What does one prolong blast of a horn mean?

On a ship, a prolonged blast of a horn can be used for various reasons. It is primary used to alert other ships to the ship's presence.

The ship is leaving it's berth or approaching a blind bend in a channel or river.

In restricted visibility a power driven vessel making way.

In sight warning signal of approaching a bend in a channel.

Generally the "Horn" is called a Ship's whistle. A prolonged blast could mean several different things, all being dependent on the situation.

One prolonged blast could mean:

Rule 34 (e) "A vessel nearing a bend of a channel or fairway, where other vessels may be obscured by an intervening obstruction shall sound ONE PROLONGED BLAST. This signal shall be answered with a prolong blast by any approaching vessel that maybe within hearing around the bend or behind the intervening obstruction."

OR

Rule 35 "In or near an area of restricted visibility, whether by day or night, the signals prescribed in this rule shall be used as follows: A power driven vessel making way through the water, shall at intervals of not more than two minutes, sound ONE PROLONGED BLAST.

OR

A power driven vessel when leaving a dock or berth will sound ONE PROLONGED BLAST.