Sound and light signals let vessels in sight of one another communicate their intentions, and let vessels in restricted visibility announce their presence.
Definitions (Rule 32)
- Short blast: about one second.
- Prolonged blast: four to six seconds.
Manoeuvring and warning signals (Rule 34)
Power-driven vessels in sight of one another and underway, when manoeuvring, sound:
- 1 short blast: "I am altering my course to starboard."
- 2 short blasts: "I am altering my course to port."
- 3 short blasts: "I am operating astern propulsion."
A vessel in doubt about the other's intentions or whether sufficient action is being taken sounds at least five short and rapid blasts (doubt signal).
Overtaking in a narrow channel
- Two prolonged followed by one short: "I intend to overtake you on your starboard side."
- Two prolonged followed by two short: "I intend to overtake you on your port side."
- Prolonged, short, prolonged, short: the agreement of the vessel to be overtaken.
Bends
A vessel nearing a bend where other vessels may be hidden sounds one prolonged blast; the reply is also one prolonged blast.
Restricted visibility (Rule 35)
- Power-driven vessel making way: one prolonged blast at intervals of not more than 2 minutes.
- Power-driven vessel underway but stopped and making no way: two prolonged blasts at intervals of not more than 2 minutes.
- NUC, RAM, constrained by draught, sailing, fishing, or towing/pushing: one prolonged followed by two short blasts at intervals of not more than 2 minutes.
- Vessel at anchor: rapid ringing of the bell for about 5 seconds at intervals of not more than 1 minute; vessels of 100 m or more also sound a gong aft.
Attracting attention and distress (Rules 36-37)
A vessel may make light or sound signals to attract attention, provided they cannot be mistaken for any signal authorised elsewhere in the Rules. Distress signals are listed in Annex IV.
Memory aid: 1 short = starboard, 2 short = port, 3 short = astern.