How long is a prolonged blast under COLREGs Rules of the Road?

What’s the sound telling you at sea? A foggy morning, a gray horizon, and the whistle of a ship cutting through the murk. Signals like this aren’t decoration—they’re life-saving clues that help everyone stay clear of danger. When we talk about a prolonged blast in the COLREGs—the Rules of the Road at sea—we’re touching on one of the most dependable audible cues a mariner can rely on in low visibility.

What exactly is a prolonged blast?

Here’s the thing: a prolonged blast is a whistle signal that lasts four to six seconds. That specific duration isn’t random. It’s long enough to cut through the fog of sound and distinguish it from shorter blasts, yet not so long that it becomes confusing or overbearing. If you’ve ever heard a ship’s whistle stretch for a moment longer than a quick puff, you’ve probably sensed why those extra seconds matter. In practical terms, it’s the audible gateway to safe navigation when sightlines are poor. The other vessels listening know, “Okay, something is nearby, and I should pay closer attention to that actual vessel’s position and maneuvering.”

Why does the 4-6 second window matter so much?

First, timing creates clarity. In busy waterways or maze-like coastal zones, you can’t rely on eye contact alone. Sound travels differently than light, and in fog or drizzle it travels farther and more reliably than a quick flash of a light. A four-to-six-second signal gives nearby skippers time to localize where the sound is coming from and how that vessel might be moving. It’s a standardized cue that reduces guesswork, which is exactly what you want when you’re navigating through restricted visibility.

Second, consistency builds trust. The COLREGs aren’t a random set of tips; they’re a shared language. When one vessel uses a prolonged blast for four to six seconds, others recognize it quickly and respond in a predictable way. Predictability is safety at sea. Think of it like a predictable traffic signal for pilots of the water—no drama, just a clear call-and-response that helps everyone stay out of each other’s way.

Finally, it’s about presence. A strong, steady prolonged blast announces a vessel’s presence and position. That’s crucial when you’re not sure who is around you, which direction they’re moving, or whether they’re on a collision course. The signal isn’t about fault or fault-finding; it’s about giving others enough information to make good decisions fast.

How this signal fits with the broader sound-signaling world

In the world of maritime sound signals, a prolonged blast is the backbone of audible communication in restricted visibility. It’s often introduced into a broader sequence of signals, but even on its own it carries weight. For example, a vessel may use a single prolonged blast to indicate its presence in fog, or it might be followed by short blasts under certain procedures or when indicating crossing intentions. The key point to take away: the four-to-six-second duration is the standard, widely recognized cue that helps other vessels interpret your location and actions.

If you’re new to listening for these cues, you might notice how other signals contrast with the prolonged blast. Short blasts—brief, quick bursts—convey different messages, like warnings or specific maneuver intentions. The distinction between “I am here” (the prolonged blast) and “I am changing course” or “I am passing you on your port/starboard” (shorter, more tactical blasts) is subtle but vital. It’s a bit like learning a new language where tempo, length, and cadence carry meaning as much as the words themselves.

A quick mental shortcut to keep it straight

Let me explain with a simple memory nudge: imagine counting out a steady beat. Four seconds, six seconds—think of it as a musical measure. If you’re hearing a whistle that sits in that four-to-six-second zone, you’ve got the prolonged signal. If it’s shorter, you’re hearing something else. This isn’t a trick; it’s a practical cue you can rely on during the foggy morning watch.

Real-world flavors and a little color from the water

I’ve been on decks where fog rolled in like a thick blanket and the world narrowed to the edge of a radar screen and a faint, stubborn whistle. In those moments, the prolonged blast is like a lighthouse at sea: a steady, unmistakable message that someone else is there and planning to move. It buys you time to adjust speed, steer, or simply take the safe route around a possible crossing. And yes, the sea still has its poetry—there’s something almost human about a captain recognizing that sound and thinking, “All right, let’s give ourselves room to breathe.”

Practical tips you can apply on deck

• Listen for the duration, not just the sound. If you hear a whistle that lasts in the four-to-six-second range, treat it as a deliberate presence signal from another vessel.

• Don’t assume orientation from one signal alone. Use radar, AIS, and, when possible, visual confirmation to triangulate where the sound is coming from and what the other ship might be doing.

• When you’re the one signaling, make your intent clear with a steady prolonged blast if visibility is bad. If conditions allow, you may supplement with short blasts to indicate maneuvers—but the key is the prolonged portion, lasting four to six seconds.

• Remember the limits. A different duration—say, one, two, or seven to ten seconds—doesn’t fit the standard definition of a prolonged blast. Treat those durations as signals with different meanings, and respond accordingly.

A tiny tangent that actually helps the main point

Fog isn’t the only time these signals matter. In busy harbors, near river mouths, or during nighttime transits, the sea can throw a mix of signals at you—all afloat, all with their own rules about who does what and when. The four-to-six-second rule for a prolonged blast is a sturdy anchor in that tidal sea of signals. It helps you stay calm, interpret correctly, and act decisively. When you hear it, you acknowledge that another vessel is nearby and that you should maintain a safe distance and speed. It’s less about “winning” a close call and more about choosing the safer, smarter option for everyone aboard.

Common misunderstandings—and how to clear them up

One sneaky misunderstanding is thinking that any long whistle is always a warning. The reality is subtler: the duration is a defining feature, but context matters. The same four-to-six-second blast can be part of different signaling sequences depending on the situation and the vessel’s course. That’s why you don’t rely on a single signal in isolation; you assess the full set of clues—sound, sight, radar, and AIS data—before you decide how to respond.

Another pitfall is assuming shorter blasts are irrelevant in fog. Short blasts carry their own messages and are used in specific maneuver situations. They aren’t interchangeable with the prolonged five-second-plus signal. The maritime rulebook lays these signals out so crews can preempt confusion and avoid collisions, even when the weather tests everyone’s nerves.

Wrapping it up: the core takeaway

In the end, the reason this particular duration matters is straightforward: four to six seconds is the standardized window that reliably signals a vessel’s presence and position when visibility is compromised. It’s a simple, practical tool that makes the water safer for everyone who uses it. If you ever find yourself on a deck where the sea is a gray wall and sound is the only clue you’ve got, this rule is your compass. Recognize the prolonged blast, hold your course with caution, and let the sea do its part to keep things orderly.

A few friendly reminders as you go

• The four-to-six-second rule is a defining feature of a prolonged blast. This is the standard you’ll hear described and expected in real-world navigation.

• The signal isn’t just about loudness or length; it’s about a shared expectation of how boats communicate under stress and limited sight.

• Combine audible signals with other navigation tools—radar, AIS, and visual observations—for the clearest picture of what’s happening around you.

• Stay curious about the other signals you hear. A bit of context about what each tone and duration means makes you a safer mariner and a better teammate on any vessel.

If you’ve ever wondered how something as simple as a whistle can carry such weight, you’re not alone. The sea rewards attention to small details—the kind you notice when you’re present, listening, and ready to react. The four-to-six-second prolonged blast isn’t flashy, but it’s one of the most reliable stones in the fortress of maritime safety. And when you recognize it, you’re part of a long, shared tradition of seaworthy conduct: clear signals, calmer crossings, and safer journeys for everyone who sails.