Definitions and classifications
| Whistle | Means of making short or long blasts |
| Short blast | About one second duration  |
| Prolonged blast | 4-6 seconds duration  |
| Vessels of 100 m or more in length | Use bell, whistle and gong  |
| Vessels of 20 m or more in length | Use bell and whistle  |
| Vessels of 12 m or more in length | Use whistle |
| Vessels less than 12 m in length | Use any efficient sound signal |
Manoeuvring and warning signals when vessels are in sight of one another
Whistle signals below may be supplemented by light signals using the same code.
| Description | Signal |
|---|---|
| I am altering my course to starboard | |
| I am altering my course to port | |
| I am operating astern propulsion | |
| I am unsure of your intentions, or doubt whether you are taking enough action to avoid collision | |
Warning signals – vessels in narrow channels
When the vessel overtaking or being overtaken must take action to permit safe passing.
| Description | Signal |
|---|---|
| I intend to overtake on your starboard | |
| I intend to overtake on your port | |
| Agreement by overtaken vessel | |
| A vessel in doubt about signals, intentions or safety of the proposed manoeuvre of an overtaking vessel | |
| Vessel nearing blind bend in channel | |
| Vessel other side of bend repeats | |