Rule 20 requires vessels to display lights and shapes according to their type and activity

Outline (skeleton for flow and clarity)

• Hook and purpose: why Rule 20’s lights and shapes matter on the water

• Core idea: vessels must display proper lights and shapes according to their type and activity

• Lights in practice: what you’ll see at night on different vessels (sidelights, masthead, stern, towing)

• Day shapes: the signals used in daylight to indicate status (anchor, not under command, restricted maneuverability, sailing, fishing)

• Real-world examples: how signals help avoid collisions and keep everyone safe

• Quick takeaways: how to apply this knowledge on the water

• Resources and further learning: where to look for precise configurations

Rule 20: Let there be light, and shapes that tell a story

Let me explain this in plain terms: Rule 20 isn’t a secret code. It’s the safety net that says, at night or when visibility drops, every vessel must show the right lights and shapes so other mariners can read what’s going on. The idea is simple—if you can’t see the whole boat, you should still be able to deduce its type and what it’s doing. That way, you can steer clear of danger the moment you spot a light or a shape in the dark.

Why lights and shapes matter

Think about driving in dense fog. You rely on taillights, headlights, and blinking signals to know where cars are and what they’re doing. On the water, the same logic holds, but with a maritime twist. Lights indicate whether a vessel is under power, under sail, anchored, or maneuvering in a restricted way. Day shapes—those symbols you see on the hull in daylight—convey status when conditions aren’t shiny and dark. Together, lights and shapes help everyone maintain a safe distance, predict actions, and avoid collisions that could end badly.

What you’ll see on a vessel at night

Let’s start with the basics you’re most likely to notice:

• Sidelights: A red light on the left (port) and a green light on the right (starboard). They tell you which side of the boat you’re approaching from.

• Masthead light: A bright white light forward, usually visible from a long distance on a power-driven vessel.

• Stern light: A white light at the back, helping you see the boat from behind.

• Towing light: A white light that some boats use when they’re towing a long line or another vessel.

These lights aren’t decorative. They’re placed to be visible at specific angles and distances so you can judge speed, course, and risk from a distance. And yes, there are rules about intensity, visibility range, and the exact height at which to mount them. If you’re curious about the precise specs, a trusty navigation rules book or a formal pocket guide will spell them out in neat, exact terms.

Day shapes: signals you read in daylight

When the sun’s out, lights fade away, but shapes rise to the occasion. Day shapes are standardized markers that tell other vessels what you’re up to. They’re usually simple silhouettes—think balls, diamonds, and cones—arranged in specific patterns on the vessel’s fore and aft sides or stacked in a vertical line. The key idea is consistency: every mariner should instantly recognize what a particular arrangement means.

You don’t need to memorize every possible combination to understand the gist. The important takeaway is this: there are day shapes corresponding to common activities and statuses—anchored, not under command, restricted in ability to maneuver, sailing, fishing, and more. Recognizing that a vessel is, for example, anchored because you see the classic day shape helps you adjust speed and course early rather than reacting at the last moment.

How Rule 20 works in practice: common scenarios

Let’s walk through a few everyday situations to ground this in reality.

• A power-driven vessel at night: You’ll typically see the standard white masthead light forward, a white stern light at the back, and red/green sidelights on the sides. If you’re approaching head-on, you’ll notice both vessels showing their sidelights and perhaps a dimmer, consistent speed indication. The message is simple: this is a moving vessel under power; keep clear.

• A sailing vessel at night: In addition to the usual sidelights and stern light, a sailing vessel may display a special signal that indicates it’s under sail and not relying on engine power alone. This helps you assess whether you should pass with extra caution, especially if wind shifts could affect tack and course.

• A vessel fishing in restricted visibility: If the boat is actively fishing and below deck, the lights and shapes are chosen to convey activity and vulnerability. Other mariners should give extra space, recognizing that fishing gear or nets can extend into a path you’d normally use.

• A vessel at anchor after dark: A strong, unambiguous signal is shown by day shapes (and a white all-around light at night). Other boats will know this is not a moving target and should steer clear, allowing the anchored vessel its space.

• A vessel not under command or restricted in maneuverability: These situations are particularly critical. The not-under-command status is signaled by distinct day shapes, while restricted maneuverability uses a specific combination of shapes that abruptly communicates the vessel’s limited ability to move out of your way. The takeaway for you is simple—allow extra distance and plan a wider pass.

Putting it all together: safety comes from clarity

One of the neat things about Rule 20 is that it aligns expectations across diverse waters. A fishing boat and a cargo ship might have very different missions, but both are bound to use lights and shapes to tell other mariners how they’re operating. That shared language reduces guesswork. It’s not just a set of rules; it’s a practical map for navigating in crowds, dodging fog banks, and staying on the same page as your fellow captains.

Practical tips you can use

• Keep a mental checklist: If you’re approaching another vessel, scan for the lights first, then check for any day shapes. The order matters because at night, lights are your quickest read on intent.

• Don’t rely on a single cue: A light can be out of place or dim. If something looks off, give more space and slow down to reassess.

• Learn a few core combinations well: Sidelights plus masthead and stern lights? A grounded ball in the day? A diamond-and-ball setup? Familiarize yourself with the most common signals you’ll encounter on busy waterways.

• Use real-world practice: If you operate near harbors or busy routes, pay attention to how nearby vessels signal. It’s a great way to build intuition without overthinking every move.

• Keep resources handy: A simple, up-to-date guide to COLREGs or a pocket reference can be incredibly useful when you’re trying to confirm a signal you’re not sure about.

What this means for you on the water

Rule 20 isn’t about memorizing a long list so you can recite it verbatim. It’s about building a shared toolkit of signals that make the water safer for everyone. When you spot the right light, you know whether a vessel is coming toward you, moving away, or parked for the night. When you see day shapes, you instantly know whether a boat is anchored, fishing, sailing, or restricted in maneuverability. And when you act with that knowledge—slowing down, altering course, or passing at a safe distance—you’re taking part in a simple, powerful habit: clear communication through signals.

A few memorable takeaways

• Rule 20 centers on proper lights and shapes that convey a vessel’s type and activity.

• Lights (sidelights, masthead, stern, towing) are the daytime and nighttime language of the sea.

• Day shapes communicate status when visibility is good, complementing the lights.

• The goal is to prevent collisions by making intentions obvious to nearby mariners.

• Practice spotting these signals in real-world settings, and keep a reliable reference within reach.

Resources to deepen your understanding

• A current Navigation Rules reference book or a reputable online marine safety guide can provide exact configurations for different vessel types and activities.

• Local maritime authorities often publish simplified guides for crews, captains, and boaters that map signals to common scenarios.

• If you ever get curious about the science behind the signals, marine safety organizations and training centers offer approachable explanations and quick-reference sheets.

In the end, Rule 20 isn’t a mysterious code so much as a practical, daylight-and-dark communication system. It helps you read the sea’s traffic just as clearly as you’d read road signs on a crowded city street. When you’re out there, it’s not about chasing perfection; it’s about embracing a reliable universal language that keeps you, your crew, and every other mariner safer as you share the same water.

If you ever find yourself in a moment of doubt, ask this: what signal am I seeing, and what does it tell me about the vessel’s status? Answering that question quickly can be the difference between a smooth passage and a tense wake. And that safety margin—well—that’s the real prize of understanding lights and shapes on Rule 20.