Official rule text as published in the U.S. Coast Guard Navigation Rules
| International | Inland |
|---|
| (a) When vessels are in sight of one another, a power-driven vessel underway, when maneuvering as authorized or required by these Rules, shall indicate that maneuver by the following signals on her whistle: | (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: |
One short blast — [International: “I am altering my course to starboard”] [Inland: “I intend to leave you on my port side”]
Two short blasts — [International: “I am altering my course to port”] [Inland: “I intend to leave you on my starboard side”]
Three short blasts — “I am operating astern propulsion” (same meaning in both).
| Inland |
|---|
| (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 Rule 34(d) and each vessel shall take appropriate precautionary action until a safe passing agreement is made. |
| International | Inland |
|---|
| (b) Any vessel may supplement the whistle signals prescribed in (a) by light signals, repeated as appropriate, while the maneuver is being carried out: (i) these signals shall have the following significance: (ii) the duration of each flash shall be about one second, the interval between flashes shall be about one second, and the interval between successive signals shall not be less than ten seconds; (iii) the light used for this signal shall, if fitted, be an all-round white light, visible at a minimum range of 5 miles, and shall comply with Annex I. | (b) A vessel may supplement the whistle signals prescribed in (a) by light signals: (i) these signals shall have the following significance: (ii) the duration of each flash shall be about one second; (iii) the light used for this signal shall, if fitted, be an all-round white or yellow light, visible at a minimum range of 2 miles, synchronized with the whistle, and shall comply with Annex I. |
One flash — [International: “I am altering my course to starboard”] [Inland: “I intend to leave you on my port side”]
Two flashes — [International: “I am altering my course to port”] [Inland: “I intend to leave you on my starboard side”]
Three flashes — “I am operating astern propulsion” (same meaning in both).
Color note: black text applies generally; blue is International Waters; green is Inland Waters.
MyCOLREGS explanation
Summary
When power-driven vessels are in sight of each other and maneuvering, they announce it on the whistle. Under the International Rules, one short blast means “I am altering my course to starboard,” two short blasts mean “I am altering my course to port,” and three short blasts mean “I am operating astern propulsion.” On U.S. inland waters, within about half a mile of another power-driven vessel, one short blast instead proposes “I intend to leave you on my port side” and two short blasts propose “I intend to leave you on my starboard side” — these are proposals the other vessel answers by sounding the same signal back, not just an announcement of a maneuver already underway. The same 1/2/3 pattern can be repeated as light flashes to supplement or substitute for the whistle.
Real World Example
Under the Inland Rules, one short blast is not permission to turn - it's only a proposal, and the maneuver isn't supposed to start until the other vessel echoes the same signal back. Treat it like the international version, an announcement of a turn already underway, and you can end up committing to a maneuver nobody else has actually agreed to.
Practice Questions
Q1. Operating under the Inland Rules, within about half a mile of another power-driven vessel, you sound one short blast. What are you telling her?
A. I am altering my course to starboard.
B. I intend to leave you on my port side.
C. I am altering my course to port.
D. I am operating astern propulsion.
Q2. You're on an inland river, closing head-on with a tow (a barge pushed by a towboat) per your AIS. You call the tow on VHF, and the captain says "pass on the one whistle." Which side does he want you to pass on?
A. Port to port.
B. Starboard to starboard.
C. It's informal radio slang with no fixed meaning - pass however is safest.
D. He wants you to sound your whistle before passing, but the side is still up to you.
Practice Question Answers
Q1.
B. I intend to leave you on my port side. Under the Inland Rule 34(a), one short blast proposes "I intend to leave you on my port side" - a passing proposal the other vessel answers by sounding the same signal back, unlike the International meaning (altering course to starboard) for that identical single blast. A is the INTERNATIONAL meaning for one short blast, not the Inland one - the same signal means something different depending on which set of Rules applies. C reverses the direction. D describes a different maneuver (astern propulsion), which uses three short blasts, not one.
Q2.
A. Port to port. On inland waters, "pass on the one whistle" is standard towing-industry shorthand for the traditional default meeting arrangement - port to port. It traces back to Rule 34(a)'s own signal: one short blast means "I intend to leave you on my port side," which results in each vessel passing with the other on her port side. The alternative, "two whistles," means starboard to starboard instead. B is the "two whistle" arrangement - the opposite of what was actually requested. C underestimates how standardized this convention actually is among working vessels on U.S. rivers, not open-ended slang. D misreads the phrase - agreeing to a passing side over VHF is explicitly recognized under Rule 34(h) as settling the arrangement, not just a prelude to sounding a literal whistle.