Rule 18 Explained: The Vessel Hierarchy Every Skipper Must Know

Every skipper learns early that some vessels get out of the way of others. But the logic behind that pecking order, and the situations where other rules take precedence, is where exam candidates stumble and, more importantly, where real collision risk lives. Rule 18 of the COLREGS is the rule that formalises the hierarchy, and understanding why it is structured the way it is makes it far easier to apply under pressure.

The underlying principle is straightforward: vessels with the least ability to manoeuvre get the greatest protection. A laden tanker in a dredged channel cannot simply turn 30 degrees to avoid you, and a vessel with gear streamed across the seabed cannot accelerate clear. The rules ask those who can manoeuvre to bear the burden of keeping clear of those who cannot.

One essential caveat before we begin: this article is educational support for exam preparation and practical understanding. It does not replace formal training, an MCA or RYA instructor, or the official COLREGS text. With that said, let us work through the rule from top to bottom.

The Hierarchy in Order

Rule 18 applies except where Rules 9, 10 and 13 otherwise require, and its paragraphs (a) to (c) set out who keeps out of the way of whom.

Not Under Command (NUC) sits at the top. Rule 3(f) defines a NUC vessel as one which, through some exceptional circumstance, is unable to manoeuvre as required by the rules and is therefore unable to keep out of the way of another vessel. Machinery failure or loss of steering are the classic cases. Power-driven vessels, sailing vessels and fishing vessels all keep out of her way.

Restricted in Ability to Manoeuvre (RAM) comes next. Rule 3(g) defines it as a vessel which from the nature of her work is restricted in her ability to manoeuvre as required by the rules. The examples in Rule 3(g) are broader than many candidates realise: servicing navigation marks, cables or pipelines; dredging, surveying or underwater operations; replenishment or transfers while underway; aircraft operations; mine clearance; and a tow that severely restricts the ability of tug and tow to deviate from their course. Power-driven, sailing and fishing vessels keep out of the way of a RAM vessel. Rule 18 places no duty between NUC and RAM vessels in either direction; neither can manoeuvre, and Rule 2 governs.

Vessels Engaged in Fishing rank above sailing vessels and power-driven vessels. Rule 3(d) limits the term to vessels fishing with nets, lines, trawls or other apparatus that restricts manoeuvrability, and expressly excludes trolling lines; a vessel trolling at speed has no special status. Under Rule 18(c) a fishing vessel underway shall, so far as possible, keep out of the way of NUC and RAM vessels, the qualifier acknowledging that a vessel with gear down has limited options.

Sailing Vessels sit below fishing vessels. Many recreational sailors are surprised by this: a yacht under sail does not have priority over everything except power. Under Rule 18(b) she keeps out of the way of NUC, RAM and fishing vessels.

Power-Driven Vessels sit at the bottom. Under Rule 18(a) a power-driven vessel underway keeps out of the way of all four categories above. She bears the greatest responsibility precisely because she is generally the most able to manoeuvre quickly and precisely.

Constrained by Draught: A Different Kind of Duty

A vessel constrained by her draught is deliberately not placed in the list above, and the distinction is one of the most examinable points in the rule. Rule 3(h) defines her as a power-driven vessel which, because of her draught in relation to the available depth and width of navigable water, is severely restricted in her ability to deviate from the course she is following. Only a power-driven vessel can hold the status.

Rule 18(d)(i) then says that 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. Three features of that sentence matter. First, the duty falls on any vessel other than NUC and RAM, which includes sailing and fishing vessels, not only other power-driven vessels. Second, the duty is to "avoid impeding", which Rule 8(f) explains as taking early action to leave sufficient sea room, rather than the fuller "keep out of the way" of paragraphs (a) to (c). Third, it is triggered by the signals: the protection attaches when she is exhibiting the Rule 28 signals. Rule 18(d)(ii) then requires her to navigate with particular caution. So a RAM vessel is not required to keep clear of a vessel constrained by her draught, and it is equally wrong to say that only power-driven vessels need avoid impeding her.

Where Rule 18 Fits Alongside Rules 9, 10 and 13

Rule 18 does not operate in isolation. Its opening words, "except where Rules 9, 10 and 13 otherwise require", mean that where those rules impose a specific obligation, the specific rule takes precedence.

Rule 9(b) says a vessel under 20 metres or a sailing vessel shall not impede a vessel that can safely navigate only within a narrow channel, and Rule 10(j) says the same about a power-driven vessel following a traffic lane. Both are near-reversals of the Rule 18 order, and they catch candidates who assume the hierarchy overrides everything.

Rule 13 governs overtaking, and this is where a common exam trap hides. Any vessel overtaking any other keeps out of the way of the vessel being overtaken, whatever the types involved. The direction that bites is the one candidates miss: a sailing vessel overtaking a power-driven vessel must keep clear of her, even though Rule 18 would otherwise rank her higher. None of this means Rule 18 "does not apply" in those situations; the other rules take precedence where they speak, and Rule 18 governs everything they do not.

