COLREG Rule 18 Explained — Responsibilities Between Vessels

Plain-language guide to COLREG Rule 18 — the rule that decides who gives way when two different vessel types meet. Power-driven, sailing, fishing, RAM, NUC, CBD, seaplane — the pecking order at sea, with the exceptions in Rules 9, 10 and 13 explained.

What does Rule 18 actually say?

COLREG Rule 18 — formally titled "Responsibilities Between Vessels" — is the rule that sorts out which vessel gives way when two different types of vessel meet. It exists because vessels with different capabilities and constraints can't sensibly be treated the same way in a collision-avoidance decision. A loaded tanker can't dodge a sailing dinghy the way the dinghy can dodge the tanker.

Rule 18 establishes a priority hierarchy: the more constrained a vessel's ability to manoeuvre, the higher her priority. A power-driven vessel underway has the most freedom of manoeuvre, so she has the lowest priority — she gives way to almost everyone. A vessel not under command has zero ability to manoeuvre, so she has the highest priority — everyone gives way to her.

The Rule 18 priority hierarchy (the pecking order)

Listed from lowest priority (gives way to almost everyone) to highest priority (kept clear of by almost everyone):

A vessel constrained by her draught (CBD) sits outside this simple ladder. She's a PDV, so by Rule 18(a) she'd give way to sailing, fishing, RAM and NUC. But Rule 18(d) adds that other vessels shall avoid impeding her safe passage. That makes CBD effectively a special category: smaller and more manoeuvrable vessels keep out of her way, while she still owes way to RAM and NUC.

Rule 18(a) — Power-driven vessel responsibilities

The text of Rule 18(a) (paraphrased for clarity): a power-driven vessel underway shall keep out of the way of:

• (i) a vessel not under command;
• (ii) a vessel restricted in her ability to manoeuvre;
• (iii) a vessel engaged in fishing;
• (iv) a sailing vessel.

This is the rule most yacht skippers think of when they remember "power gives way to sail." But notice it goes much further than just sail. A power-driven yacht meeting a trawler hauling nets, a cable-laying vessel, or a drifting freighter with steering failure must keep clear of all of them.

Practical example: You're cruising under engine in a 12 m motor yacht at 8 knots. Ahead, a 9 m sailing yacht is on a converging course. Rule 18(a)(iv) puts you, the PDV, in the give-way position — provided no overtaking, narrow channel or TSS rule overrides. You alter course or speed early and substantially (Rule 8), passing safely astern of the sailing vessel.

Rule 18(b) — Sailing vessel responsibilities

A sailing vessel underway shall keep out of the way of:

• (i) a vessel not under command;
• (ii) a vessel restricted in her ability to manoeuvre;
• (iii) a vessel engaged in fishing.

"Right of way" doesn't exist in COLREG, and Rule 18(b) is one of the rules that proves it. A sailing yacht meeting a trawler under sail must give way to the trawler — even though she has a sail up. The trawler's fishing operations restrict her manoeuvrability; the sailing yacht's free-sailing condition does not.

Practical example: Day-sailing in a 7 m dinghy, you spot a trawler towing a beam-trawl about a mile off your port bow. Rule 18(b)(iii) puts you in the give-way position. You alter course to pass well clear of the trawler and (importantly) clear of any towed gear that extends behind her — Rule 26 lights and shapes give clues to the trawler's gear extent.

Rule 18(c) — Fishing vessel responsibilities

A vessel engaged in fishing when underway shall, so far as possible, keep out of the way of:

• (i) a vessel not under command;
• (ii) a vessel restricted in her ability to manoeuvre.

Note the phrase "so far as possible" — fishing vessels are themselves constrained by their gear and operations, so the requirement to keep clear is softer than for PDVs and sailing vessels. But the priority is unambiguous: fishing gives way to RAM and to NUC.

