International boating rules

Manoeuvring and warning signals

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.

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.

diver below flag Blue rectangle with a smaller blue and white flag with a blue outlined sideways chevron

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Page last updated 29 November 2016

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