masthead light, abaft of and higher than the forward one, and excuses only a vessel of less than 50 metres, which 'shall not be obliged to exhibit such light but may do so'.

So, once she is close enough for both to be in view, one masthead light points to a vessel under 50 metres; but two masthead lights do not prove she is 50 metres or more — a smaller vessel may choose to show both. At long range the higher, after light may be the only one you see at first.

How far apart are the two masthead lights?

Annex I sets it. On a power-driven vessel of 20 metres or more, when two masthead lights are carried the after one must be at least 4.5 metres vertically higher than the forward one (section 2(a)(ii); section 13 lets high-speed craft of 50 metres or more use a smaller separation, set by a formula). When two are prescribed, the horizontal distance between them must be not less than half the vessel's length but need not be more than 100 metres, and the forward light must be not more than a quarter of the length from the stem (section 3(a)).

How do I read a power-driven vessel's lights?

The arcs in Rule 21 tell you where you are relative to her. A green sidelight means you are on her starboard side; a red one, her port side. Only her sternlight, and neither sidelight, means you are more than 22.5 degrees abaft her beam — and if you are coming up with her, Rule 13(b) makes you the overtaking vessel.

If you see her ahead or nearly ahead with her masthead lights in a line or nearly in a line, and/or both sidelights, Rule 14(b) treats it as a head-on situation; for two power-driven vessels meeting so as to involve risk of collision, Rule 14(a) requires each to alter course to starboard.

In practice the two masthead lights also work as a pair of range lights: when they open out, the lower, forward light is on the side she is heading towards, and a change in their spacing is often the first sign of an alteration.

What does a masthead light with a red sidelight on my starboard bow mean?

You are on her port side and she is on your starboard side. If you are both power-driven vessels crossing so as to involve risk of collision — and under Rule 7 risk is deemed to exist if her compass bearing does not appreciably change, and may exist even when it does — Rule 15 makes you the vessel that keeps out of the way, and if the circumstances of the case admit you must avoid crossing ahead of her.

What extra light does a hovercraft show?

Under Rule 23(b), an air cushion vessel 'when operating in the non-displacement mode' — riding on her cushion — shows an all-round flashing yellow light in addition to the Rule 23(a) lights. Off the cushion, in the displacement mode, she shows the Rule 23(a) lights only. A flashing light flashes at 120 flashes or more per minute (Rule 21(f)), which sets it apart from the steady yellow of a towing light.

What lights does a WIG craft show?

Rule 3(m) defines a WIG (Wing-in-Ground) craft as a 'multimodal craft' whose main operational mode is flight close to the surface, using surface-effect action; Rule 3(a) counts it as a vessel. Rule 23(c) requires it, 'only when taking off, landing and in flight near the surface', to show a high intensity all-round flashing red light in addition to the Rule 23(a) lights.

What lights can a power-driven vessel under 12 metres show?

Either the full Rule 23(a) lights, or — under Rule 23(d)(i), in lieu of them — an all-round white light and sidelights. The single all-round white takes the place of both the masthead light and the sternlight.

If centreline fitting is not practicable, Rule 23(d)(iii) lets the masthead light or all-round white light be fitted off the fore and aft centreline, provided the sidelights are combined in one lantern carried on the centreline, or as nearly as practicable in the same fore and aft line as the white light.

What about a vessel under 7 metres?

Rule 23(d)(ii) applies only to a power-driven vessel of less than seven metres 'whose maximum speed does not exceed seven knots' — both conditions. She may show an all-round white light in lieu of the Rule 23(a) lights, and must also show sidelights if practicable.

A fast tender under 7 metres does not qualify. She must show either the Rule 23(a) lights or, being under 12 metres, the Rule 23(d)(i) alternative of an all-round white light and sidelights. Either way her sidelights are required, not merely 'if practicable'.

How might Rule 23 come up in an MCA oral?

It may come up as a lights picture to identify, or as a question about the vessel you serve on. Examples of the kind of question to be ready for:

“What lights does a 40-metre motor yacht show underway at night? Must she show a second masthead light?”

“Right ahead, you see two masthead lights nearly in line and both sidelights. What is the situation and what do you do?”

“You're motoring a 14-metre sailing yacht with the sails down. Which lights do you show?”

“Your 6-metre tender can make 30 knots. Which lights must she show at night?”

“You see an all-round flashing yellow light together with a vessel's masthead light and sidelights. What is she?”

Where do candidates go wrong on Rule 23?

The common slips: putting the second masthead light threshold at 20 metres instead of 50; saying a vessel stopped in the water switches off her masthead light; showing a tricolour when a sailing yacht is under engine; applying the 7-metre alternative to a tender without checking that her maximum speed does not exceed 7 knots; and confusing the hovercraft's flashing yellow with the WIG craft's high intensity flashing red.