Rule 18(d) — Vessel constrained by draught (CBD)

Rule 18(d) is a slightly different beast. It says: "Any vessel other than a vessel not under command or a vessel restricted in her ability to manoeuvre shall, if the circumstances of the case admit, avoid impeding the safe passage of a vessel constrained by her draught, exhibiting the signals in Rule 28."

So CBD is protected — but only down to the level of RAM and NUC, which still have priority over her. A loaded VLCC transiting a deep-water channel through shallow surrounding water displays Rule 28 lights (three red, vertically) and the cylinder shape; other PDVs, sailing vessels, and fishing vessels avoid impeding her. But a NUC or RAM vessel does not have to give way to her.

Rule 18(e) — Seaplanes and WIG craft

Rule 18(e): "A seaplane on the water shall, in general, keep well clear of all vessels and avoid impeding their navigation. In circumstances, however, where risk of collision exists, she shall comply with the Rules of this Part."

Seaplanes sit at the bottom of the pecking order. The same general principle applies to Wing-In-Ground (WIG) craft — Rule 18(f) — when taking off, landing and in flight near the surface. When operating as displacement craft on the water, WIG craft are subject to the standard Rules of the Road like any other vessel.

When Rule 18 does NOT apply — the overrides

Three other rules override Rule 18. Memorising these is exam gold:

The mnemonic: 9, 10, 13 trump 18.

Why "right of way" is the wrong term

Many sailors say "I have right of way." COLREG never uses that phrase. Instead, the rules talk about stand-on and give-way vessels:

• The stand-on vessel shall keep her course and speed (Rule 17(a)(i)) until it becomes apparent that the give-way vessel is not taking sufficient action. Then she may, and eventually shall, take action herself.
• The give-way vessel shall take early and substantial action to keep clear (Rule 16), passing astern of the stand-on vessel if circumstances permit.

Rule 18 tells you which is which when vessel types differ. But the stand-on vessel never has the "right" to plough on through a collision — she has a duty to keep course and speed up to the point that doing so becomes dangerous, then a duty to take action.

Exam tips and common confusions

Rule 18 appears in Day Skipper, Coastal Skipper, Yachtmaster and most STCW deck-officer examinations. Common ways examiners trip candidates up:

• Mixing up RAM and NUC. RAM = restricted by the nature of work (cable-laying, dredging). NUC = exceptional circumstances (engine failure, drifting). Lights and shapes differ: RAM red-white-red vertical; NUC two-red vertical.
• Forgetting CBD is itself a PDV. CBD doesn't outrank RAM or NUC — Rule 18(d) is a "do not impede" requirement on other vessels, not a vessel-type priority elevation.
• Assuming sailing always has priority. A sailing vessel gives way to fishing, RAM and NUC (Rule 18(b)) — and to anyone she is overtaking (Rule 13).
• Forgetting the 9/10/13 overrides. Examiners love narrow-channel and TSS scenarios where Rule 18 priorities are reversed.
• "Right of way" language. Examiners may directly ask you to correct a candidate who uses the term. Use "stand-on" and "give-way."
• Misreading lights at night. Knowing which vessel is which (PDV, sailing, fishing, RAM, NUC, CBD) starts with correctly identifying her lights — Rules 23 through 31.

Practise Rule 18 on the simulator

Reading the rule and answering a quiz is one thing. Applying Rule 18 in real time on a moving bridge picture is another. SkipperCheck's AIS/radar/COLREG bridge simulator has 54 scenarios — many of them directly testing Rule 18 priorities:

• Power-Driven vs Sailing Vessel — Rule 18(a)(iv) in clean form
• Power-Driven vs Fishing Vessel — Rule 18(a)(iii)
• Power-Driven vs RAM — Rule 18(a)(ii)
• Power-Driven vs CBD — Rule 18(d)
• Identifying NUC and RAM by lights — Rule 27
• CBD lights identification — Rule 28
• Fishing vessel light identification — Rule 26

The simulator is part of the Skipper Refresher Course. Three free demo scenarios let you try the bridge interface before deciding.

