Rule 20 of COLREGs focuses on establishing the display of lights and shapes to keep vessels visible and safe

Outline:

• What Rule 20 is all about: the visual language ships use on the water

• Why lights and shapes matter: safety, recognition, and quick decisions

• The basic lighting everyone's meant to display

• Day shapes vs. night signals: what they communicate and to whom

• Reading the sea’s silent language in real life

• Gentle reminders for staying visible and clear on the water

• A closing thought: why this rule keeps sailors safe and ships in harmony

Lights on the water aren’t just pretty glows at dusk. They’re a code—one that every skipper, mate, or student navigator learns to read without thinking. Rule 20 of the COLREGs puts this code front and center. Its primary focus? Establishing the display of lights and shapes. Put simply, it’s about making vessels visible and their intentions legible, so the sea doesn’t spit surprises at us when visibility dims or the night grows thick.

Why this matters more than you might think

Picture yourself steering through a busy harbor as day fades. The water isn’t quiet; it’s a moving mosaic of wakes, swells, and shadows. In that transition, lights become signposts. A green sidelight here, a white stern light there, a quick glance at a day shape—these tell you, at a glance, what kind of vessel you’re dealing with and what it might do next. The rule isn’t about fancy signaling; it’s about predictable behavior to prevent collisions. If everyone’s lights and shapes follow a common standard, you gain precious seconds to adjust speed, change course, or hold your position.

The basic language: what you’ll actually see

Rule 20 sets the stage for two kinds of visual signals: lights (used from sunset to sunrise and in restricted visibility) and day shapes (used in daylight when lights aren’t enough or aren’t required for some vessels). Here’s the gist:

• Navigation lights: These are the colored beacons you’ll recognize easily. Red on the port (left) side, green on the starboard (right) side, plus white lights that indicate the vessel’s stern and sometimes masthead signals for larger ships. The specific arrangement depends on vessel type and activity (power-driven, sailing, fishing, not under command, etc.), but the core idea is simple: the colors and positions tell other mariners how to predict your path.

• The day shapes: In daylight, ships display shapes that convey status or constraint. Think of shapes like balls, diamonds, and cones. They aren’t mere decorations; they’re quick-reads about what a vessel can or cannot do in the near future. They help other skippers anticipate actions even when the sky is bright.

The why behind the distinctions

You might wonder, “Why not just rely on a horn or radar?” The answer is: there are times when sound signals or radar info isn’t enough. Sound travels differently than light, and it can be muffled by fog, rain, or engine noise. Visual signals cut through some of that ambiguity. They also provide a universal shorthand across languages and flags. Whether you’re at a windy harbor or far offshore, those lights and shapes are a shared vocabulary.

Night signals and limited visibility: what to expect

When visibility drops—night, fog, rain—the lights take on even greater importance. A vessel’s position, size, and potential maneuvering needs become a lot clearer when you can observe the status indicated by the lights. That whitening glow on the horizon isn’t just a light; it’s a beacon that helps you gauge distance and trajectory. In calm, clear weather you’ll still rely on lights and shapes, but the stakes feel higher when the darkness closes in.

Day shapes: a quick primer you can rely on

If you ever find yourself on deck when the sun is high, day shapes save you from guesswork. They provide a snapshot of the vessel’s condition or constraints. A simple rule of thumb: shapes communicate immediate, practical information about what the vessel can do. They’re not fancy; they’re utilitarian, and that’s exactly why they’re so effective. If you’re ever unsure what a shape means, remember this: it’s a signal about capability, not intention alone.

Reading the sea’s silhouette: a few practical scenarios

Let me explain with a couple of real-world vibes.

• Meeting head-on at night: You see a green sidelight on one vessel and red on the other. If those colors face you head-on, you know you’re in a crossing situation—each vessel must hold its course or alter course to pass safely. Quick recognition keeps the risk of a near-miss low, even when you’re tired or anxious.

