A seaplane can't fit standard lights. Rule 31 says:

Show lights as closely similar as possible

Rule 31: Seaplanes (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 31 says that where it's impracticable for a seaplane or WIG craft to show lights and shapes with the characteristics or positions the rules require, she shows lights and shapes as closely similar in characteristics and position as possible. It's a practical fallback, not a separate light pattern.

The rule text

she shall exhibit lights and shapes as closely similar in characteristics and position as is possible.
International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 31

What it means on the water

Applies to seaplanes and WIG craft.
Only where the standard lights or positions are impracticable.
Show the closest possible equivalent.
Seaplanes on the water generally keep well clear of vessels (Rule 18(e)).

International vs Inland

Same in both International and Inland.

Common exam traps

Inventing a special seaplane light. Rule 31 asks for the closest equivalent to the standard lights.

Try 3 questions on Rule 31

A seaplane can't fit standard lights. Rule 31 says:
1. Show no lights
2. Show lights as closely similar as possible
3. Show a flashing red
4. Stay ashore
Show answer
B. Show lights as closely similar as possible Rule 31.
Rule 31 also covers:
1. WIG craft
2. Submarines
3. Barges
4. Tugs
Show answer
A. WIG craft Rule 31.
Which rule says a seaplane on the water generally keeps clear of vessels?
1. Rule 13
2. Rule 18(e)
3. Rule 31
4. Rule 9
Show answer
B. Rule 18(e) Rule 18(e).

Frequently asked questions

What lights does a seaplane show?

The standard lights where practicable; otherwise lights as closely similar as possible (Rule 31).