Vessels must slow down in restricted visibility under COLREGs Rule 19.

Outline (skeleton)

• Opening: Why restricted visibility isn’t just “being careful”—it’s a formal safety requirement.

• The core rule: In restricted visibility, a vessel must proceed at a safe speed and reduce speed as necessary to avoid collisions.

• Why speed matters: Limited sight, longer stopping distances, less time to react.

• Distinguishing facts from myths: Other situations may demand caution, but the primary obligation in restricted visibility is to slow down appropriately.

• How to apply it: Practical ways to judge a safe speed—using radar, lookouts, sound signals, and speed adjustments.

• Real-life scenarios: Fog on the water, heavy rain, twilight along a coastline—and what you should do.

• Quick checklist: A compact, handy reminder for navigating in poor visibility.

• Wrap-up: Safety, clarity, and steady judgment as boats share the same quiet, dim sea.

Now, the full article.

Seeing less? Slow down. A simple idea, but it sits at the heart of COLREGs when visibility isn’t on your side. The rules aren’t just guidelines; they’re a framework that helps every vessel share the water politely and safely. When fog closes in, or rain hangs low in the air, or the sun vanishes into a murky horizon, your conscience as a mariner should kick in before your speed does. In restricted visibility, you must proceed at a safe speed and, if needed, reduce speed to keep yourself and others out of trouble.

What does “restricted visibility” really mean? It’s not a color code or a badge; it’s a condition. It’s when sight is impaired to a degree that you can’t clearly see other vessels, navigational marks, or dangers in time to avoid a collision. Fog, mist, heavy rain, snow, or even a thick screen of spray can be enough to trigger it. In those moments, the COLREGs set a clear expectation: slow down. Not because you’re being cautious in a hazy moment, but because you’re aligning your speed with what you can actually perceive and respond to.

If you’ve ever tried to drive through a dense fog, you know the feeling. You’re moving, but your perception of distance and speed is off. On the water, that same uncertainty multiplies. You may suddenly realize you’re closer to a buoy or another vessel than you thought. The goal isn’t to reach a destination faster; it’s to reach it safely and with the best possible chance to avoid a collision.

Why speed matters in restricted visibility is straightforward: you have less time and fewer chances to react. Your stopping distance grows because your ability to identify and interpret hazards is compromised. Even if another vessel is nearby, you might not have a clear, timely sense of its course and speed. A slower speed helps you buy those precious seconds to verify, to alter course, or to stop if necessary. It’s practical prudence turned into rulebook reality.

Let’s separate the idea from a few tempting alternatives. If another vessel is in sight, that’s important information, but it doesn’t automatically demand a speed change in every scenario. If waters are congested, you should exercise extra caution too, but the essential legal obligation in restricted visibility isn’t about sighting or traffic density alone. It’s specifically about adjusting your speed to the conditions so you can take proper and effective action to avoid collision. In other words, visibility drives the speed decision: the worse the visibility, the slower you should go, and go only as fast as you can safely manage.

Practical ways to apply the rule when fog, rain, or dim light hits

• Lookout and perception: In restricted visibility, you want a sharp lookout, ideally with a second pair of eyes. One person scanning the radar or AIS can be a lifesaver, especially in low visibility.

• Safe speed as a moving target: Picture your speed as something you adjust, not a fixed number. You should reduce it to a level where you can stop within the distance you can clearly observe, given your current conditions.

• Use all tools at hand: Radar, AIS, charts, and depth sounders aren’t luxuries; they’re the tools that help you gauge where you are and where hazards might be. If you’re uncertain, slow down more and rely on these aids to confirm what you can’t see.

• Communicate clearly: Use sound signals or radio discipline to indicate your presence and intent. A light or horn might be your only steady allies in fog, so don’t skip a chance to send a cautious message to others.

• The “safe speed” mindset: Safe speed isn’t a fixed pace; it’s a dynamic assessment. How fast you go depends on visibility, traffic, weather, and the maneuverability of your vessel. In rough seas or with a high aspect, you’ll want to slow more.

Real-life feel: fog rolling in by the harbor, rain tapping the window on a coastal run, or twilight that makes markers hard to spot. In these moments, the rule isn’t some dry paragraph; it’s a practical shield. You’re balancing momentum with caution, much like driving to pass a stalled car on a rain-slick street—except here, the waterline is your boundary and the other vessels are unpredictable neighbors sharing a single channel. You’re not trying to rush; you’re trying to keep others—barges, sailboats, ferries, fishing vessels—from colliding. That’s the essence of Rule 19 in restricted visibility: to proceed safely by reducing speed as needed so you can take proper and effective action.

Common misconceptions, cleared up

• Misconception: “If I can just see the other boat, I’m fine.” Reality: Even if another vessel becomes visible, reduced speed might still be necessary. Visibility can change quickly, and you must be ready to act.

• Misconception: “Congested waters are the same as restricted visibility.” Not quite. You still adjust speed for safety, but the primary obligation in restricted visibility is tied to how poor the sight is, not just how many boats are around.

• Misconception: “Slowing down is a sign of weakness.” On the water, it’s a sign of maturity and safety. It shows you’re prioritizing prevention over urgency.

• Misconception: “All rules are about boat size.” The rules apply to every vessel, everywhere, with adjustments for speed and maneuverability as needed. Small boats aren’t exempt from the hazard; they’re often the most vulnerable when visibility drops.

A compact checklist you can carry on deck

• Do I have restricted visibility right now? If yes, slow to a safe speed and reassess continuously.

• Can I stop within the distance I can clearly observe? If not, slow further or alter course.

• Am I using radar, AIS, and depth information to supplement sight? Use them to confirm what you sense visually.

• Do I have a proper lookout and clear communication with other vessels? Keep your signals steady and predictable.

• Is my course consistent with safe passage for everyone around me? If there’s doubt, adjust.

Why this matters beyond a test answer

Think about a calm morning on a busy estuary, with sails and powerboats sharing a narrow channel. The fog bank isn’t dramatic, but visibility is down. You’re not just steering to stay on a path; you’re actively managing risk. Reducing speed isn’t about slowing to a crawl; it’s about keeping options open—the ability to turn, stop, or maneuver out of trouble if another vessel appears unexpectedly. That’s the practical magic of Rule 19: it turns theory into a habit you can trust when the world seems a shade grayer than usual.

A few more thoughts that keep the concept human

• It’s easy to treat speed as a personal preference. On the water, it’s a shared boundary. Your reduced speed protects you and others—the same waterway serves a dozen journeys at once.

• The sea has a quieter tempo in fog. You may feel a little lonely out there, but it’s a good time to practice focus and patience. Slowing down invites calmer decision-making.

• If you ever doubt the rule, remember this: in restricted visibility, the purpose of reducing speed is to ensure you can act properly and effectively if something appears. It’s about responsible seamanship, not bravado.

Final thoughts

So, under which condition must a vessel reduce speed? The honest, clear answer is: when navigating in limited or restricted visibility. It’s not merely about feeling cautious; it’s about adhering to a rule that’s designed to save lives when sight is poor and responses must be swift and precise. In those moments, speed becomes a variable, not a fixed number. Your job is to tune it to what you can actually perceive and respond to—consistently, calmly, and with a steady hand.

If you’re anchoring yourself in this rule, you’re not just memorizing a line from a handbook. You’re embracing a habit of safety that keeps every sailor, passenger, and crew member safer on the water. And that, in the end, is what makes the sea feel less like a gamble and more like a shared journey.