Reprinted from:

COMDTINST, M16672.2, Page 119 Inland Sound and Light Signals

RULE 35

Sound Signals in Restricted Visibility

In or near an area of restricted visibility, whether day or night, the signals prescribed in this rule shall be used as follows:

1. A power-driven vessel making way through the water shall sound, at regular intervals of not more than two minutes, one prolonged blast.
2. A power-driven vessel underway,  but stopped and making no way through the water shall sound, at intervals of not more than two minutes, two prolonged blasts in succession with an interval of about two seconds between them.
3. A vessel not under command; a vessel restricted in her ability to maneuver, whether underway or at anchor; a sailing vessel; a vessel engaged in fishing, whether underway or at anchor; and a vessel engaged in towing or pushing another vessel shall, instead of the signals prescribed in paragraphs 1 or 2 of this Rule, sound at intervals of not more than two minutes, three blasts in succession; namely one long blast followed by two short blasts.