Frequently asked questions

What is COLREG Rule 18?

Rule 18 — "Responsibilities Between Vessels" — defines the priority hierarchy when different vessel types meet. Power-driven vessels give way to sailing, fishing, RAM and NUC vessels. Sailing vessels give way to fishing, RAM and NUC. Fishing vessels give way to RAM and NUC. CBD vessels get a "do not impede" status from other PDVs, sailing and fishing vessels.

Does Rule 18 always apply?

No. Rule 9 (narrow channels), Rule 10 (TSS) and Rule 13 (overtaking) all override Rule 18. In a narrow channel a sailing yacht gives way to a coaster restricted to the channel. Inside a TSS, smaller and sailing vessels do not impede PDVs in lanes. When overtaking, the overtaker keeps clear regardless of types.

What's the difference between RAM and NUC?

RAM (Restricted in Ability to Manoeuvre) — restricted by the nature of the work being done: cable-laying, dredging, replenishment, mineclearance. Lights: red-white-red vertical (Rule 27(b)). Day shape: ball-diamond-ball.

NUC (Not Under Command) — exceptional circumstances make the vessel unable to manoeuvre: engine failure, steering failure, drifting. Lights: two red all-round vertical (Rule 27(a)). Day shape: two black balls vertical.

Does a sailing vessel always have priority over a power-driven vessel?

No. Rule 18(a)(iv) gives sailing vessels priority over PDVs in standard encounters, but only when Rules 9, 10 and 13 do not override. A sailing vessel overtaking a PDV gives way to the PDV (Rule 13). A sailing vessel in a TSS gives way to PDVs in the traffic lanes (Rule 10). A sailing yacht meeting a fishing vessel gives way to the fishing vessel (Rule 18(b)(iii)).

What is a Vessel Constrained by her Draught (CBD)?

A power-driven vessel whose draught restricts her ability to deviate from her course — typically a large vessel transiting a dredged channel through shallow surrounding water. Lights: three red all-round vertical (Rule 28). Day shape: a cylinder. Other PDVs, sailing vessels and fishing vessels shall not impede her safe passage (Rule 18(d)), but she still gives way to RAM and NUC.

What does "shall not impede" actually mean?

"Shall not impede" is weaker than "keep out of the way." It means: position and operate yourself so that the other vessel does not have to alter course or speed to avoid you. Rule 8(f) elaborates the language and how this interacts with the give-way / stand-on framework.

Is there a memory aid for the Rule 18 priority order?

One common mnemonic for the giving-way order (from PDV up): "Power gives way to Sail; Sail gives way to Fishing; Fishing gives way to RAM; RAM gives way to NUC." Or top-down: NUC outranks RAM outranks Fishing outranks Sailing outranks Power. CBD slots between PDV/sailing/fishing (which avoid impeding her) and RAM/NUC (which she still gives way to). Always remember the 9/10/13 overrides.

How do I tell what vessel type I'm dealing with at night?

Lights. Each vessel type has a defined light pattern under Rules 23–31. PDV: masthead + sidelights + sternlight. Sailing: sidelights + sternlight (no masthead). Trawling: green-over-white plus sidelights/sternlight when making way (Rule 26(b)). Other fishing: red-over-white. RAM: red-white-red vertical plus PDV lights when making way. NUC: two red vertical, plus sidelights/sternlight when making way. CBD: three red vertical plus PDV lights. Glossary and the simulator's light scenarios drill these.

Related reading

• How to Send a MAYDAY Distress Call on Marine VHF Radio — step-by-step Mayday procedure
• Short Range Certificate (SRC) — Complete Guide — the marine VHF qualification
• All 69 simulator scenarios — COLREG and VHF practice
• Maritime glossary — 392 sailing, COLREG, VHF and navigation terms