Understanding what not under command means in COLREGs and how it affects navigation

Not Under Command: a term that sounds dry, but it’s loaded with shipboard reality. If you’ve ever watched a busy waterway from the deck of a ferry or spotted a strange signal from a distant hull, you’ve probably wondered what that phrase actually means when it’s applied to real ships. In the world of COLREGs—the Rules of the Road for the sea—it’s a precise status, not a mood. Here’s the practical gist, plus what it looks like to other vessels on the water.

What does it mean to be “not under command”?

Let me answer plainly: the correct interpretation is B. The vessel is unable to maneuver due to exceptional circumstances. That’s a mouthful, but the idea is simple: this is a vessel that, for reasons beyond its control, cannot take the action expected under the rules to avoid a collision. Think engine failure, a broken steering system, or some other serious problem that makes it impossible to steer away from danger or to alter speed safely.

It’s not about choosing not to move. It’s not about being anchored. It’s not about being towed. Those are distinct situations with their own signals and rules. A vessel that’s deliberately not moving can still maneuver in theory; a vessel under tow has power available to others via its towing arrangement; a vessel at anchor can, in principle, maneuver if needed. Not Under Command is different—it signals an emergency of capability, not just a temporary pause.

Two kinds of signals carry the message

When a vessel is Not Under Command, it must make sure other mariners understand its predicament. In practice, that means both visual and, where appropriate, sound signals.

• Visual signals (daylight and nighttime): The classic day signal for Not Under Command is two circular shapes displayed vertically. If you’re spotting from another vessel, you’d expect to see two round shapes lined up one above the other. At night, that same status is conveyed by two all-round red lights in a vertical line. The lights are easy to recognize in a congested horizon, but the day shapes are a handy cue in good visibility.

• Sound signals: In addition to lights or shapes, a Not Under Command vessel should use appropriate sound signals. The goal is to make sure nearby traffic understands that this ship cannot maneuver. The exact pattern isn’t something you should rely on in isolation, but the idea is clear: your ears should cue you to exercise extra caution and give the vessel a wide berth when you hear an unusual, persistent signal in conjunction with visual indicators.

If you’ve ever heard a pair of distant, persistent signals echoing across a busy harbor, you’ve felt the practical impact of this rule. The crew isn’t trying to bluff their way through; they’re communicating a real limitation.

What Not Under Command looks like at the moment of truth

Here’s where the everyday workplace of a mariner meets the law book. A Not Under Command vessel is, for all practical purposes, a “must-give-way” vessel in any collision risk scenario, unless another constraint comes into play (like weather, traffic density, or the other vessel’s right-of-way status).

• The give-way rule still applies. The other vessel must take early and substantial action to keep well clear of the NUC vessel.

• The NUC ship keeps its course and speed unless it’s forced to maneuver to avoid imminent danger. It can’t rely on its own ability to dodge or weave to stay safe, because it cannot maneuver as required by the rules.

• In crowded channels or crossing situations, the presence of a Not Under Command vessel changes the dynamic sharply. Other mariners must anticipate that the NUC vessel will not respond to the usual steering or speed adjustments.

To put it simply: if you’re approaching a vessel signaling NUC, you don’t expect a textbook swerving dodge. You slow down, keep clear, and adjust your own course early to avoid potential contact.

What it isn’t, and why that matters

Let’s clear up the common misunderstandings. The term isn’t a badge of laziness or a signal that the crew simply forgot to steer. It indicates a genuine incapacity to maneuver safely. So:

• Not under command is not “deliberately not moving.” Deliberately not moving is a different situation altogether, and it doesn’t carry the same legal signal or the same mandatory response from others.

• Not under command is not the same as being anchored. An anchored vessel can still maneuver by letting out the anchor or by swinging with the current; a Not Under Command vessel cannot.

• Not under command isn’t the same as being towed. A vessel that’s being towed generally retains some maneuverability controlled by the towing arrangement. Not Under Command signals a loss of the fundamental ability to comply with navigation rules.

A quick mental model you can hold onto

If you’re ever unsure in a crossing scenario, try this mental check: “Is the other boat physically unable to maneuver to avoid me?” If the answer is yes, you’d expect that boat to show the NUC signals, and you should be prepared to give it extra space and time. It’s a courtesy—and, frankly, a necessity—for safe navigation.

