Official rule text as published in the U.S. Coast Guard Navigation Rules
| International | Inland |
|---|
| (c) When in sight of one another in a narrow channel or fairway: (i) a vessel intending to overtake another shall, in compliance with Rule 9(e)(i), indicate her intention by the following signals on her whistle — two prolonged blasts followed by one short blast to mean “I intend to overtake you on your starboard side”; two prolonged blasts followed by two short blasts to mean “I intend to overtake you on your port side”. (ii) the vessel about to be overtaken, when acting in accordance with Rule 9(e)(i), shall indicate her agreement by sounding one prolonged, one short, one prolonged and one short blast, in that order. | (c) When in sight of one another: (i) a power-driven vessel intending to overtake another power-driven vessel shall indicate her intention by the following signals on her whistle — one short blast to mean “I intend to overtake you on your starboard side”; two short blasts to mean “I intend to overtake you on your port side”. (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 (d). |
(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 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.
| Inland |
|---|
| (g) When a power-driven vessel is leaving a dock or berth, she shall sound one prolonged blast. |
| (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 (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. |
MyCOLREGS explanation
Summary
If a vessel does not understand what another vessel is doing, or doubts that the other vessel is taking enough action to avoid a collision, Rule 34(d) requires her to immediately sound at least five short and rapid blasts on the whistle - the universal signal for danger or doubt. On U.S. inland waters this same signal also answers a proposed passing arrangement you are not comfortable with, so both vessels know to hold off and reassess rather than one side going along with a maneuver it never actually agreed to.
Real World Example
Sounding the danger signal isn't optional venting - Rule 34(d) makes it a mandatory duty the instant you're in doubt, not a discretionary courtesy, and on inland waters it specifically obligates both boats to hold off and reassess rather than one side proceeding with a maneuver the other never actually agreed to.
Practice Questions
Q1. You are approaching another power-driven vessel and cannot tell whether she intends to pass port-to-port or starboard-to-starboard; she has not sounded a signal you understand. What should you do under Rule 34(d)?
A. Turn hard to starboard without signaling.
B. Hold your course and speed and wait for her to signal first.
C. Sound at least five short and rapid blasts to indicate doubt.
D. Sound one prolonged blast to request she stop.
Q2. On U.S. inland waters, a vessel proposes a passing arrangement with one short blast, but you are not comfortable with it. What is the correct response?
A. Sound the same one-short-blast signal back, indicating agreement you do not actually have.
B. Sound two prolonged blasts to request she leave the channel.
C. Ignore the proposal and maintain your course.
D. Sound the Rule 34(d) five-short-blast signal to indicate doubt, and take precautionary action.
Practice Question Answers
Q1.
C. Sound at least five short and rapid blasts to indicate doubt. Rule 34(d) requires a vessel that is in doubt about another vessel's intentions or actions to immediately sound at least five short and rapid blasts, which may be supplemented by five short, rapid flashes. A skips the required doubt signal and turns blind. B just waits, which doesn't address the uncertainty. D names a signal (one prolonged blast) that means something else entirely, not doubt.
Q2.
D. Sound the Rule 34(d) five-short-blast signal to indicate doubt, and take precautionary action. Under the Inland Rules, if you doubt the safety of a proposed passing maneuver you must sound the Rule 34(d) danger/doubt signal rather than silently disagreeing or going along with it. A has you falsely signaling agreement, which is worse than doing nothing. B invents a signal the rule doesn't prescribe for this situation. C just ignores the proposal, leaving the uncertainty unresolved - the doubt signal exists precisely so you don't have to guess or stay silent.