Power-driven vessels under 50m are distinguished by two masthead lights.

Outline you can skim before we dive in

• Why lights matter at sea and what COLREGs aim to do

• The key distinction: power-driven vessels under 50m have two masthead lights

• Why the other options aren’t reliable identifiers

• A quick mental map of navigation lights you’ll see at night

• Practical tips to read vessels at a glance, plus a few friendly digressions

• Wrap-up: how this simple rule keeps nights safer for everyone

Two lights, one simple idea: telling power-driven boats apart at night

Let me explain it this way: navigating at sea is a story told in lights. Boats don’t speak; their lamps do the talking. The COLREGs—the international “rules of the road” for vessels—set up a vocabulary so that every skipper can understand what kind of boat is nearby just by looking. When you’re on watch after dark, that quick visual read can spell the difference between a smooth pass and a close call.

For power-driven vessels under 50 meters, the distinctive signal is surprisingly straightforward: two masthead lights, arranged in a vertical line, white and steady. One masthead light sits above the other, both clearly visible to any observer ahead and to the sides. It’s not fancy lighting, but it’s incredibly effective. Think of it as the maritime version of a two-story sign that says, plainly, “I am a power-driven vessel, moving ahead.” This simple arrangement helps you quickly differentiate a power-driven craft from its sailing counterparts, from fishing boats, or from other types of craft that share water with you.

Why two masthead lights, not something else?

Here’s the thing about the other options you might come across in questions or in casual conversations:

• A: By the presence of additional port lights. Extra port lights aren’t a universal, standardized signal for identifying power-driven vessels. The rules don’t rely on a crowd of extra lamps to convey identity. In other words, extra port lights aren’t the dependable, across-the-board cue that the two masthead lights provide.

• C: By its sternlight color. Stern lights are typically white on many vessels, but color alone isn’t a reliable discriminator for classifying a vessel as power-driven versus sailing, fishing, or other types. Colors and arrangements can vary with vessel size, operation, and even local practice, so you shouldn’t count on stern light color as a stand-alone identifier.

• D: By its all-around light. An all-around white light is used in several contexts and doesn’t single out power-driven vessels. Some boats may display an all-around light due to size, operation, or maneuvering, but it isn’t the crisp, universally recognized signal that two masthead lights provide for the purpose of identification under 50 meters.

That leaves B—the two masthead lights—as the reliable hallmark the rules emphasize. It’s a clean, unambiguous cue that helps mariners anticipate behavior on the water, especially at night or in reduced visibility when you can’t rely on other sensory cues.

Reading the lights like a map in the dark

If you’ve ever looked out over a harbor at midnight and tried to make sense of the silhouettes dotting the horizon, you know lighting can be a language in itself. Here are a few mental anchors to help you read the scene:

• Masthead lights signal a vessel’s underway status on a forward path. When you see two white lights stacked vertically, that’s a classic marker that you’re looking at a power-driven vessel making way.

• The spacing matters. The lights aren’t glued at the same height as a decorative touch; they’re deliberately vertically separated so observers can tell there are two distinct masthead lamps, even at a distance or in choppy water.

• The color isn't the drama here. Masthead lights are white. The color drama you should notice more readily comes from the sides—red on the port (left) side and green on the starboard (right) side. Those sidelights help you judge which way the vessel is heading relative to you, but the masthead pair is the quick badge of identity.

• Sail-powered vessels have a different signature. While sailing ships may still show white lights, their masthead configuration is not the same two-headliner signal as a power-driven vessel. Recognizing the two-masthead cue helps you separate the specialists from the general traffic, at least after dark.

A practical mental model you can carry on deck

Imagine you’re scanning the horizon with a pair of binoculars, keeping a constant yaw on the scene. You notice a vessel approaching in the distance. You pick out its lights:

• If you see two white lights stacked, you’ve got a power-driven vessel. You know it’s likely engine-powered and moving, which usually implies a certain predictability in maneuver—though not a guarantee.

• If you only see one light, or if you see lights that don’t align in a neat vertical pair, you pause and reassess. It could be a vessel at a strange angle, or perhaps a different type altogether. The key is not to jump to conclusions; the two masthead lights are the anchor.

• White sternlight and red/green sidelights complete the picture. The combination of colors and positions can tell you which side of the waterway the other vessel is on, and how fast it might be moving. But the two masthead lights are the sure-fire identifier for a power-driven vessel under 50 meters.

A quick note on context and safety

We’re talking about a rule that keeps night navigation clear and predictable. It’s not just about memorizing a fact; it’s about building a reliable habit. When you’re out there—whether you’re steering, observing, or just learning—the ability to read lights quickly gives you time to plan safe crossing, overtaking, or passing strategies. It reduces the guesswork and increases the margin for safety, which matters a lot when visibility is limited.

A few friendly tangents that still stay on topic

• Weather and light conditions change the game. Fog, rain, or haze can dull sight lines. Two masthead lights are especially useful because their vertical arrangement helps observers pick them out in imperfect visibility. That said, always couple light-reading with speed, bearing, and sound signals. Don’t rely on lights alone to judge another vessel’s intentions.

• Modern vessels may have extra signaling gear, like AIS indicators or radar reflectors. While those tools are invaluable, the visual cue remains king on the water. The two masthead lights are universal and don’t depend on electronics, which is a big plus when electronics fail or power is momentarily disrupted.

• Training, drills, and routine watchkeeping matter. It’s one thing to know the rule in theory; it’s another to apply it on a busy night. Regular practice in recognizing light patterns—alongside standard watch routines—helps crews react quickly and calmly.

Putting the pieces together: a simple rule with real-world impact

Here’s the takeaway you can carry with you into any voyage or training scenario: Under 50 meters, a power-driven vessel is identified by two masthead lights, arranged vertically and sharing the same white hue. It’s a straightforward cue that helps you distinguish power-driven craft from sailing vessels and other traffic. While the all-around light, sternlight color, or extra port lights can appear in various configurations, they don’t offer the universal, unambiguous identifier that two masthead lights provide.

Let’s ground this in a real-world moment. You’re navigating a busy channel at dusk. A small tug with a barge is chugging along ahead of you. If you spot the two white masthead lights in a tight vertical line, you know you’re watching a power-driven vessel likely maneuvering under power. You’ll expect it to respond to your actions with a level of predictability typical of powered craft, but you’ll still keep a safe distance and monitor its speed. Your next move—whether you pass behind it, overtake, or cross ahead—will be guided by standard passing rules and the normal courtesy of marine traffic. The lights give you the first piece of the puzzle, and your training fills in the rest.

A final note on clarity and confidence

The beauty of these lighting rules lies in their clarity. They aren’t clever tricks—just a practical language that ships speak when the sea is dark. When you’re on deck, that shared language helps you see, anticipate, and respond with confidence. And that confidence is what keeps nights on the water safer for everyone.

If you’re curious to learn more about COLREGs and how the different lights interact with other signals on board, a few accessible resources can be handy. Look for official navigation-light diagrams, reflect on vessel types and their typical lighting, and, if you’re near water, take a nighttime harbor walk with a seasoned skipper so you can see these patterns in action. The more you observe, the more natural it feels to read the sea like a living map.

In short: two masthead lights, vertical and white, are the telltale mark of a power-driven vessel under 50 meters. Remember that image when you’re on the water, and the night becomes a lot less mysterious. It’s one small rule with a big payoff in safety, clarity, and calm seamanship.

If you’d like, I can tailor a few quick, light-weight checks you can run on a night watch to reinforce this identification habit—little routines that fit neatly into your existing practices at the helm.