One short blast means 'I am changing course to starboard.'
Two short blasts mean 'I am changing course to port.'
Three short blasts mean 'I am operating astern.'
Two prolonged blasts followed by one short blast mean 'I intend to overtake you on your starboard side';
Two prolonged blasts followed by two short blasts mean 'I intend to over-take you on your port side.'
A long blast every two minutes is used when operating in fog.
Ten short blasts means "I was leaning on the horn button"
There are others, including responses to the above.
e.g.
- a vessel being overtaken is expected to reply with one prolonged, one short, one prolonged and one short blast.
- When vessels in sight of one another are approaching and from any cause either vessel fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle. Such signal may be supplemented by at least five short and rapid flashes.
- A vessel nearing a bend or an area of a channel or fairway where other vessels may be obscured by an intervening obstruction shall sound one prolonged blast. Such signal shall be answered with a prolonged blast by any approaching vessel that may be within hearing around the bend or behind the intervening obstruction.
The term "short blast" means about one second's duration.
The term "prolonged blast" means about four to six seconds' duration.
More details in the International Maritime Organization (IMO) 1972 COLREGS (Collision Avoidance at Sea Regulations).
http://www.boatsafe.com/nauticalknowhow/boating/colregs.html
