Three red all-around lights vertically stacked signal a vessel constrained by its draft

Navigating the seas isn’t just about steering—it's about reading signals that keep everyone safe. Among the many identifiers you’ll notice on a vessel, the lights and their arrangement tell you a lot about its capabilities and constraints. One of the most telling signals is the set of lights that indicates a vessel is constrained by her draft. Let me explain what that looks like, why it matters, and how it shapes your decisions on the water.

What signals a vessel is constrained by her draft?

Short answer: three red all-around lights stacked vertically. Yes, you read that right—the color is red, and there are three of them, aligned one above the other. This specific configuration is a dedicated signal under the COLREGs to communicate that a vessel’s depth requirements limit its ability to maneuver. The water depth and the ship’s own draft leave her no easy option to deviate from her course, so she flags that constraint to others to prevent risky close-quarters situations.

Now, you might be wondering how this looks in practice and how it differs from other signals you might see at sea. Let’s break down the choices you might encounter and why the three-red-light signal stands out for a vessel constrained by draft.

• A. Three red all-around lights vertically stacked — this is the correct signal for a vessel constrained by her draft. It’s a clear, visual cue that the vessel cannot easily change course to avoid a collision.

• B. Two masthead lights if she is over 70 meters — this isn’t the draft signal. Masthead lights are part of standard navigation lighting for larger vessels, but their purpose is about visibility and presence in darkness, not about draft constraints. The exact number and arrangement of masthead lights don’t communicate a draft limitation by themselves.

• C. One white light on the bow — that would be unusual for signaling draft constraints. A simple forward white light might be seen on certain small craft or as part of specific lighting schemes, but it doesn’t convey the “restricted maneuverability due to draft” status that three red lights stacked vertically do.

• D. Green light on the starboard side — the starboard green light is part of the standard navigation light system indicating the vessel’s right-hand side. It helps other ships determine bearing and point of approach, but it doesn’t tell anyone about draft constraints.

Why this particular signal matters

Safety on the water hinges on clear communication. The three red all-around lights tell other vessels, immediately and unambiguously, that the ship cannot easily change her course because of her depth needs. Picture a large container ship or a vessel carrying heavy cargo whose deep draft keeps her tied to a narrow corridor of water. If another boat tries to squeeze past or cross in front, the constrained vessel might not be able to maneuver quickly enough to avoid a collision. The triple red signal is a calm, predictable cue that shifts expectations—other vessels should give more space, slow down, and avoid risky passes.

That clarity is crucial because questions about signals aren’t just trivia; they translate into real-world decisions. When you’re at sea and you spot those three red lights, you know there’s more at stake than simply keeping to your course. You’re looking at a moving reminder that safety depends on everyone understanding each other’s limitations and choices.

How the other options fit into the bigger picture

Let’s zoom out a bit and keep this practical. In the world of COLREGs, every light pattern and every signal has a purpose. Here’s a quick mental map of where the other options fit (or don’t) when you’re trying to read a situation:

• The two masthead lights signal a large vessel, often one that’s underway and visible from a distance. They don’t automatically tell you anything about draft or maneuverability. They’re about visibility and indicating the vessel’s presence in darkness.

• A single white light on the bow—that’s not a standard, standalone indicator you’d rely on for draft status. It’s more about general forward visibility or, in some contexts, specific equipment or local traffic schemes.

• The green light on the starboard side is part of the navigation lighting system that helps you determine a vessel’s orientation. It’s vital for risk assessment, especially in crowded waterways, but it doesn’t encode a vessel’s draft constraint.

So, when you’re honing your understanding of COLREGs, the triple red lights are the standout symbol for a vessel that’s constrained by her draft. Everything else is informative in its own right, but it doesn’t convey the same critical limitation in one instantly recognizable cue.

Bringing the concept to life: what this means on the water

Imagine you’re piloting a smaller craft near a busy harbor approach. A massive ship glides in, and you notice three red lights in a tight vertical line. The pattern is unmistakable: this vessel is not just moving; she’s constrained—she can’t swing wide or swerve to miss you as easily as a less restricted boat might. Your next moves become a little more deliberate.

