Rule 18 makes it clear that all other vessels must yield to a vessel constrained by its draft.

Title: Rule 18 Unpacked: When the Draft-Constrained Vessel Has the Right of Way

Let’s imagine this scene: you’re threading a busy shipping lane or slipping through a snug harbor entrance. A massive vessel sits there, its hull gently resting in shallower water than its long, wide frame would prefer. It can’t swing and sway with the same freedom as a smaller boat. This is what mariners call a vessel constrained by its draft. And under Rule 18 of the COLREGs, all other vessels are required to keep out of its way.

A simple answer, a big responsibility

If you’re ever tempted to think this rule is about who’s in charge on the water, here’s the takeaway: all other vessels must yield to a vessel constrained by its draft. That means power-driven boats, sailing vessels, fishing boats, and vessels towing a line—all of them share the obligation to give way when they encounter a draft-constrained ship. It’s not a matter of speed or seamanship alone; it’s a safety condition born from physics. In shallow water, a big ship with a heavy draft just can’t maneuver as nimbly as its deeper-drafted cousins. Letting it hold course could mean grounding or a collision. So the rule is less about who is bigger and more about who can’t move as freely.

What exactly is a vessel constrained by its draft?

Think of it this way: the draft is the vertical distance from the waterline to the hull’s bottom. Some ships, like certain container ships, bulk carriers, or tankers, ride with a lot of concrete depth beneath them. In shallow channels, river mouths, or near port approaches, that depth becomes a hydraulic chain held tight. The draft-constrained vessel has limited ability to alter its course or speed quickly. It can’t swing wide, it can’t suddenly dart aside, and it certainly can’t stop on a dime. Weather, current, and the ship’s own momentum all conspire to restrict its options.

Rule 18 isn’t a lecture on who gets to choose first; it’s a safety protocol built on the physics of movement at sea. When a big ship is tucked into a shallow area, the margin for error shrinks. The smaller vessel, which could normally maneuver with relativa ease, is the one expected to yield. It’s a practical rule that helps avoid close-quarters drama in places where maneuverability is a premium.

Why this matters in practice

You might ask, “How do I know I’m dealing with a draft-constrained vessel?” The short answer is: identify the situation and act with caution. A vessel constrained by its draft is most common in:

• Narrow channels or entrances to harbors where depth is limited.

• Approaches to ports where dredging or sedimentation has reduced depth.

• Areas with shoals, bars, or sandbanks that reduce available draught.

• Large vessels that are unavoidable in the traffic mix but cannot easily move aside.

In these contexts, Rule 18 puts the onus on every other vessel to stay clear. It isn’t about heroics or who has the right of way in a broad sense; it’s about ensuring that the ship with limited maneuverability isn’t squeezed into a dangerous situation.

Let me explain with a mental picture: imagine trying to squeeze a bus through a narrow alley. You (the smaller craft) have to pick a path carefully, leaving room for the bus to pass at a comfortable pace. The bus is constrained by the alley’s width and the weight of its own mass. In marine terms, the “alley” is the shallow water and the bus is the draft-constrained vessel. The result? You pause, you slow down, you adjust course early, and you give it a wide berth.

Real-world scenarios you’ve probably thought about

• Harbor entrances during dredging: A large bulk carrier nears the entrance while the depth is just enough for it to pass. Other vessels keep a safe distance, timing their moves to avoid forcing the carrier to alter course abruptly.

• Narrow rivers with heavy traffic: A tanker or container ship in a tight bend isn’t going to swing wildly. Other traffic slows or changes course slightly to maintain a safe gap.

• Approaches to ports in shoal waters: A fishing vessel might be weaving along the outer edge of the channel. While it has its own responsibilities, the draft-constrained ship remains the focal point of awareness for everyone else.

What this means for mariners on deck

• Stay alert in areas where depth limits are posted or where you know dredging has reduced the channel. The signs aren’t just paperwork; they’re your roadmap.

• If you’re not draft-constrained and you see one approaching, give it space. Don’t press forward with cutting speed or a last-minute course change. Time your actions to keep a reliable margin.

• Maintain early recognition and clear communication. If your vessel has any doubt about another vessel’s status, slow down, broadcast a quick VHF advisory, and adjust as needed. A short, crisp exchange can prevent a near miss.

A note on interpretation and learning

Rule 18 is one of those parts of the Rules of the Road that rewards clear understanding more than clever interpretation. It’s not about clever maneuvers; it’s about predictable, safe behavior in tricky situations. The practical takeaway is simple: when there’s a possibility a large ship can’t maneuver easily because of depth, you yield. It’s the prudent move that keeps everyone safer.

Guidance for students and lifelong mariners

• Visual cues matter. Look for signs of depth limitations, such as posted draft restrictions, shoaling indicators, or the presence of dredging equipment in the area.

• Don’t assume that a large vessel will move just because you need to pass. Predictability wins. Ease off, keep a safe distance, and let the draft-constrained ship do the work it can do.

• Communicate clearly. If you’re in a scenario where the safe course might require negotiation, use VHF radio or other established channels to confirm the plan with the other vessel’s master or the bridge team.

• Practice situational awareness. In a busy waterway, you’ll see a mix of vessel types—from power-driven boats to sailing craft to towing operations. Each has its own set of restrictions and responsibilities. Rule 18 reminds us to spot the constraints and act accordingly.

Common misconceptions worth clearing up

• It’s not a “who has the right of way” contest. The phrase “All other vessels” might feel absolute, but it’s rooted in safety. The constrained vessel has limited maneuverability; other vessels adjust their behavior to reduce risk.

• It isn’t limited to very large ships. Any vessel with a restricted draft in a given environment can trigger this obligation on others. Sometimes you’ll see smaller ships in shallow sections that still influence the safety picture because of depth limitations.

• It isn’t a one-size-fits-all rule. The COLREGs are a big web of interlocking rules. Rule 18 interacts with speed, compass bearing, and other rules spread across the water. The smart approach is still to anticipate and adapt, not memorize a single line of text.

A little practical recap

• The rule in one line: All other vessels must keep out of the way of vessels constrained by their draft.

• Why: The draft-constrained vessel has limited maneuverability in shallow water or narrow channels; giving way reduces the risk of grounding or collision.

• How to apply: In situations with depth restrictions, slow down, create space, and communicate your intentions. Be predictable and patient.

• What to watch for: Depth charts, channel markings, dredging activity, and the physical cues of a large vessel’s limited ability to swing or stop.

A final thought

Navigation at sea isn’t just about being fast or assertive. It’s about reading the water—the currents, the depth, the channel width—and making choices that keep everyone safe. Rule 18 is a reminder that sometimes the best move isn’t the most dramatic one, but the one that preserves the space for the ship that can’t move as freely as the rest. When you’re on the bridge, or at the chart table, that awareness can be the difference between calm navigation and a close call.

If you’re curious to explore more, you’ll find plenty of real-world scenarios that illustrate how these rules play out in ports, channels, and busy waterways. It’s like learning a language—the more you listen and observe, the clearer the conversations become. And that clarity is what makes every voyage safer for everyone on board.