Why the stern light shows a 135-degree arc under COLREGs and what it means for safe night navigation

Lights on the water aren’t decoration. They’re a quick, universal language that helps boats share the night without collisions. If you’ve ever done a night sail or watched a harbor wake you’ll know how essential those signals are. Let me walk you through one of the most straightforward, yet sometimes puzzling, details: the arc of visibility for different navigation lights and why it matters.

The quick answer, right up front

Question: Which light is designed to show an arc of 135 degrees, 67.5 degrees from each side of the vessel?

• A. Stern light

• B. Masthead light

• C. All-around light

• D. Towing light

Answer: Stern light.

That “stern light” is a white light mounted at the rear of powered vessels. It’s carefully positioned to cover 135 degrees of arc—67.5 degrees to each side of the vessel’s stern. In practice, this helps boats behind you understand where you are and how you’re moving. It’s a behind-you signal, literally giving other mariners a rearward reference point at night or in poor visibility.

A quick tour of the other lights

To really appreciate the stern light, it’s helpful to compare it with the other common navigation lights you’ll see on COLREGs rules:

• Masthead light: This is the bright white light at the top of the rig or superstructure. It covers a 225-degree arc. Think of it as the forward-facing beacon, signaling a vessel’s forward course and presence to other boats in front or to the sides as you head out.

• All-around light: As its name suggests, this one shines in every direction—360 degrees. It’s common on smaller vessels or on vessels that aren’t using the full forward-and-aft light arrangement. It’s the “standalone” beacon that keeps you visible from all angles.

• Towing light: This is a specialized signal used when a vessel is towing another. It involves a yellow light (often placed in a specific position above the stern light) to indicate that the boat is engaged in towing. The exact visibility criteria are tailored to ensure other mariners understand that a tow operation is underway.

Why those arcs exist

The arc measurements aren’t arbitrary. They’re designed to convey a clear picture of where you are relative to other vessels:

• Stern light’s 135-degree arc means you’re primarily visible from behind, with the light extending out to the sides a bit. It communicates: “I’m moving in a direction toward the rear of the boat.”

• Masthead light’s 225-degree arc covers most of the vessel’s forward half, ensuring boats ahead or to the sides know you’re there as you sail or power toward them.

• All-around light’s 360 degrees keeps you visible from every angle, which is especially helpful for boats that aren’t using other navigation lights or are at anchor.

• Towing lights signal a more complex operation, letting nearby vessels know not to assume the towed vessel’s path unless they’ve accounted for the tow.

Putting it into real-world sense

Imagine you’re on a harbor inlet late at dusk. A motorboat appears in the distance, its stern light flashing as it slows to a stop. If you’re following behind or crossing from your side, that 135-degree stern signal helps you read its likely path. You know the boat is oriented with its rear toward you, which changes how you steer and pass. Without that rearward cue, you’d be guessing a lot more and risk a near-miss.

Now picture a small sailboat at anchor at a quiet cove. It displays an all-around light, visible from every direction. You’re approaching at night, and that 360-degree glow tells you, clearly, “Here I am,” even if you’re coming up from off the stern or dead ahead. There’s a sense of reassurance when the maze of silhouettes on the water starts to make a little more sense.

What to remember, practically

If you’re learning COLREGs with the goal of staying safe on the water, a few takeaways fit neatly into memory:

• Stern light = 135-degree arc, centered on the rear of the vessel. It’s a strong cue that the vessel’s primary direction is away from you, toward the stern.

• Masthead light = 225-degree arc. It’s the forward beacon that helps you gauge a vessel’s head-on or crossing intentions.

• All-around light = 360 degrees. It’s a simple, universal signal used by smaller vessels or in specific situations (like anchored or not using separate forward/stern lights).

• Towing light = yellow, indicating towing operations. It’s a specialized signal that modifies how others behave around the vessel.

If you’re ever unsure about a light, remember this quick mental map: front lights tell where a vessel is headed; rear lights tell you where it’s been; a single light that goes all around keeps you in the loop no matter where you’re standing on the water.

A few nuanced notes that help deepen understanding

• Visibility and weather matter. A bright, clear night makes these arcs easier to read. In fog or rain, the same arcs become even more important since you may have limited sight lines. The lights act like a language you’ll quickly learn to interpret.

• The same lights, different vessels. A stern light on a small motorboat looks like the same light you’d see on a larger vessel, but the angle and intensity can vary with size and mounting height. The human brain is pretty good at decoding those cues once you’ve spent a season or two on the water.

• It’s not just about seeing; it’s about predicting. The arc of visibility isn’t only about “can I see you?” It’s also “where do you think the other boat will go next?” That’s why those rules are written with path predictions in mind, not just static signals.

A tiny detour to reality: how this shows up in responsibility on board

If you’re operating a vessel, you carry responsibility to understand and comply with the NAV rules set forth by COLREGs. It isn’t about flashy equipment; it’s about predictable behavior. The lights are pieces of a shared system that, when read correctly, reduce the chance of collision. They’re like the road signs of the sea: predictable, reliable, and essential.

A few quick tips for recognizing lights at night

• Scan the horizon before you move. Note any bright white light ahead (masthead) and any rearward signal (stern light) as you reposition.

• Don’t chase a single bright light. It could be a lighthouse or a buoy. Cross-check with the color and pattern to identify a vessel’s intentions.

• Train your eye to pick out the yellow towing signal if you’re near towing operations. It’s a telltale sign that you should give extra space and plan a slower, safer pass.

• Practice makes familiarity. The more you see these patterns on calm nights, the quicker you’ll identify the vessel’s course and predict the next move.

A few thought-provoking questions you’ll recognize later

• If you see only a white light ahead and none on the sides, what might that tell you about the vessel’s status?

• How would your action change if you notice a stern light only, versus a masthead light and stern light together?

• Why is it useful to have an all-around light when you’re anchored in a busy harbor?

A closing thought

The night water has a language all its own, and navigation lights are its grammar. The stern light—the 135-degree rear signal—might be a small detail, but it’s a big piece of the puzzle. It helps tell other mariners, in a glance, where you are and where you’re likely to head next. The masthead light, the all-around light, and the towing light round out the system, each with its own purpose and arc.

If you ever wonder how to stay safer on the water, start with lights. They’re not just about visibility; they’re about intention. And intention, when shared, reduces risk for everyone on the water. So next time you’re near a vessel at night, take note of those arcs. They’re the sea’s most readable handwriting, and reading it well can turn a potentially tense moment into smooth sailing.

Short, practical recap

• Stern light: 135-degree arc, 67.5 degrees to each side; signals behind you for powered vessels.

• Masthead light: 225-degree arc; forward-facing signal, high and bright.

• All-around light: 360-degree visibility; versatile for small vessels or special cases.

• Towing light: yellow, indicates towing operations; signals a different kind of maneuver.

With this you’ve got a clear, human way to understand the essential signaling on the water. It’s not just a quiz item; it’s a real, practical skill that helps everyone share the ocean with a little more confidence and a lot more safety.