The rule in plain words
- What does a power-driven vessel underway do?It keeps out of the way of a vessel not under command, restricted in her ability to manoeuvre, engaged in fishing, or sailing: that is the complete order.
- What about sailing and fishing vessels?A sailing vessel underway keeps out of the way of vessels not under command, restricted in their ability to manoeuvre, or engaged in fishing. A fishing vessel does so, so far as possible, for the first two.
- What about a vessel constrained by her draft?Any vessel other than one not under command or restricted in her ability to manoeuvre avoids, if circumstances admit, impeding the safe passage of one showing Rule 28 signals; she navigates with particular caution.
- What about seaplanes and WIG craft?On water, a seaplane keeps well clear of all vessels and avoids impeding navigation; with risk of collision, it follows these Rules. When taking off, landing or flying near the surface, a WIG craft does likewise; on water, it complies as a power-driven vessel.
Frequent mistakes
Wrong: "A power-driven vessel always has priority."
Right: It keeps out of the way of four categories, unless Rules 9, 10 or 13 require otherwise.
Wrong: "A draft-constrained vessel always makes everyone else give way."
Right: Vessels not under command or restricted in manoeuvre are outside that no-impeding duty.
Wrong: "A seaplane only keeps clear when risk of collision exists."
Right: It generally keeps well clear and avoids impeding; with collision risk, it also complies with these Rules.
Exam keys & interpretation
- Exam order: power-driven → not under command, restricted manoeuvre, fishing, sailing; sailing and fishing have shorter lists.
- Spot the opening exception: Rules 9, 10 and 13 may require a different action.
- In draft questions, separate "if circumstances admit" from the vessel's own duty of particular caution.
How to read it
Read the list by vessel type: it assigns a keep-out-of-the-way duty underway, not a universal priority label.
For a special condition, check the express exception in Rules 9, 10 and 13 before applying the relevant paragraph.
Key points
- Power-driven → four categories; sailing → three; fishing → two.
- Fishing yields so far as possible; draft-constrained vessels retain special caution.
- Draft-constrained → Rule 28 signals · special condition.
- Aircraft on water → keep clear and do not impede; WIG on water = power-driven.
Apply Rule 18: when the exceptions control
COLREG · USCG NAVCEN · 2026-09-19A sailing vessel is in sight in open water and is overtaking an ordinary power-driven vessel. The relative bearing is more than 22.5° abaft the other vessel's beam, and the sailing vessel remains overtaking until finally past and clear.
Can the overtaking sailing vessel expect the power-driven vessel to keep out of its way because it is sailing?
No. Rule 13 controls the overtaking situation: the sailing vessel keeps clear and remains responsible until finally past and clear. Rule 18(a)(iv) does not cancel that duty or create a blanket priority for the sailing vessel.
Rule 18's opening exception leaves Rule 13 in control of an overtaking encounter. A sailing vessel's category under Rule 18 does not reverse the specific overtaking duty or end it before the vessel is finally past and clear.
A sailing vessel is in sight of a power-driven vessel following an IMO-adopted traffic lane. The sailing vessel is near the lane and considers entering or crossing it. The crossing is not necessary to avoid immediate danger, and the sailing vessel can wait without entering the lane.
Does the sailing vessel’s Rule 18 category give it priority to enter or cross the traffic lane?
No. Do not impede the power-driven vessel following the lane. Under Rule 8(f), take early action to allow it sufficient sea room and stay clear while entry or crossing can be avoided. If crossing later becomes necessary, use a heading as nearly as practicable at right angles to the traffic flow. This non-impeding duty does not relieve either vessel of its other collision-avoidance duties if risk develops.
Rule 18 applies except where Rule 10 controls. Rule 10(j) states that a sailing vessel must not impede a power-driven vessel following a traffic lane; Rule 10(c) governs a necessary crossing. Rule 8(f) requires early action and preserves the other collision-avoidance duties if risk develops. The vessel category alone does not create a right-of-way hierarchy.
Test your knowledge
3 questions to check it. Pick an answer to see why.
What does Rule 18 require of a vessel constrained by her draft?
Official text
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: