Sound signals cheat sheet
short blast, about 1 second · prolonged blast, 4 to 6 seconds
Manoeuvring and warning signals (Rule 34), vessels in sight of each other
| I am altering my course to starboard | In sight | |
| I am altering my course to port | In sight | |
| I am operating astern propulsion | In sight | |
| I am in doubt of your intentions — danger signal | In sight | |
| Approaching a bend or obscured area, or leaving a berth | Any visibility | |
| I intend to overtake you on your starboard side | Narrow channel | |
| I intend to overtake you on your port side | Narrow channel | |
| Agreement to be overtaken | Narrow channel |
Restricted visibility (Rule 35)
| Power-driven vessel making way | Every 2 minutes | |
| Power-driven vessel underway but stopped | Every 2 minutes | |
| Sailing, fishing, towing, NUC, RAM or CBD vessel | Every 2 minutes | |
| Last vessel of a manned tow | Every 2 minutes | |
| 🔔 | At anchor — rapid ringing of the bell for 5 seconds | Every minute |
| At anchor — optional warning to an approaching vessel | As needed | |
| Pilot vessel on duty (in addition to her other signals) | Optional |
A vessel under 12 m need not give these signals but must make some other efficient sound signal at least every 2 minutes. Five or more short blasts means “I don’t understand your intentions”: be ready to act.