Rule 17 also stays in view: the hierarchy decides who gives way, but the stand-on vessel must still act when it becomes apparent that the give-way vessel is not taking appropriate action. And Rule 2 runs through everything: no skipper can cite Rule 18 as justification for standing on into danger. You can explore how Rules 9, 10, 13 and 18 interrelate in the rule library.

Lights and Shapes: Identifying Where a Vessel Sits

A NUC vessel shows two all-round red lights in a vertical line, or by day two balls in a vertical line (Rule 27(a)). When making way through the water she also shows sidelights and a sternlight.

A RAM vessel shows three all-round lights in a vertical line, red over white over red, or by day a ball, a diamond and a ball (Rule 27(b)). When making way she also shows the masthead light or lights, sidelights and a sternlight. A dredger adds two red lights or balls on the obstructed side and two green lights or diamonds on the side another vessel may pass (Rule 27(d)), so red-white-red is the core of RAM identification, not the whole picture.

A vessel constrained by her draught may show, in addition to her lights as a power-driven vessel under Rule 23, three all-round red lights in a vertical line, or by day a cylinder (Rule 28). The word "may" matters: the signal is optional, and Rule 18(d) attaches to a vessel that is exhibiting it.

Fishing vessels are a common area of confusion. Rule 26 distinguishes trawlers from other vessels engaged in fishing. A trawler shows two all-round lights, green over white (Rule 26(b)). A vessel fishing by any other method, such as longlining or netting, shows red over white (Rule 26(c)). Both show two cones with apexes together by day, and both show sidelights and a sternlight when making way. Treating green over white as the universal fishing signal is a regular exam mistake, and misidentifying any of these at sea is a serious hazard.

A Practical Memory Aid

A reliable sequence to memorise for paragraphs (a) to (c) is NUC, RAM, Fishing, Sailing, Power, with constrained by draught remembered separately as the "avoid impeding" case. The more durable habit is to reason from principle: what is that vessel's realistic ability to manoeuvre? The more constrained she is, the higher she sits, and the more responsibility you carry to keep out of her way, or in the case of a vessel constrained by her draught, to avoid impeding her.

Limits Worth Drilling In

A few points about Rule 18 are worth fixing in memory before any exam or passage plan:

• Rules 9, 10 and 13 take precedence where they apply; in particular, an overtaking vessel keeps clear regardless of type.
• Constrained by draught is not a tier in paragraphs (a) to (c). The Rule 18(d) duty is to avoid impeding, it falls on every vessel except NUC and RAM, and it attaches to a vessel exhibiting the Rule 28 signals.
• Only a power-driven vessel can be constrained by her draught, and the Rule 28 signals are optional.
• Fishing vessels outrank only sailing and power-driven vessels, and keep out of the way of NUC and RAM so far as possible.
• Fishing lights differ by method, green over white for trawling and red over white for other fishing, but the day shape is the same.
• Rule 17 and Rule 2 still apply: the stand-on vessel must act when the give-way vessel is not doing enough.

The practice quiz includes scenario questions on Rule 18 modelled on MCA and RYA exam formats, a useful way to test whether these distinctions hold up under timed conditions.

FAQ

Does a sailing vessel always have right of way over a motorboat? Not always. Under Rule 18(a) a power-driven vessel keeps out of the way of a sailing vessel, but Rule 10(j) requires the sailing vessel not to impede a power-driven vessel following a traffic lane, Rule 9(b) requires her not to impede a vessel confined to a narrow channel, and Rule 13 makes her the give-way vessel when she is overtaking. She must also keep out of the way of fishing, RAM and NUC vessels, and avoid impeding a vessel constrained by her draught.

What makes a vessel "not under command"? Rule 3(f) defines a NUC vessel as one which through some exceptional circumstance is unable to manoeuvre as required by the rules and is therefore unable to keep out of the way of another vessel. The key word is exceptional: planned slowdowns, routine stops and a reluctance to alter course do not qualify.

Can a RAM vessel ever be required to give way? Rule 18 itself imposes no duty on a RAM vessel towards any other category, including NUC. That does not make her free of obligation: Rule 2 requires every vessel to have regard to the ordinary practice of seamanship and to any special circumstances, and a RAM vessel that can take action to avoid a collision is expected to.

What is the difference between a vessel aground and a vessel not under command? They are distinct categories with distinct signals. A vessel aground is covered by Rule 30(d): her anchor lights plus two all-round red lights in a vertical line, or by day three balls in a vertical line. A NUC vessel shows two red lights or two balls under Rule 27(a). A vessel aground is not automatically NUC. If in any doubt about a vessel's status, give her a wide berth and establish contact if possible.