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...

▶

Try this scenario in the live simulator now Hands-on practice — runs in your browser, no install needed.

Launch scenario →

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

Why it matters

COLREG knowledge is the foundation of safe ship handling. Most maritime collisions happen because watchkeepers either misidentified the situation, applied the wrong rule, or acted too late. This scenario lets you train pattern recognition + correct response under time pressure — the exact skill the rules are designed to support. RYA, Yachtmaster, STCW and most national exams expect you to apply COLREGs reflexively, not just recite them.

Related scenarios

About SkipperCheck simulators

SkipperCheck offers two browser-based maritime training simulators:

ARPA · AIS · COLREG Bridge Simulator — 54 scenarios covering Rules 2, 5–10, 12–19, 23–30, 34 and 35.
VHF SRC Radio Simulator — 15 scenarios: voice Mayday, DSC distress, Mayday Relay, Pan-Pan, Sécurité, routine.

Both run in any modern browser, on desktop or mobile. No install, no plugins.