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Rule 37: Distress Signals (COLREGS)

Last reviewed October 21, 2026

The short answer

Rule 37 says a vessel in distress and requiring assistance uses or exhibits the signals in Annex IV. They include a continuous sounding of any fog-signal apparatus, SOS, the spoken word Mayday, red flares, orange smoke, and slowly raising and lowering outstretched arms. Inland adds a high intensity white light flashing 50 to 70 times a minute.

The rule text

When a vessel is in distress and requires assistance she shall use or exhibit the signals described in Annex IV to these Regulations.

International Rules, U.S. Coast Guard Navigation Rules and Regulations Handbook (NAVCEN, 2024 corrected edition), Rule 37

A high intensity white light flashing at regular intervals from 50 to 70 times per minute.

Inland Rules, 33 CFR Part 83 (eCFR), Rule 37

What it means on the water

  • Annex IV lists the signals; Rule 37 just points to it.
  • Examples: a gun or explosive signal about every minute, continuous fog signal, rockets throwing red stars, SOS, Mayday, NC flags, square flag with a ball, flames, red flare, orange smoke, raising and lowering arms, DSC and EPIRB alerts.
  • Using these signals except for distress is prohibited (Annex IV).

International vs Inland

Inland Annex IV (33 CFR Part 87) adds a high intensity white light flashing 50 to 70 times per minute. The NAVCEN handbook notes this is the only difference.

Common exam traps

  • Calling the Rule 27 NUC lights a distress signal. Rule 27(h) says they aren't.
  • Using the flashing white distress light in International waters.

Try 3 questions on Rule 37

  1. Where are the distress signals listed?

    1. Annex I
    2. Annex III
    3. Annex IV
    4. Rule 36
    Show answer

    C. Annex IV Rule 37.

  2. Which is a distress signal?

    1. Two all-round reds
    2. Orange smoke
    3. Five short blasts
    4. One prolonged
    Show answer

    B. Orange smoke Annex IV.

  3. Inland-only distress signal?

    1. Mayday
    2. High intensity white light flashing 50 to 70 per minute
    3. SOS
    4. Red flare
    Show answer

    B. High intensity white light flashing 50 to 70 per minute Inland Annex IV, 33 CFR 87.01(p).

Frequently asked questions

What are the distress signals?

Those in Annex IV, including continuous fog signal, SOS, Mayday, red flares, orange smoke, and raising and lowering arms.

Is continuous sounding of a horn a distress signal?

Yes. A continuous sounding with any fog-signaling apparatus is listed in Annex IV.

Inland difference?

Inland adds a high intensity white light flashing 50 to 70 times per minute.

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