33 CFR § 83.34 - Maneuvering and warning signals (Rule 34).
(a) When power-driven vessels are in sight of one another and meeting or crossing at a distance within half a mile of each other, each vessel underway, when maneuvering as authorized or required by these Rules:
(i) Shall indicate that maneuver by the following signals on her whistle:
(1) One short blast to mean “I intend to leave you on my port side”;
(2) Two short blasts to mean “I intend to leave you on my starboard side”; and
(3) Three short blasts to mean “I am operating astern propulsion”.
(ii) Upon hearing the one or two blast signal of the other shall, if in agreement, sound the same whistle signal and take the steps necessary to effect a safe passing. If, however, from any cause, the vessel doubts the safety of the proposed maneuver, she shall sound the signal specified in paragraph (d) of this Rule and each vessel shall take appropriate precautionary action until a safe passing agreement is made.
(b) A vessel may supplement the whistle signals prescribed in paragraph (a) of this Rule by light signals:
(i) These signals shall have the following significance:
(1) One flash to mean “I intend to leave you on my port side”;
(2) Two flashes to mean “I intend to leave you on my starboard side”;
(3) Three flashes to mean “I am operating astern propulsion”;
(ii) The duration of each flash shall be about 1 second; and
(iii) The light used for this signal shall, if fitted, be one all-round white or yellow light, visible at a minimum range of 2 miles, synchronized with the whistle, and shall comply with the provisions of Annex I to these Rules (33 CFR part 84).
(c) When in sight of one another:
(1) One short blast to mean “I intend to overtake you on your starboard side”;
(2) Two short blasts to mean “I intend to overtake you on your port side”; and
(ii) The power-driven vessel about to be overtaken shall, if in agreement, sound a similar sound signal. If in doubt she shall sound the signal prescribed in paragraph (d) of this Rule.
(d) When vessels in sight of one another are approaching each other and, from any cause, either vessel fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle. Such signal may be supplemented by a light signal of at least five short and rapid flashes.
(e) A vessel nearing a bend or an area 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 prolonged blast by any approaching vessel that may be within hearing around the bend or behind the intervening obstruction.
(f) If whistles are fitted on a vessel at a distance apart of more than 100 meters, one whistle only shall be used for giving maneuvering and warning signals.
(h) A vessel that reaches agreement with another vessel in a head-on, crossing, or overtaking situation, as for example, by using the radiotelephone as prescribed by the Vessel Bridge-to-Bridge Radiotelephone Act (85 Stat. 164; 33 U.S.C. 1201 et seq.), is not obliged to sound the whistle signals prescribed by this Rule, but may do so. If agreement is not reached, then whistle signals shall be exchanged in a timely manner and shall prevail.