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31

Seaplanes

Left: seaplane afloat; right: WIG craft in near-surface flight. Seaplanes and WIG craft unable to use the prescribed positions must comply as closely as possible. WIG craft also show the prescribed flashing red light during take-off, landing and flight near the surface.

1The rule in plain words

In one sentence…

When a seaplane or WIG craft cannot show the prescribed lights and shapes, it must use ones as closely similar as possible.

When does it apply?

When it is impracticable for a seaplane or WIG craft to exhibit the prescribed lights and shapes, she shall exhibit similar lights and shapes.

Similar in what way?

They must be as closely similar as possible in characteristics and position to those prescribed in the Rules of this Part.

2See it in 3D

Turn the vessel or pick bow, stern, port or starboard: this is how her lights look at night.

An interactive 3D illustration is shown here. The same content is described in the rule text and key takeaways below.

3Key points

  1. 1.

    COLREG · Rule 31 → seaplane · WIG craft.

  2. 2.

    Trigger → prescribed display is impracticable.

  3. 3.

    Substitute → lights and shapes.

  4. 4.

    Test → closest possible match in characteristics + position.

4Frequent mistakes

  • Wrong: "If the prescribed display is impracticable, the vessel is exempt."

    Right: It must exhibit lights and shapes as closely similar as possible.

  • Wrong: "Any alternative lights and shapes are acceptable."

    Right: They must be as similar as possible in characteristics and position.

  • Wrong: "Rule 31 covers seaplanes, not WIG craft."

    Right: It covers both a seaplane and a WIG craft.

5Exam keys & interpretation

Exam keys

  • "Impracticable" means it cannot exhibit them, not that it simply chooses another arrangement.

  • "Characteristics and position": the answer must address both, not just one.

  • "As closely similar as possible" asks for the nearest match, not a random alternative.

How to read it

  • Read it as a conditional adaptation: first the prescribed display is impracticable; then choose the closest possible match.

  • Compare the substitute with what the Rules of this Part prescribe, in both characteristics and position.

6Official text

IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)

Full wording of the rule, reproduced from the official source below.

Where it is impracticable for a seaplane or a WIG craft to exhibit lights and shapes of the characteristics or in the positions prescribed in the Rules of this Part she shall exhibit lights and shapes as closely similar in characteristics and position as is possible.
Source:

7Test your knowledge

Question 1/4

When does Rule 31 apply?

More questions on this rule →

Keep studying…

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Previous: Rule 30 · Anchored Vessels and Vessels Aground

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Frequently asked questions about COLREG Rule 31

Are seaplanes exempt from showing navigation lights?

No. Rule 31 requires a seaplane to exhibit lights and shapes as closely similar in characteristics and position as is possible whenever the characteristics or positions prescribed in Part C are impracticable for her.

Does COLREG Rule 31 apply to WIG craft?

Yes. Rule 31 covers seaplanes and WIG craft alike, allowing the closest practicable equivalent when the prescribed light and shape characteristics or positions cannot be met.

What must a seaplane show when the prescribed light positions are impracticable?

Lights and shapes as closely similar in characteristics and position as is possible to those prescribed in Part C. Practical limitations are acknowledged, but the obligation to be lit is not waived.