These are International COLREG signals. Rule 34(a) concerns power-driven vessels underway in sight of one another; 34(e) also covers an obscured bend. Rule 35 applies in or near restricted visibility, day or night. Listed intervals are maxima. Check the alternatives for vessels under 20 m and under 12 m.
| Signal | Meaning | Rule |
|---|---|---|
| 1 short blast (•) | I am altering course to starboard | 34(a) |
| 2 short blasts (• •) | I am altering course to port | 34(a) |
| 3 short blasts (• • •) | I am operating astern propulsion | 34(a) |
| 5+ short blasts (• • • • •) | Doubt about the other vessel's intentions or collision-avoidance action | 34(d) |
| 1 prolonged blast (—) | Power-driven vessel making way through the water | 35(a) |
| 2 prolonged blasts (— —) | Power-driven vessel underway, stopped with no way through the water | 35(b) |
| 1 prolonged + 2 short (— • •) | NUC, RAM, draught-constrained, sailing, fishing, towing or pushing vessel | 35(c) |
In or near restricted visibility, Rule 35 applies by day and night. These examples use 30 m vessels and the International Rules. A short blast lasts about 1 second; a prolonged blast lasts 4–6 seconds. Each playback demonstrates one sequence; the written answer gives the maximum repeat interval.
COLREG · USCG NAVCEN · 2026-09-10A 30 m power-driven vessel is moving through the water in fog, with no special operational status.
One prolonged whistle blast (4–6 s), repeated at intervals no longer than 2 minutes.
Use movement through the water, not speed over the ground. Stopping the engine alone does not mean the vessel has stopped making way.
The same 30 m vessel has stopped and now makes no way through the water in fog. It is not anchored, made fast to shore or aground.
Two prolonged whistle blasts (4–6 s each), with about 2 seconds between them. Repeat the pair at intervals no longer than 2 minutes.
It is still underway: that describes its status, not its speed. No way through the water is different from being at anchor, so the anchor bell signal does not apply.
A 30 m sailing vessel is underway in fog under sail alone. Its engine is not being used for propulsion.
One prolonged whistle blast followed by two short blasts. Repeat the group at intervals no longer than 2 minutes.
A sailing vessel uses this signal even when making way. With the engine propelling it, it counts as power-driven even with sails set. Other Rule 35(c) categories share this pattern, so hearing it alone does not identify a sailing vessel.
A 30 m vessel is anchored in fog. It is not engaged in fishing and is not restricted in its ability to manoeuvre while carrying out work at anchor.
Ring the bell rapidly for about 5 seconds, repeated at intervals no longer than 1 minute.
At 30 m, the bell requirement applies and no gong is required; the additional gong starts at 100 m. The optional short–prolonged–short whistle warning does not replace this bell signal. Fishing or working with restricted manoeuvrability at anchor follows Rule 35(d) instead.
An ordinary 30 m vessel has grounded on a shoal in fog. It is resting on the bottom, not floating at anchor.
Three distinct bell strokes, rapid bell ringing for about 5 seconds, then three distinct bell strokes. Repeat the full sequence at intervals no longer than 1 minute.
The three strokes before and after the rapid ringing distinguish grounding from anchoring. They are bell strokes, not three whistle blasts. At 30 m no gong is required; from 100 m the gong is also required.
These are International COLREG signals. Rule 34(a) concerns power-driven vessels underway in sight of one another; 34(e) also covers an obscured bend. Rule 35 applies in or near restricted visibility, day or night. Listed intervals are maxima. Check the alternatives for vessels under 20 m and under 12 m.
COLREG 32 · U.S. Coast Guard — International Navigation Rules (2024): (a) The word whistle means any sound signaling appliance capable of producing the prescribed blasts and which complies with the specifications in Annex III to these Regulations. (b) The term short blast means a blast of about one second’s duration. (c) The term prolonged blast means a blast of from four to six seconds’ duration. COLREG 34(a) · U.S. Coast Guard — International Navigation Rules (2024): (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: − one short blast to mean “I am altering my course to starboard”; − two short blasts to mean “I am altering my course to port”; − three short blasts to mean “I am operating astern propulsion”.
COLREG 35(d) · Official source excerpt · U.S. Coast Guard — International Navigation Rules (2024): (d) A vessel engaged in fishing, when at anchor, and a vessel restricted in her ability to maneuver when carrying out her work at anchor, shall instead of the signals prescribed in paragraph (g) of this Rule sound the signal prescribed in paragraph (c) of this Rule. COLREG 35(c,e,g,i,j) · U.S. Coast Guard — International Navigation Rules (2024): (c) A vessel not under command, a vessel restricted in her ability to maneuver, a vessel constrained by her draft, a sailing vessel, a vessel engaged in fishing and a vessel engaged in towing or pushing another vessel shall, instead of the signals prescribed in paragraphs (a) or (b) of this Rule, sound at intervals of not more than 2 minutes three blasts in succession, namely one prolonged followed by two short blasts. (e) A vessel towed or if more than one vessel is towed the last vessel of the tow, if manned, shall at intervals of not more than 2 minutes sound four blasts in succession, namely one prolonged followed by three short blasts. When practicable, this signal shall be made immediately after the signal made by the towing vessel. (g) A vessel at anchor shall at intervals of not more than one minute ring the bell rapidly for about 5 seconds. In a vessel of 100 meters or more in length the bell shall be sounded in the forepart of the vessel and immediately after the ringing of the bell the gong shall be sounded rapidly for about 5 seconds in the after part of the vessel. A vessel at anchor may in addition sound three blasts in succession, namely one short, one prolonged and one short blast, to give warning of her position and of the possibility of collision to an approaching vessel. (i) A vessel of 12 meters or more but less than 20 meters in length shall not be obliged to give the bell signals prescribed in paragraphs (g) and (h) of this Rule. However, if she does not, she shall make some other efficient sound signal at intervals of not more than 2 minutes. (j) A vessel of less than 12 meters in length shall not be obliged to give the above-mentioned signals but, if she does not, shall make some other efficient sound signal at intervals of not more than 2 minutes.
