What does 5 short blasts of the horn mean?

Answer 1

It means "Man overboard".

Note:

1 long blast = 5 or 6 seconds

1 short blast = 1 or 2 seconds

Three short blasts means "My engines are going Astern". (It doesn't necessarily mean that the ship itself is going astern).

Answer 2

Oh, dude, 5 short blasts of the horn means that someone is probably having a horn-playing party in their car. Just kidding! It's actually the signal for danger or that you're about to collide with another vessel. So, like, maybe pay attention if you hear that, unless you're into boat collisions, I guess.

Answer 3

International and Inland Navigational rules. Sound and Light Signals.

Rule 34

Maneuvering Signals(a) When vessels are in sight of one another, a power driven vessel underway, when maneuvering as authorized or required by these Rules, shall indicate than maneuver by the following signals on her whistle: -Three short blasts to mean "I am operating astern propulsion".

Answer 4

Rules of the Road

Sound & light signals

Rule 34 Maneuvering Signals and warning signals

(d) When vessels in sight of one another are approaching each other and from any cause either vessel fails to understand the intentions or action of the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least 5 short and rapid blast of the whistle. Such signal maybe supplemented by a light signal of at least five short and rapid flashes.