Why this distinction matters in real life

Navigation is a stream of split-second decisions when boats converge. The Not Under Command label is a powerful caution flag for anyone piloting a vessel. A few everyday touches help keep the water safer:

• Early recognition: Spotting those two vertical day shapes or the twin red lights should trigger a slower pace and greater vigilance. You can’t rely on a single signal alone, but it’s a strong pointer that something unusual is going on.

• Predictive planning: In busy waterways, you’ll often be dealing with multiple potential conflicts. If a NUC vessel is nearby, you’ll plan your course wide, anticipate abrupt changes in the other vessel’s status, and be prepared to give that craft a generous margin.

• Clear communication: If you’re in command aboard a vessel that’s signaling NUC, don’t hope everyone will “get it.” Use proper sound signals and, if necessary, radio contact to ensure you’re heard by nearby traffic. Clarity of intent reduces the chance of an accidental close call.

A few memorable scenarios

• Engine trouble in a congested harbor: A small freighter loses propulsion in a narrow channel. It sets two balls up the masthead and blasts a polite, persistent warning to others. The channel slows, the traffic stream separates, and the vessel crawls toward a safe haven. It’s not dramatic, but it’s real—the kind of calm, procedural response that makes the difference between a near-make and a near-miss.

• Severe weather pushing a hull into a bad angle: A sailing vessel loses steering in gusty seas and can’t steer to avoid a crossing. The other vessel slows, gives a wide berth, and waits for the weather to ease. The NUC vessel isn’t “free” to maneuver out of danger, so everyone else plays a careful waiting game.

• Mechanical failure out of the main channel: A tug or barge experiences steering issues just outside the main traffic lane. The crew deploys day signals and intermittently uses sound signals to alert others, buying time for a safe towing arrangement or a rescue plan. It’s messy, yes, but it still follows a predictable rhythm that prevents chaos.

Tips for learners who want to remember this clearly

• Mnemonic moment: Two balls, two red lights—NUC signals. Day shapes or night lights, same message. If you can remember the two-for-two motif, you’ve got a quick cue for both day and night.

• Link to actions: Not Under Command means “I can’t maneuver safely.” Others respond with “I will maneuver to avoid.” That simple cause-and-effect helps most learners connect the dots during tests and on water alike.

• Context helps memory: Think about what makes a vessel unable to maneuver—engine failure, steering gear breakage, severe weather. When you picture those scenarios, the signals become less abstract and more practical.

A few gentle reminders for the curious mind

• The sea doesn’t rely on charm or luck. It relies on rules, signals, and clear communication. Not Under Command is a quintessential example of that.

• The signals are designed to be quickly recognizable from a distance, in calm seas or in a foggy harbour. That’s why you’ll see those simple, universal indicators—two circular shapes or two red lights—standing out against a busy skyline of vessels.

• For mariners, safety is not about who has priority but about who can move safely given the situation. Not Under Command changes the calculus, and that’s exactly why the rule exists.

Bringing it all together

So, what does it mean when a vessel is not under command? It means exactly what the term says, carried out in concrete, navigable signals: the ship cannot maneuver due to exceptional circumstances and must signal that status to others. It’s a crucial distinction that shapes how nearby vessels respond, how routes are adjusted, and how conversations on deck unfold in real time. The two circular shapes or the pair of red lights aren’t decorative—those signals are a lifeline, a language of safety spoken across the water.

If you’re exploring COLREGs for the first time or revisiting them with fresh eyes, keep this scenario in your pocket: Not Under Command isn’t about a ship choosing to stall; it’s about a ship telling you, with both light and sound, that it’s currently unable to take the action most of us would take instinctively to avoid a collision. When you hear that signal—or see those two balls—give plenty of space, and let the rules guide your next move with calm confidence.

And that’s the heart of it: a simple status that carries a serious responsibility, understood by mariners worldwide. It’s one more thread in the intricate tapestry of safe navigation, a reminder that the sea rewards clear signals, steady judgment, and a little bit of patience when every second matters.