1The rule in plain words
In one sentence…
Rule 3 turns COLREG words into clear definitions: sailing, fishing, command, draft and visibility.
Sailing or power-driven?
"Vessel" includes every water craft used or capable of being used for transportation on water. If a sailing vessel uses her propelling machinery, she is power-driven because using machinery changes the label in the classification.
Does fishing with lines always count?
A vessel is engaged in fishing when she is fishing with apparatus that restricts maneuverability. Nets, lines and trawls can do so; trolling lines are excluded when they do not restrict maneuverability.
How do you separate the three categories?
Not under command comes from an exceptional circumstance; restricted in her ability to maneuver from the nature of her work; constrained by her draft is a power-driven vessel whose draft, relative to available depth and width, severely restricts deviation.
What do "underway" and "in sight" mean?
A vessel is underway when not at anchor, made fast to the shore or aground; a drifting vessel can still be underway. Vessels are in sight only when one can be observed visually from the other; restricted visibility includes fog and similar causes.
2Key points
- 1.
Machinery in use → power-driven vessel.
- 2.
Trolling lines ≠ engaged in fishing when maneuverability is not restricted.
- 3.
Not under command = exceptional circumstance · restricted ability = nature of work.
- 4.
Constrained by draft = draft vs available depth + width; drifting vessel = underway.
3Frequent mistakes
Wrong: "A sailing vessel stays one while her propelling machinery is running."
Right: Using the machinery makes her a power-driven vessel.
Wrong: "Any vessel fishing with lines is engaged in fishing."
Right: Trolling lines are excluded when the apparatus does not restrict maneuverability.
Wrong: "A drifting vessel is never underway."
Right: Underway means not at anchor, made fast to shore or aground.
4Exam keys & interpretation
Exam keys
Engine running on a sailing vessel → choose "power-driven vessel"; the sailing-vessel definition requires machinery not to be used.
"Trolling" is not enough: check whether the fishing apparatus restricts maneuverability.
Drifting is not anchoring: not at anchor, made fast or aground means "underway".
How to read it
Classify the vessel by the condition Rule 3 names: exceptional circumstance, nature of work, draft, or physical status.
"In sight" requires eyes: a contact detected only by radar does not meet that definition.
5Official text
IMO COLREG 1972U.S. Coast Guard — International Navigation Rules (2024)
Full wording of the rule, reproduced from the official source below.
For the purpose of these Rules, except where the context otherwise requires:
(a) The word vessel includes every description of water craft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water.
(b) The term power-driven vessel means any vessel propelled by machinery.
(c) The term sailing vessel means any vessel under sail provided that propelling machinery, if fitted, is not being used.
(d) The term vessel engaged in fishing means any vessel fishing with nets, lines, trawls or other fishing apparatus which restrict maneuverability, but does not include a vessel fishing with trolling lines or other fishing apparatus which do not restrict maneuverability.
(e) The word seaplane includes any aircraft designed to maneuver on the water.
(f) The term vessel not under command means a vessel which through some exceptional circumstance is unable to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel.
(g) The term vessel restricted in her ability to maneuver means a vessel which from the nature of her work is restricted in her ability to maneuver as required by these Rules and is therefore unable to keep out of the way of another vessel. The term vessels restricted in their ability to maneuver shall include but not be limited to:
(i) a vessel engaged in laying, servicing or picking up a navigation mark, submarine cable or pipeline;
(ii) a vessel engaged in dredging, surveying or underwater operations;
(iii) a vessel engaged in replenishment or transferring persons, provisions or cargo while underway;
(iv) a vessel engaged in the launching or recovery of aircraft;
(v) a vessel engaged in mine clearance operations;
(vi) a vessel engaged in a towing operation such as severely restricts the towing vessel and her tow in their ability to deviate from their course.
(h) The term vessel constrained by her draft means a power-driven vessel which, because of her draft 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.
(i) The word underway means that a vessel is not at anchor, or made fast to the shore, or aground.
(j) The words length and breadth of a vessel mean her length overall and greatest breadth.
(k) Vessels shall be deemed to be in sight of one another only when one can be observed visually from the other.
(l) The term restricted visibility means any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms or any other similar causes.
(m) The term Wing-In-Ground (WIG) craft means a multimodal craft which, in its main operational mode, flies in close proximity to the surface by utilizing surface-effect action.6Test your knowledge
A sailing vessel has her engine running. Which term applies?
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