Large vessels sound a series of specific signals to indicate their intentions to other vessels in the vicinity. Some of the more common signals are listed below.
| Description | Signal |
|---|---|
| Short blast - about one second duration | |
| Prolonged blast - four to six seconds duration | |
| I am altering my course to starboard | One short blast on a horn or similar device. |
| I am altering my course to port | Two short blasts on a horn or similar device. |
|
I am operating astern propulsion engines in reverse - vessel slowing down, stopping or intends going astern | Three short blasts on a horn or similar device. |
| Signal by vessel in doubt as to the intentions of the other vessel | Five short blasts on a horn or similar device. |
| Vessel nearing a blind bend in a channel or river | One long blast on a horn or similar device. |
Diver below flag
The signal flag for the letter 'A' shown below is used internationally to indicate 'I have a diver below - keep well clear at slow speed'.
The flag is coloured white and blue and may be displayed either from a vessel or floating buoy. It is an offence in South Australia to exceed four knots within 50 metres of a vessel or buoy displaying this flag.
