Horn and bell

Short and prolonged blasts, the meeting and overtaking signals, the danger signal and what you hear in fog.

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Lights tell you what a vessel is; her whistle tells you what she is about to do. From the U.S. Inland Navigation Rules (33 CFR Part 83, Rules 32-35).

Two lengths of blast

A short blast lasts about 1 second. A prolonged blast lasts 4 to 6 seconds. Every signal below is built from those two.

Meeting or crossing within half a mile (Inland)

One short: "I intend to leave you on my port side".
Two short: "I intend to leave you on my starboard side".
Three short: "I am operating astern propulsion".

The other vessel answers with the same signal if she agrees. On inland waters these are proposals that need an answer; under the International Rules one and two short blasts instead announce a turn to starboard or port that is already being made.

Overtaking (Inland)

In sight of each other, the overtaking power-driven vessel sounds one short blast to pass on your starboard side, two short to pass on your port side. Agree by repeating it.

Doubt, bends and docks

Five or more short and rapid blasts: the danger (doubt) signal. Slow down and sort it out.
One prolonged: nearing a bend where vessels may be hidden; any vessel around the bend answers with one prolonged. Under the Inland Rules a power-driven vessel leaving a dock or berth also sounds one prolonged blast.

In fog, every two minutes or less

Sound: prolonged, short, short (every 2 minutes or less).

One prolonged and two short: a vessel that cannot easily get out of your way.

One prolonged: power-driven, making way.
Two prolonged: power-driven, underway but stopped.
One prolonged, two short: not under command, restricted in ability to maneuver, sailing, fishing, or towing/pushing.
One prolonged, three short: a manned vessel being towed.
Bell rung rapidly for about 5 seconds, every minute: at anchor. Aground adds three distinct strokes before and after.

Small craft get some relief: under 12 m a vessel need not carry a whistle or bell, but must have some other means of making an efficient sound signal.