What a vessel over 100 meters must display when anchored

What a big ship looks like at anchor isn’t something most of us think about until you’re in the thick of a harbor at night. But for mariners, lighting isn’t just about looking pretty. It’s about safety, visibility, and staying out of trouble on busy waterways. When a vessel over 100 meters sits at anchor, there’s a specific lighting requirement that’s all about making the ship unmistakable to nearby boats after dark. The correct answer to this rule is simple: all deck lights on.

Let me explain what that means in everyday terms and why it matters.

All deck lights on: the practical meaning

If a vessel is more than 100 meters long and it’s anchored, it must display every light that lights up the deck. Think of it as turning on the whole package of illumination you’d use if you were preparing for a night shift on deck. The intent is to make the anchored ship highly visible from every direction, turning a potentially dim silhouette into a bright, recognizable shape that other vessels can gauge quickly.

Why does the size matter here? Larger ships take up more space on the water and can be harder to spot, especially in congested areas, in fog, or when there’s glare from city lights. When a big vessel lights up all the deck, it creates a clear, continuous outline of the ship’s length. Other mariners can see not only where the ship is, but roughly how long it is and where it sits in the water. That visibility helps everyone plot a safe course, avoid close encounters, and respond in time if something changes—like a tugboat sneaking in with a tow or a fishing vessel drifting near the anchor chain.

What the other options get wrong

• B. Only one all-around light

That would give observers a finite, narrow glimpse of the ship. It creates a single point of reference, not the full picture. In heavy traffic or low visibility, a lone light can be easy to misinterpret or miss entirely, especially when other vessels approach from different angles. It reduces situational awareness rather than enhancing it.

• C. Flashing red lights

Flashing red lights have their own set of meanings in nautical signaling, but they aren’t the indicator used to signal an anchored vessel’s presence or size. Red flashes can be used in restricted visibility or as safety signals in particular contexts, but they don’t convey the comprehensive, forward-and-around visibility that all deck lights provide.

• D. Only its stern light

A stern light alone tells you where the back of the ship sits, not where the bow is, not how long the ship is, and certainly not what the entire vessel is doing for the next several hundred feet of water. It’s a partial picture at best and could provoke misjudgments about distance and heading.

Where this sits in the bigger picture of COLREGs

COLREGs exist to prevent collisions at sea, especially in busy zones and along routes with heavy traffic. Lighting is one of the most practical, low-tech tools the rules rely on. For large vessels, the standard is to maximize visibility when anchored to make it easy for other mariners to determine position, length, and orientation. It’s a simple, human-centered principle: the bigger the vessel, the more visibility you owe to others.

A quick mental model you can use

• If a vessel is very long (over 100 meters) and not moving, picture the deck as a runway of light. The more lights you see, the more confident you can be about the vessel’s location and the direction it’s facing.

• If you’re the captain or crew on a large anchored vessel, think of every light as a helper, guiding nearby boats to keep their distance and give you room to maneuver should the wind or current shift.

• If you’re piloting or passing nearby, scan for a bright line of deck lights first. That’s your cue to adjust your approach so you don’t surprise the anchored ship with a sudden turn or acceleration.

A little discipline goes a long way

The rule isn’t just about turning on lights and forgetting about it. It’s about discipline on deck. When visibility is poor—think rain, fog, or a misty night—the difference between several lights and a sea of darkness can be a matter of life and death. Crews must ensure lights are functional, shields and glass are clean, and nothing blocks the visibility of those lights. It’s surprisingly easy for a small fuse to blow or a navigation light to dim in heavy weather, and that’s exactly when you want everything shining as it should.

Digging a little deeper into why the rule exists

• It reduces ambiguity. A bright, continuous line of deck lights signals the presence of a large vessel in a way that a single light or a few lights never can. Other mariners don’t have to guess where the ship begins and ends.

• It speeds judgment. In a busy harbor, decisions need to be made quickly. Clear lighting helps pilots, tug crews, and small craft estimate distance and heading at a glance.

• It aligns with human perception. Humans are drawn to continuous silhouettes. The eye can lock onto the length and position of a big ship if the illumination is there, which brings an intuitive sense of safety.

tips for absorbing this rule without turning it into a memory trap

• Create a mental image: a long, bright raft of lights spanning the deck of a giant ship at anchor.

• Tie it to a real-world scene: imagine docking a cruise ship or container vessel at night in a busy harbor. The deck lights are the visible heartbeat of the ship, pulsing along its length.

• Use a simple cue: “Large ship, big light show.” It’s quirky, but it helps recall that bigger ships require more illumination when anchored.

• If you’re studying, pair this rule with a quick visual checklist: check for deck-wide brightness, verify ongoing functionality, and ensure there are no lighting gaps along the vessel’s length.

A few practical context notes

• This rule applies specifically to vessels of a certain size. Smaller vessels aren’t necessarily required to cast the same broad, continuous deck glow when anchored, though they must still comply with the general navigation-light requirements of COLREGs.

• The intent isn’t to waste energy; it’s to maximize safety. In many harbors, power systems are robust enough to support the lighting needs of large ships without compromising safety or creating avoidable hazards.

• Weather and local regulations can influence deck- lighting standards in practice. Mariners should always stay aware of harbor-specific guidance and any temporary lighting restrictions that might come into play during maintenance or special operations.

A closing note on navigation and culture

Rules like this one aren’t about punishing mariners; they’re about making the water safer for everyone who uses it. Think of the ocean like a crowded street at night: the more lights you can see, the better you can judge distance, speed, and intent. The “all deck lights on” rule for big anchored vessels is a clear signal to the world: I am here, I am large, and I am resting in this spot. Please pass with care.

If you’re curious about how these lighting rules evolve, you’ll find that COLREGs are the product of collaboration among international maritime authorities and ship operators. The goal is consistent safety across diverse seas, ports, and weather. It’s one of those practical corners of maritime law that quietly keeps everything moving smoothly—like a good lighthouse that doesn’t overpromise but always delivers a clear signal when you need it.

Takeaway for the road (or the water)

For vessels over 100 meters at anchor, all deck lights on is the standard. It’s a straightforward rule with a big impact: it makes the ship’s presence unequivocal and helps everyone around it navigate safely through the night. So next time you’re near a large anchored ship, look for that continuous glow along the deck. It’s a small detail with a big purpose, and it’s a perfect example of how precise the COLREGs can be when it comes to keeping the sea a safer place for all who travel it.

If this sparked your curiosity, you’re not alone. The rules can feel like a labyrinth until you see how each clause fits into the bigger picture of maritime safety. And if you want to keep the momentum going, I’m here to explore more scenarios, share memorable ways to recall the rules, and connect the dots between the technical details and real-world situations you’ll actually encounter on the water.