• Overtaking or being overtaken: You’ll notice a series of lights that tell you which ship is the “give way” vessel. Understanding these signals helps you decide whether to maintain speed, slow down, or change lane in the water just like you would merge lanes on a busy highway.

• In restricted visibility: Fog rolls in and you’re surrounded by muffled engine noise and the dim glow of distant vessels. Here, the lights aren’t cosmetic; they’re your primary cues to where someone else might be headed. It’s a bit like driving with really good headlights in a fog bank—every shadow is a potential hazard unless you’ve got clear signals ahead.

A few practical tips to keep the signal clear

The best signal is a clearly visible signal. Here are a few straightforward ways to keep Rule 20 meaningful in real life:

• Check your own lights first: Fog or rain can dull lenses, and bulbs burn out. A quick walk-around to confirm clean lenses and functioning bulbs goes a long way.

• Keep the right lights on when they’re supposed to be on: Sunset to sunrise isn’t just a convention; it’s when other mariners count on your lights to be visible.

• Understand the general idea of day shapes: Even if you don’t memorize every shape’s exact meaning, recognizing that a vessel is constrained or not under command gives you a heads-up to adjust your plan.

• Watch for blinking or inconsistent signals: If a light sequence looks off, treat the vessel as uncertain and give extra room. When in doubt, slow down and communicate with a sound signal if necessary.

• Practice keeping situational awareness: Regularly scanning both lights and shapes, and cross-referencing them with radar or AIS where available, makes your decisions faster and safer.

A practical mindset you can carry

Rule 20 isn’t just a rule in a handbook; it’s a mindset for watching, predicting, and acting. Think of it as the sea’s etiquette. If you can read the lights like you read street signs—glance, interpret, respond—you’ll move more confidently through busy waters. And while the specifics may feel technical, the core idea is simple: visibility is safety, and clear signaling is the language that keeps everyone moving in the same direction.

A few friendly reminders as you navigate

• The sea is a living thing; conditions change quickly. Even with Rule 20 in place, stay adaptable. A vessel that seems far away can close in faster than you expect in a heavy chop.

• Communication matters. If a signal isn’t clear or you’re unsure about another vessel’s course, you can use sound signals to confirm intent or to request space.

• Invest in good gear. Clean optics, properly mounted lights, and a basic understanding of where each signal should appear will serve you well in busy waters.

Why Rule 20 matters in the big picture

Rule 20 sits at the intersection of visibility and understanding. The other rules guide your actions on the water, but Rule 20 makes sure others can read your actions in the first place. It’s the visual backbone that supports safer navigation, especially when conditions threaten to blur lines of sight. Without clear lights and predictable day shapes, the more complex rules would be harder to apply. In a world where a single mistake can ripple across the deck, Rule 20 offers a steady, universal language to avoid collisions and keep crews safe.

A closing reflection

Next time you observe a vessel at dusk or hear the faint hum of engines in fog, take a moment to notice its signals. The red and green of a sidelight, the white glow of a stern light, or the simple silhouette of a day shape is more than color and form. It’s a message from one navigator to another: I’m here, I’m moving, and I intend to do so with care. That mutual recognition—that shared discipline—is what keeps the ocean from becoming a place of guesswork.

Resources and further reading you might find helpful

• The COLREGs themselves offer the official language for lights and shapes and the many scenarios sailors encounter.

• Coast guard guides and navigation manuals often include diagrams of typical light configurations and day shapes.

• Weather and visibility tools, while not part of Rule 20 itself, complement signal reading by giving you a sense of when lights will be your main guide.

In the end, Rule 20 is about clarity in a world of motion. It’s a practical, accessible rule that translates into safer voyages, fewer near-misses, and a calmer crew. When you glance across the harbor at night and see that familiar glow, you’re watching a shared standard at work—a quiet reminder that the sea, for all its beauty and vastness, relies on simple, reliable signals to keep us all moving in harmony.