What is 1 prolonged blast from a vessel?

Rules of the Road.

Rule 34

Maneuvering and Warning signals

International

(e) A vessel nearing a bend or an area of a channel or fairway where other vessels maybe obscured by an intervening obstruction shall sound one prolonged blast. Such signal shall be answered with a prolonged blast by any approaching vessel that maybe within hearing around a bend or behind the intervening obstruction

Rule 34

Inland

(g) When a power driven vessel is leaving a dock or a berth, she shall sound one prolonged blast.