What should you do in response? Here are practical takeaways rooted in good seamanship:

• Give extra space. If you can, don’t cut in front of a CSD (Constrained by Draft) vessel. They need room to continue along their planned path without abrupt changes.

• Reduce speed when visibility is limited. Slower speeds give you more time to react and help prevent surprises in tight quarters.

• Maintain a clear bearing. Keep your trajectory predictable, and avoid sudden shifts that could force a maneuver the other vessel cannot execute.

• Use communications tools if needed. If there’s a risk of collision or a significant uncertainty about intentions, sound signals or radio communications can help clarify.

• Stay aware of other constraints. Draft isn’t the only limitation to consider—weather, currents, and harbor traffic all play a role in how you navigate near a constrained-by-draft vessel.

A few practical tips to sharpen your signal-reading skills

If you’re trying to become confident in reading signals, here are some easy ways to build intuition without turning it into a chore:

• Practice light-pattern recognition. When you’re near marinas or busy shipping lanes, take a moment to observe the lights of passing ships. See how the patterns align with different statuses: underway, not under command, restricted in ability to maneuver, and yes, constrained by draft.

• Use a quick mental checklist. If you see three red lights in a vertical line, you know the vessel is constrained by draft. If you see red lights arranged differently, or two red lights, you’re looking at something else—and you’ll adjust your actions accordingly.

• Read the water context. Signals live in a broader scene—bow waves, vessel size, and the traffic density all color what you should do next.

• Think through potential misreads. In rough seas or crowded channels, a signal might be partially obscured or misinterpreted. Slow down, widen your margin, and verify whenever possible.

• Pair theory with real-world tools. Radar overlays, AIS data, and up-to-date charts help you corroborate what you’re seeing with what you know about a vessel’s likely status and route.

A quick, human moment to tie it all together

Let’s be honest: the sea is a place where tension meets skill. The triple red lights are a reminder that not all ships can bend a little to avoid you; some ships are limited by their own design and depth. That’s when good judgment matters, not just knowledge. You don’t want to stretch the line with a vessel that literally can’t move in a direction you expect. By recognizing this signal, you’re helping keep the water safer for everyone—skippers, crews, and even the folks who ride the ferries and tugboats in between.

Connecting the dots with COLREGs at large

The COLREGs aren’t a random set of rules; they’re a living framework built to reduce risk as traffic on the water grows more complex. Signals like the three red all-around lights in a vertical stack are specific tools in a broader toolkit—one that includes right-of-way rules, crossing situations, and safe passing distances. When you understand what each signal communicates, you’re better equipped to interpret the intentions of other vessels and decide on actions that minimize risk.

A few words on context and nuance

You’ll hear a lot of emphasis on visibility and predictability in maritime safety discussions. That’s not just theory; it’s the backbone of seamanship. In practice, recognizing a constrained-by-draft signal is part of reading the room, so to speak. It’s about knowing when to go with the flow and when to hold back, to keep momentum steady and conversations clear between vessels. And yes, there are rare moments when you’ll encounter signals that seem to clash with real-world conditions. In those moments, it’s prudent to rely on instrument data, communications, and your own training to guide smart, safe choices.

Closing thoughts—keep your eyes open and your mind sharp

If you take one thing away from this, let it be this: three red all-around lights stacked vertically are the tell-tale sign a vessel is constrained by her draft. That single cue carries a lot of weight on the bridge, and rightly so. It informs how other vessels approach, how speed is managed, and how navigational decisions unfold in tight spaces.

As you continue learning about the COLREGs and the rules that govern vessels of every size, keep this image in mind. It’s a reminder that the ocean is a shared space, filled with different kinds of ships, each carrying its own limitations. When we tune into those signals—whether three red lights, the position of a starboard green light, or a pair of masthead lamps—we participate in a safer, more predictable voyage for everyone aboard.