Sound signals under the ColRegs divide into two distinct categories: maneuvering and warning signals (Rule 34), which are used when vessels are in sight of each other, and fog signals (Rule 35), which are used in restricted visibility when you may not see another vessel until it is too late to respond. Both categories are legally required, and neither is superseded by the presence of VHF radio, AIS, or radar — though those systems change the practical landscape considerably.
A short blast is approximately one second in duration. A prolonged blast is four to six seconds. These definitions matter because the distinction between a short and a prolonged blast is the entire vocabulary of marine sound signals.
Maneuvering Signals: When Vessels Are in Sight
When power-driven vessels are in sight of one another and are close enough that the actions of one require the understanding of the other, Rule 34 requires them to signal their intentions with the ship's horn. These signals apply to power-driven vessels maneuvering in accordance with Rule 34.
One short blast: I am altering my course to starboard. This signal accompanies the helm movement — it does not request permission, it states what you are doing. The other vessel should acknowledge with the same signal if it understands and agrees with the action.
Two short blasts: I am altering my course to port. Again, a statement of action, not a question. Two blasts to port; one blast to starboard.
Three short blasts: I am operating astern propulsion. Note that this means the engine is in reverse, not necessarily that the vessel is moving astern — a vessel moving forward under its own momentum but with the engine in reverse will still show a sternward propulsion signal. This signal alerts nearby vessels that your speed and direction may change unpredictably.
In European waters, Rule 34 also allows for an agreement signal: vessels approaching each other in a narrow channel may signal their intention with the above blasts and receive the same signal back as acknowledgment. If the other vessel does not understand or considers the maneuver unsafe, it signals five short blasts. In North American inland waters (where the Inland Navigation Rules apply rather than International ColRegs), the signaling scheme is a question-and-answer system rather than a statement-of-action system — a practical difference that matters to mariners who transit both regimes.
The Five-Blast Signal: The Most Important Signal at Sea
Five or more short blasts is the danger/doubt signal. It means: I am in doubt of your intentions, or I am not sure you are taking sufficient action to avoid collision. The correct response to hearing five short blasts is immediate: alter course to open the distance, and do so obviously. This signal can be given by any vessel to any other vessel — it carries no implication about who has right of way. It is a warning that the situation is developing in a dangerous direction.
Many collisions in good visibility occur precisely because neither vessel sounded this signal when doubt existed. If you are approaching another vessel and you are genuinely uncertain what it is doing, sound five blasts before the situation becomes critical. If you hear it, act immediately — don't assume the other vessel will manage.
Fog Signals: When Visibility Is Restricted
Rule 35 specifies the fog signals for various vessel types. These signals must be sounded in conditions of restricted visibility — fog, heavy rain, snow, dust, or any condition that limits sight distance — regardless of whether it is day or night. They are required whether or not you have radar or AIS. They are required even in daylight. The trigger is restricted visibility, not darkness.
Power-driven vessels making way: one prolonged blast at intervals of not more than two minutes. This is the signal you will hear most often in fog near shipping lanes — a single long blast every two minutes from a vessel that is moving.
Power-driven vessels under way but stopped and making no way: two prolonged blasts in succession at intervals of not more than two minutes. The two-blast signal tells you a power vessel is stopped — a vessel that may be an unpredictable hazard because it is occupying your path without forward motion.
Sailing vessels under way (regardless of point of sail): one prolonged blast followed by two short blasts, at intervals of not more than two minutes. This distinctive pattern — long, short, short — identifies a sailing vessel. Learn it. If you hear it while motoring in fog, there is a sailboat somewhere in the vicinity.
Vessels not under command, vessels restricted in their ability to maneuver, vessels constrained by draft, and fishing vessels: one prolonged blast followed by two short blasts — the same as a sailing vessel under way. The distinguishing light signals apply at night; the fog signal is the same pattern for these vessel types. This means that in fog, you cannot determine from the sound signal alone whether you are hearing a fishing vessel, a RAM vessel, or a sailing yacht. Visual identification — if visibility allows — is required.
Pilot vessels on duty: one prolonged blast followed by four short blasts, in addition to any other required signal.
At Anchor in Fog
A vessel at anchor in restricted visibility is required to ring a bell rapidly for approximately five seconds at intervals of not more than one minute. This is the anchor bell signal — one of the oldest sound signals in maritime practice and one that many recreational sailors entirely neglect.
On vessels over 100 meters, the bell must be sounded forward, and a gong must be sounded aft. On vessels under 12 meters, the bell or any other efficient sound signal may be substituted, though the five-second-per-minute rhythm remains required.
The practical issue is that few recreational sailing yachts carry a proper fog bell. Many carry a compressed-air horn that can produce the signal, which is compliant if the signal can be heard at a reasonable distance. If you are anchored in a fog bank in a fairway or anchorage, you are a hazard to approaching vessels. Making the anchor signal audible and consistent is a serious safety requirement.
A vessel at anchor may also sound one short, one prolonged, and one short blast as a warning to an approaching vessel — three blasts that say, essentially, "I am here, I am anchored, do not run into me."
Why VHF Does Not Replace Sound Signals
VHF radio and sound signals serve different functions. VHF allows communication and coordination; sound signals provide mandatory public warnings that require no response and no agreement. A vessel that does not have its VHF on — or is not monitoring the right channel, or whose crew is asleep — will not receive your radio call. The sound signal, if powerful enough and properly timed, may reach a vessel whose crew is below and whose electronics are off.
There is also a legal dimension: the ColRegs require sound signals. A vessel that fails to make required fog signals and subsequently collides with another vessel may find its insurance coverage and legal position affected by that failure, regardless of who was at fault for the underlying collision. The requirement to sound fog signals in restricted visibility is unconditional.
In practice, a vessel making an offshore passage in fog should: sound the required fog signal on schedule, maintain a proper watch, monitor VHF channel 16, use radar or AIS if available, and reduce speed to allow stopping in half the visible distance. None of these measures replaces the others. Together they constitute the proper lookout and precautionary approach the ColRegs require.