1.13 Lights for Power-Driven Vessels Underway (Rule 23)

Rule 23: Lights for Power-Driven Vessels Underway

Rule 23 is the baseline every mariner must know by heart, because almost every other light pattern is built by adding lights to this one. It answers a single question: what does an ordinary power-driven vessel underway — including your six-pack charter — show at night?

The standard set

A power-driven vessel underway exhibits:

So a small power vessel at night shows, at minimum, one masthead light, two sidelights, and a sternlight — "masthead, sides, stern." A large ship (≥50 m) shows two masthead lights in a fore-and-aft line, the after one higher than the forward one. That second, higher light is a huge tell at night: two white masthead lights in a vertical-ish line means a big power-driven vessel, and the line between them reveals her heading — if the after (higher) light is to the right of the forward one, she is heading across you left-to-right.

The small-vessel options

Small boats get relief from carrying a full masthead-plus-sternlight rig:

• A vessel less than 12 metres may, instead of the masthead light and sternlight, show one all-round white light plus the two sidelights. (This is the classic recreational/small-commercial arrangement: an all-round white at the top of a pole, red and green on the bow.)
• A vessel less than 7 metres whose maximum speed does not exceed 7 knots may show just one all-round white light and, if practicable, also exhibit sidelights. But do not over-rely on this: if that little vessel can exceed 7 knots, she needs the sidelights, and in practice showing sidelights is always the safer choice because it lets others read your aspect.
• On a vessel less than 12 metres, the all-round white light and the masthead light need not be exactly on the centerline if that is impracticable, but they must be as close to it as possible.

Note the two thresholds are different: the all-round-white-plus-sidelights option is keyed to 12 metres, while the all-round-white-only (sidelights optional) concession requires both under 7 metres and 7 knots or less.

Air-cushion craft and WIG

Rule 23 adds two modern cases you may see on the exam:

• An air-cushion vessel (hovercraft) operating in the non-displacement mode shows, in addition to her power-driven lights, an all-round flashing yellow light.
• A WIG (wing-in-ground) craft on take-off, landing, and in flight near the surface shows, in addition, a high-intensity all-round flashing red light.

Remember the color pairing: hovercraft = flashing yellow, WIG = flashing red. Both are flashing all-round lights layered on top of the normal power-driven set.

An Inland wrinkle: the second masthead light and Western Rivers

Under the Inland Rules, a power-driven vessel of less than 20 metres is not required to carry the second (after) masthead light, and vessels operating on the Western Rivers (the Mississippi system and connected waters) and certain other specified waters get further relief — for example, they may show a single masthead light and are not required to carry the after masthead light. The practical point for a six-pack operator is that the exact rig your boat must show depends on its length and the waters you run; when in doubt, more lights and full sidelights are always safer than the bare minimum. Do not assume every large vessel you meet will carry two masthead lights — a 15-metre workboat on inland waters may legitimately show only one.

Building intuition — this rule is the "base layer"

Keep Rule 23 in your head as the default, then read every later rule as a modification:

• A tug (Rule 24) is a power-driven vessel that adds masthead lights and a yellow towing light.
• A vessel not under command (Rule 27) is a power-driven vessel that has switched off her masthead lights and added two all-round reds.
• A pilot vessel (Rule 29) adds a white-over-red all-round pair at the masthead.

If you truly know the base set, you diagnose the others by spotting what was added or removed.

Worked example

At night you see, dead ahead, two white lights in a vertical line, a red light below and to the left, and a green light below and to the right. Diagnose it: two masthead lights (the after one higher) plus both sidelights means a power-driven vessel 50 m or more, roughly head-on to you. Both sidelights showing tells you her aspect is near head-on — apply Rule 14, and both of you alter to starboard.

Now a simpler one: a single all-round white light low over the water with a red and a green just below it, closing slowly. That is a small power-driven vessel under 12 metres, bow-on. She is entitled to that simplified arrangement — do not mistake the single all-round white for an anchored vessel; the presence of sidelights tells you she is underway and making way, not at anchor.

Traps

• The second masthead light is mandatory at 50 m and above, optional below — a frequent multiple-choice discriminator.
• All-round white + sidelights is the under-12-metre option; all-round white alone (sides if practicable) needs under 7 m and ≤7 knots. Different thresholds.
• Hovercraft = flashing yellow; WIG = flashing red. Do not swap them.
• Sidelights present = underway; a lone all-round white with no sidelights is an anchored vessel (Rule 30), not a small power-driven vessel.