COLREG / ARPA / AIS Bridge Rule 23(a) ✓ Free demo

Rule 23 — Power-Driven Vessel Underway Lights

NIGHT. You observe the following lights approaching from ahead: two MASTHEAD lights in a vertical line (fore lower, after higher); RED sidelight on the...

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Scenario briefing

NIGHT. You observe the following lights approaching from ahead: two MASTHEAD lights in a vertical line (fore lower, after higher); RED sidelight on the left, GREEN sidelight on the right; white sternlight not visible (you are ahead of her beam); vessel appears > 50 m in length. What type of vessel is this?

Applicable COLREG rule(s)

Rule 23(a) — referenced in this scenario. Practising this scenario reinforces correct application under realistic time pressure.

📸 Night recognition — 8 aspects

The same vessel rendered every 45° of aspect — bow, starboard bow, beam, quarter, stern, port quarter, beam, bow. Use this strip to learn how the lights present from each approach angle. Click any image to view full size.

Power-driven vessel under way viewed head-on (bow aspect) — COLREG Rule 23 night-recognition diagram — Bow

Power-driven vessel under way viewed from her starboard bow — COLREG Rule 23 night-recognition diagram — STBD bow

Power-driven vessel under way viewed from her starboard beam — COLREG Rule 23 night-recognition diagram — STBD beam

Power-driven vessel under way viewed from her starboard quarter — COLREG Rule 23 night-recognition diagram — STBD qtr

Power-driven vessel under way viewed dead astern (stern aspect) — COLREG Rule 23 night-recognition diagram — Stern

Power-driven vessel under way viewed from her port quarter — COLREG Rule 23 night-recognition diagram — Port qtr

Power-driven vessel under way viewed from her port beam — COLREG Rule 23 night-recognition diagram — Port beam

Power-driven vessel under way viewed from her port bow — COLREG Rule 23 night-recognition diagram — Port bow

Key teaching points

Rule 23(a): power-driven vessel underway shows masthead light(s) forward, sidelights (red port, green starboard), sternlight (white aft).
Vessels of ≥50 m show a second masthead light aft and higher than the forward one.
Masthead light shows from 112.5° on each side (225° arc forward); sidelights show from dead ahead to 22.5° abaft the beam (112.5° each).
Sternlight shows over 135° (67.5° each side of dead astern).

Common mistakes to avoid

Confusing one masthead light + sidelights (under-50 m PDV) with two masthead lights (over-50 m PDV) — the count is your distance/size cue.
Forgetting the sternlight arc — at certain aspects, only the sternlight is visible and the vessel can be misread as anchored.

Why it matters

The standard power-driven vessel underway light pattern is the foundation of all other Rule 21–31 light recognition. Get this baseline wrong and every other identification cascades into error — RAM, fishing, NUC, towing all reference deviations from PDV.

Exam relevance

PDV lights are the first light-recognition question in nearly every yachting and STCW exam; mastering the arcs distinguishes a competent candidate from a hesitant one.

Related scenarios

About SkipperCheck simulators

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