COLREG / ARPA / AIS Bridge Rule 24(a) 🔒 Course / Premium

Rule 24 — Towing Vessel Lights

NIGHT. You see an approaching vessel showing: TWO masthead lights in a vertical line (forward mast); red & green sidelights; white sternlight + a YELLOW...

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Scenario briefing

NIGHT. You see an approaching vessel showing: TWO masthead lights in a vertical line (forward mast); red & green sidelights; white sternlight + a YELLOW "towing light" directly above the sternlight; length of tow astern is less than 200 m. What is this vessel doing?

Applicable COLREG rule(s)

Rule 24(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.

Towing vessel — tow under 200 m viewed head-on (bow aspect) — COLREG Rule 24 night-recognition diagram — Bow

Towing vessel — tow under 200 m viewed from her starboard bow — COLREG Rule 24 night-recognition diagram — STBD bow

Towing vessel — tow under 200 m viewed from her starboard beam — COLREG Rule 24 night-recognition diagram — STBD beam

Towing vessel — tow under 200 m viewed from her starboard quarter — COLREG Rule 24 night-recognition diagram — STBD qtr

Towing vessel — tow under 200 m viewed dead astern (stern aspect) — COLREG Rule 24 night-recognition diagram — Stern

Towing vessel — tow under 200 m viewed from her port quarter — COLREG Rule 24 night-recognition diagram — Port qtr

Towing vessel — tow under 200 m viewed from her port beam — COLREG Rule 24 night-recognition diagram — Port beam

Towing vessel — tow under 200 m viewed from her port bow — COLREG Rule 24 night-recognition diagram — Port bow

Key teaching points

Two masthead lights vertically on the forward mast = tow length under 200 m (Rule 24(a)(i)). Three vertical = tow over 200 m.
Yellow towing light sits directly above the white sternlight — this is the single most reliable identifier from astern.
Sidelights are normal red/green — same as any power-driven vessel.
The towed object may show only sidelights and a sternlight (Rule 24(e)), or be unlit if "inconspicuous and partly submerged" (Rule 24(g)).
Treat a tug + tow as a single long obstacle. Pass well clear astern of the tow, not between tug and tow.

Common mistakes to avoid

Crossing astern of the tug without verifying tow length — the line lies just below the surface and will sever rigging or wrap a propeller.
Confusing the yellow towing light with a stern range light. The towing light is ONLY shown by vessels engaged in towing.

Why it matters

Towing vessels are among the most dangerous traffic to misread at night. The towed object — barge, dredger spoil, log raft — is often unlit or shows only minimal lights, sits low in the water, and trails 50–200 m astern of the tug. Watchkeepers who don't recognise the tug's light pattern have steered through the tow line, a mistake that has caused multiple fatal incidents documented by MAIB and NTSB.

Exam relevance

Rule 24 light recognition is a recurring oral exam topic in RYA Yachtmaster Coastal/Offshore and STCW Officer of the Watch exams; examiners describe the light pattern and ask for vessel type, tow length category, and recommended action.

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