Official rule text as published in the U.S. Coast Guard Navigation Rules
(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.
MyCOLREGS explanation
Summary
Rule 3 draws a hard line between a "power-driven vessel" (any vessel propelled by machinery) and a "sailing vessel" (a vessel under sail, but only if any propelling machinery it has is not being used). The moment a sailboat engages its engine — even with the sails still up — it drops out of the sailing-vessel definition and is treated as a power-driven vessel for steering-and-sailing purposes.
Real World Example
The definition doesn't care whether the sails are still doing most of the work - the instant the engine goes in gear, a boat that spent the last hour as the stand-on sailing vessel can find herself reclassified as the give-way vessel in a crossing situation, with no announcement to the other boat that anything changed.
Practice Questions
Q1. A sailboat has her sails up and is also motoring, with the engine engaged and pushing the boat forward. Under Rule 3, what is she for steering-and-sailing purposes?
A. A sailing vessel, because the sails are up.
B. Neither — Rule 3 has no category for a motor-sailing vessel.
C. A power-driven vessel, because propelling machinery is being used.
D. Whichever category the skipper chooses to declare.
Q2. A sailboat has an engine installed belowdecks, but it's switched off and out of gear - she's making way under sail alone, wind in her sails, propeller not turning. Under Rule 3(b)-(c), is she a "power-driven vessel"?
A. Neither category applies; a vessel with an engine aboard falls outside both definitions unless she is under 12 meters, in which case Rule 3 defers entirely to state small-vessel registration rules.
B. Yes - any vessel with propelling machinery installed is a "power-driven vessel" under Rule 3(b), whether or not that machinery is in use, since Rule 3(b) turns on equipment carried rather than what's actually running.
C. It depends on the size of the engine relative to the sail area, since a large enough engine is presumed to be the vessel's primary means of propulsion regardless of whether it's engaged.
D. No - Rule 3(c) defines a "sailing vessel" as one under sail provided that propelling machinery, if fitted, is not being used; simply having an engine aboard doesn't matter if it isn't running.
Practice Question Answers
Q1.
C. A power-driven vessel, because propelling machinery is being used. Rule 3(c) defines "sailing vessel" as a vessel under sail provided that propelling machinery, if fitted, is not being used — the moment the engine is engaged for propulsion, that condition fails and Rule 3(b)'s power-driven definition applies instead. A focuses on the sails, which is exactly the trap Rule 3(b) closes. B assumes there's no answer when there clearly is. D treats the category as discretionary when it's actually determined by whether machinery is propelling the boat.
Q2.
D. Being propelled by machinery. Rule 3(b)'s definition turns on propulsion, not equipment installed: "any vessel propelled by machinery" — an idle engine on a sailboat under sail alone does not change her category. A invents a "neither category applies" exception, along with a made-up small-vessel registration deferral Rule 3 doesn't contain. B and C each substitute a measurable but irrelevant fact - equipment installed, engine size - for the actual test in Rule 3(b)/(c): whether machinery is doing the propelling right now.