When a vessel is in distress and requires assistance she shall use or exhibit the signals described in Annex IV to these Regulations. Among these, the following are made by sound:
Sound signals
Played at true length: a short blast lasts about 1 second and a prolonged blast 4 to 6 seconds (Rule 32). Here a prolonged blast plays for 5 seconds.
🔊Sound
A gun or other explosive signal fired at intervals of about a minute.
🔊Sound
A continuous sounding with any fog-signalling apparatus.
📢Whistle
The group · · · – – – · · · (SOS) in the Morse Code, made by any signalling method. A Morse dash is three times the length of a dot, not a prolonged blast.
Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules
Read the whole of Rule 37 on one page
Rule 37: Distress signals
When a vessel is in distress and requires assistance she shall use or exhibit the signals described in Annex IV to these Regulations. Among these, the following are made by sound:
A gun or other explosive signal fired at intervals of about a minute.
A continuous sounding with any fog-signalling apparatus.
The group · · · – – – · · · (SOS) in the Morse Code, made by any signalling method. A Morse dash is three times the length of a dot, not a prolonged blast.
A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.