What do horn signals mean on a boat?

One short blast means I am turning to starboard, two short blasts I am turning to port, three short blasts I am going astern, and five or more short blasts mean I doubt your intentions (COLREGs Rule 34). Fog has its own signals.

A short blast lasts about one second and a prolonged blast four to six seconds (Rule 32). The main signals are:

Manoeuvring signals (Rule 34), made by power-driven vessels in sight of one another:

• one short blast: I am altering my course to starboard;
• two short blasts: I am altering my course to port;
• three short blasts: I am operating astern propulsion.

These describe what the vessel is doing now, not a request. A sailing boat under sail does not make them, but must understand them.

Warning signals, for any vessel:

• five or more short and rapid blasts: I fail to understand your intentions, or I doubt that you are taking sufficient action to avoid a collision;
• one prolonged blast: I am approaching a bend or an area where other vessels may be hidden. A vessel hearing it round the bend answers with one prolonged blast.

In restricted visibility (Rule 35), at intervals of not more than two minutes:

• one prolonged blast: a power-driven vessel making way through the water;
• two prolonged blasts: a power-driven vessel under way but stopped;
• one prolonged followed by two short: a sailing vessel, or a vessel not under command, restricted in her ability to manoeuvre, fishing or towing;
• rapid ringing of a bell for about five seconds every minute: a vessel at anchor.

A vessel under 12 metres need not give these exact fog signals, but must make some other efficient sound signal at intervals of not more than two minutes. A small boat should carry a horn, and a gas horn is the usual choice.

In fog, sound alone cannot tell you reliably where another vessel is, so slow down, listen, and keep out of shipping channels.

More in the book: Chapter 13, The rules of the road