Print this out and tape it to your fog horn canister.  

When in sight of one another, a power-driven vessel underway, when manoeuvring sounds the following (can be supplemented with all arond white light with visible range of 5 miles 1 second flashes with 1 second between and 2 seconds between signals)

– one short blast to mean “I am altering my course to starboard”.
– two short blasts to mean “I am altering my course to port”.
– three short blasts to mean “I am operating astern propulsion”. 

In sight of one another in a narrow channel or fairway when intending to overtake

– two prolonged blasts followed by one short blast to mean “I intend to overtake you on your starboard side”.
– two prolonged blasts followed by two short blasts to mean “I intend to overtake you on your port side”.

To indicate agreement of being overtaken

– one prolonged, one short, one prolonged and one short blast, in that order.

– five short and rapid blasts on the whistle or light signal – fail to understand your intentions and are in doubt whether you are taking sufficient action to avoid collision,

– one prolonged blast vessel nearing a bend or an area of a channel or fairway where other vessels may be obscured and response if you hear it and don’t see the vessel