COLREG / ARPA / AIS Bridge Rule 31 🔒 Course / Premium

Rule 31 — Seaplane on the Water

NIGHT. You see on the water a vessel which is unable to comply fully with Rule 23 due to its construction.

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Scenario briefing

NIGHT. You see on the water a vessel which is unable to comply fully with Rule 23 due to its construction. She exhibits lights of the characteristics, positions and visibilities as closely as possible to those prescribed for a power-driven vessel but not exactly. What is she most likely to be?

Applicable COLREG rule(s)

Rule 31 — 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.

Seaplane on the water viewed head-on (bow aspect) — COLREG Rule 31 night-recognition diagram — Bow

Seaplane on the water viewed from her starboard bow — COLREG Rule 31 night-recognition diagram — STBD bow

Seaplane on the water viewed from her starboard beam — COLREG Rule 31 night-recognition diagram — STBD beam

Seaplane on the water viewed from her starboard quarter — COLREG Rule 31 night-recognition diagram — STBD qtr

Seaplane on the water viewed dead astern (stern aspect) — COLREG Rule 31 night-recognition diagram — Stern

Seaplane on the water viewed from her port quarter — COLREG Rule 31 night-recognition diagram — Port qtr

Seaplane on the water viewed from her port beam — COLREG Rule 31 night-recognition diagram — Port beam

Seaplane on the water viewed from her port bow — COLREG Rule 31 night-recognition diagram — Port bow

Key teaching points

Rule 31: a seaplane shall, where it is impracticable to comply with all rules, comply as far as practicable.
In practice, expect to see sidelights and a tail/wing arrangement approximating a sternlight.
Treat a seaplane on water like a vessel of small size with limited manoeuvring — she cannot reverse easily and turns wide.
When taking off or landing, she may not be controllable.

Common mistakes to avoid

Treating a seaplane as a navigational hazard outside COLREG — she is a vessel and Rule 31 covers her.
Approaching too close during take-off run — propeller wash, wingtip vortex and unpredictable steering.

Why it matters

A seaplane on the water is a vessel for COLREG purposes — and Rule 31 says she shall, so far as practicable, exhibit the lights of her size and type. In practice this is a complex compromise because the aircraft's lights are designed for aviation, not navigation.

Exam relevance

Rule 31 seaplane treatment is a niche topic in STCW OOW orals; not commonly examined in yachting unless in seaplane-heavy waters.

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