Red on red all-around lights indicate a grounded vessel under COLREGs rules

Lights tell a story at sea. When you’re out after sunset, those tiny glows are more than pretty. They’re a live safety language that keeps ships from colliding. If you’ve ever seen a pair of red lights in a vertical line, you’ve seen a crucial message about a vessel’s condition. Let’s unpack what that message is, and why the other light configurations don’t carry the same meaning.

What the red-on-red signal means, plain and simple

On the water, the phrase “not under command” is a fancy way to say a vessel can’t maneuver as it normally would. It could be stuck on a sandbank, it could be dead in the water, or the engine might be out of action. Whatever the reason, the ship isn’t making decisions about steering or speed at will.

To broadcast this state clearly, the COLREGs require two all-around red lights, arranged one above the other, in a vertical line. The lights must be visible in all directions. If you’re navigating near a vessel displaying that red-on-red configuration, you should treat it as a red flag: keep well clear, slow down, and pass with extreme caution.

Why two red lights, vertically aligned? Here’s the logic

Two red lights in a vertical line are hard to miss. They stand out even in crowded night skies and choppy water. The red color, already associated with danger and prohibition, reinforces the message: this vessel cannot alter course or speed to avoid a collision.

A grounded vessel (or one not under command for any reason) isn’t playing the usual steering game. It can’t keep a safe distance or follow the expected traffic pattern. The red-on-red signal communicates a simple truth: “I am not moving at your pace, and I may drift.” For nearby boats, that early warning can be the difference between a close call and a safe passage.

What the other light configurations actually signal

To avoid confusion, it helps to know what those other options imply. They don’t convey the grounding state, even though they might look important in other contexts.

• Green and white lights

Green and white combinations appear on vessels that are underway and performing certain legitimate activities. They can indicate different statuses depending on the vessel type and operation, but they do not indicate grounding or not being able to maneuver. In other words, a green-and-white setup isn’t a grounding signal—it's telling you something about the vessel’s routine activity, not its maneuvering capability.

• Only one white light

A single white light is common for various reasons (a lantern, a low-power light, or a light used for a specific navigation task). It does not, by itself, communicate that a vessel is aground or not under command. It’s a cue that needs context from other signals or from the vessel’s known status.

• Blue lights on the starboard side

Blue flashing or steady lights are used for special situations. In practice, blue lights are associated with vessels that have a special status, such as law enforcement or certain fishing activities, depending on the region. This is a clear signal that the vessel is up to something different from ordinary navigation, not a message about grounding.

A quick memory aid you can lean on

If you ever find yourself wondering what a signal means, here’s a simple rule of thumb you can carry:

• Red on red = not under command. Likely aground or stuck. Give way and keep clear.

If you want a tiny mnemonic, try this: Red hands, keep hands off. It’s not perfect, but it nudges you toward caution when red lights appear in a vertical stack.

How a grounded vessel communicates beyond lights

Lights aren’t the whole story. A vessel that’s not maneuvering typically flags its status in other ways too. The crew may use radio communications to declare their situation, and they’ll often keep the engine shut down or secure, to minimize drift. In busy waters, a grounded or not-under-command vessel can cause a ripple effect—slower traffic, new passing lanes, and reorganized approaches. That’s why early recognition matters. The sooner you see the red-on-red signal, the sooner you can adjust your course and speed safely.

What to do if you spot red-on-red

If you’re piloting or navigating near a vessel showing two all-around red lights, here are practical steps:

• Reduce speed and maintain a safe distance. Give yourself space to maneuver without surprises.

• Steer well clear of the vessel’s projected track. Remember that the grounded boat isn’t steering its path the way a normal vessel would.

• Maintain a lookout for additional signals. The crew may switch to other communications or flags, and radio calls might clarify the situation.

• Communicate if appropriate. In crowded lanes or near harbour approaches, making a brief radio call to acknowledge the situation can help prevent confusion.

A moment to connect with the broader idea

Why does this signal even matter, beyond the rules on a page? Because nautical safety is a team sport. The sea is unforgiving when signals are missed or misinterpreted. The red-on-red signal is a straightforward, universally understood cue. It’s the difference between a cautious, calculated pass and a collision risk.

A few practical notes for real-world crews and students alike

• Lights must be visible at the required angles and kept clean and intact. A dirty lens or a burned-out bulb can render the signal ineffective just when it’s most needed.

• Weather doesn’t erase the rule. Rain, fog, or spray can blur lights, so many mariners keep radar and AIS handy to corroborate what they see with what they know.

• Region matters. Some areas have local practices for signaling unusual situations. It’s wise to be familiar with port authority guidance and local COLREGs interpretations.

A little historical flavor to round things out

Colregs have evolved through decades of sea travel, motorized ships, and modern navigation aids. The red-on-red rule is a nod to the idea that some conditions rob a vessel of its maneuvering agility. It’s a reminder that navigation is not just about speed and route; it’s about safety, responsibility, and clear communication. The color language—red signaling danger, white for visibility, green for safe directions—stems from centuries of seafaring lore and practical testing on the water.

Putting it all together: why this matters for anyone who spends time on the water

Whether you’re a student learning the ropes or a seasoned skipper brushing up on the rules, this signal offers a compact lesson: a single configuration can carry a lot of urgency. The two red lights tell you to look twice, slow down, and plan your move with care. It’s a simple cue with outsized impact on safety.

If you’re curious about the bigger picture, you’ll find that lights and shapes go hand in hand with sound signals, radar observations, and radio communications. The COLREGs aren’t just a checklist; they’re a culture of safety that grows when every crew member knows what each signal means and acts accordingly.

A closing thought

Next time you’re out on the water, take a moment to notice the glow of every vessel around you. If you spot two red lights stacked up high, you’ll know that the vessel is not maneuvering. That knowledge puts you in a better position to respond calmly and correctly. The sea rewards informed, patient decisions more than anything else.

If you’d like, I can help you explore more scenarios like this—different light patterns, additional signals, and the real-world steps captains and crews take to stay safe. The rules are there to guide, but the practiced eye comes from experience, observation, and a steady respect for the power of the water.