1.13 Lights for Power-Driven Vessels (Rule 23)

Why Rule 23 Is the Night Baseline for Master 100

On the ONC06 Master Less Than 100 GRT path, Q100 Rules of the Road is 50 questions at 90%. Night recognition of lights is a dense slice of that module. Rule 23 defines what an ordinary power-driven vessel underway must show. Almost every later special light picture (towing, fishing, NUC, RAM, pilot, anchored) is either an addition to or a substitution for this baseline. If you cannot recite Rule 23 cold, every other Part C rule will feel like a jumble of colors.

Most Master LT 100 platforms — day-charter, dinner cruise, water taxi, inspected small passenger vessel under 100 GRT — are power-driven vessels well under 50 meters. You must still know the ≥50 m second masthead rule because Q100 will place you meeting a freighter, ferry, or offshore supply vessel at night, not only another six-pack.

Rule 23(a) — Standard Power-Driven Picture

A power-driven vessel underway shall exhibit:

1. A masthead light forward.
2. A second masthead light abaft of and higher than the forward one — except that a vessel of less than 50 meters in length shall not be obliged to exhibit such light but may do so.
3. Sidelights (red port, green starboard).
4. A sternlight (white, 135° aft sector).

Length thresholds you must memorize

The masthead light is a white light showing over an unbroken 225° arc from dead ahead to 22.5° abaft the beam on either side — the same arc as both sidelights together. At night, seeing two white masthead lights in a line (or nearly so) plus both sidelights is a classic cue for a head-on or nearly head-on meeting with a larger power-driven vessel (Rule 14 night recognition).

Why two mastheads matter operationally

On a vessel of 50 m or more, the second, higher, after masthead gives other watchstanders range and aspect: two whites stacked in depth tell you the vessel is large and show whether you are closing on the bow or seeing her quarter. On your own Master 100 passenger boat under 50 m you are not required to carry the second masthead, but many commercial craft still fit a second light or a combination that mimics masthead geometry. Exam answers stick to the rule minimum, not what a particular builder installed.

Rule 23(b) — Air-Cushion Vessels

An air-cushion vessel when operating in the non-displacement mode shall, in addition to the lights prescribed in Rule 23(a), exhibit an all-round flashing yellow light. When the craft is floating and operating as a conventional displacement hull, the extra yellow flash is not the non-displacement signature — the rule ties the yellow to non-displacement mode. Q100 likes the color: yellow flashing, not red or blue.

Rule 23(c) — WIG Craft

A WIG craft (wing-in-ground) only when taking off, landing, and in flight near the surface shall, in addition to Rule 23(a) lights, exhibit a bright all-round flashing red light. Ordinary power-driven vessels do not get a free red flash; do not confuse WIG red flash with NUC red-over-red or with emergency blue lights (which are not Navigation Rules navigation lights).

Rule 23(d) — Small Power-Driven Alternatives

Under 12 meters

A power-driven vessel of less than 12 meters in length may, instead of the lights prescribed in Rule 23(a), exhibit an all-round white light and sidelights. The all-round white replaces the combination of masthead + sternlight geometry with a single white that can be seen from all directions, still paired with red/green sidelights for aspect.

Under 7 meters and slow

A power-driven vessel of less than 7 meters in length whose maximum speed does not exceed 7 knots may, instead of the lights prescribed in Rule 23(a), exhibit an all-round white light and shall, if practicable, also exhibit sidelights. This is the classic skiff / small launch concession. On a crowded harbor night as Master of a passenger vessel, treat that lonely white as a small power-driven craft until proven otherwise — and do not expect textbook sidelights on every dinghy.

Where to place the all-round white

The all-round white on a small vessel should be exhibited as nearly as practicable at the fore and aft centerline. Placement off-center can create confusing light geometry for other vessels reading aspect.

What Rule 23 Is Not

• Towing light (yellow above sternlight): Rule 24, not Rule 23. An ordinary underway powerboat does not show a yellow towing light.
• Anchor light: Rule 30. When you are at anchor you are not showing the Rule 23 underway power picture.
• Special flashing yellow (Inland towing/pushing contexts): not the same as the air-cushion all-round flashing yellow of Rule 23(b). Keep Inland special towing lights in the towing chapter and do not paste them onto every power-driven vessel.
• Sailing vessel under power: once machinery is engaged she is power-driven for lights and steering classification (Rule 3 + cone by day under Rule 25 when also under sail).

Master Scenario: Night Passenger Run Meeting a Freighter

You command an 85-foot (≈26 m) inspected passenger vessel on a near-coastal dinner cruise. Your lawful Rule 23 picture: one masthead light forward, sidelights, sternlight (second masthead optional because you are under 50 m). Fine on the starboard bow you see two white masthead lights nearly in a line and both red and green sidelights. That night picture supports a large power-driven vessel nearly head-on — apply Rule 14 (both alter to starboard for a port-to-port pass), not "I am stand-on because I carry passengers." If instead you see only a single white with no sidelights yet, do not assume freighter geometry; it may be a small craft under the <12 m or <7 m alternatives, an anchor light, or a stern aspect — keep looking and use radar/AIS as all available means (Rule 5).

Quick recognition table (power-driven underway)

Exam Framing for Q100 (90%)

Highest-yield Rule 23 facts: masthead + sidelights + sternlight; second masthead required at 50 m and above; <12 m all-round white + sidelights alternative; <7 m / ≤7 kn all-round white; air-cushion flashing yellow; WIG flashing red; and no yellow towing light on ordinary Rule 23 vessels. Master stems often cast you as the passenger-vessel operator reading another vessel's lights — answer from the rule table, not from "what the charter fleet usually shows."