Pibal
Probably a lot more here than you want to know…
Here’s an excerpt from http://www.safeboatingcard.com/Manual/Chapter5/Chapter5_4.jsp
Sound Signals
To let the other vessels know which way you will be heading, you signal them with either a short or long blast of your horn (or other sound signal device) or a combination of short and/or long blasts. A short blast lasts about 1 second, and a long blast lasts 4 to 6 seconds.
If you are the give-way vessel, you will give the proper signal, and if you are the stand-on vessel, you will repeat the signal to let the other vessel know you understand their intentions. If both vessels are the give-way vessel, either one can initiate the signal.
If you are ever unsure of the intentions of the other vessel or if there is danger ahead, sound five or more short blasts.
The following table lists the navigation situations and the appropriate sound signals.
Sound Signals
One short blast:
I intend to leave you on my port side (I intend to turn to the right and pass you on your port side)
Two short blasts:
I intend to leave you on my starboard side (I intend to turn to the left and pass you on my starboard side)
Three short blasts:
I am operating astern propulsion (astern propulsion is used to slow a ship by applying a force in the direction of the stern of the ship, instead of the bow), which means you are slowing down or even moving backward.
Five or more short blasts:
I don’t understand your intentions or there is danger.
One prolonged blast:
I’m coming to a bend or obstruction and you may not be able to see me.
One prolonged blast every two minutes:
I am in a power boat. Visibility is reduced (because of fog, for example), and you may not be able to see me.
One prolonged blast plus two short blasts every two minutes:
I am in a vessel constrained by her draft or restricted in her ability to maneuver, a sailing vessel, a vessel engaged in commercial fishing, or a vessel engaged in towing or pushing another vessel. Visibility is reduced, and you may not be able to see me.
Two prolonged blasts followed by one short blast:
I am in International Waters in a narrow channel or fairway and I intend to overtake you on your starboard side.
Two prolonged blasts followed by two short blasts:
I am in International Waters in a narrow channel or fairway and I intend to overtake you on your port side.
One prolonged, one short, one prolonged and one short blast:
I am in International Waters in a narrow channel or fairway and I understand your intentions to overtake me.
Hope that helps…