Official rule text as published in the U.S. Coast Guard Navigation Rules
(a) "Masthead light" means a white light placed over the fore and aft centerline of the vessel showing an unbroken light over an arc of the horizon of 225 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on either side of the vessel, except that on a vessel of less than 12 meters in length the masthead light shall be placed as nearly as practicable to the fore and aft centerline of the vessel.
(b) "Sidelights" means a green light on the starboard side and a red light on the port side each showing an unbroken light over an arc of the horizon of 112.5 degrees and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on its respective side. In a vessel of less than 20 meters in length the sidelights may be combined in one lantern carried on the fore and aft centerline of the vessel, except that on a vessel of less than 12 meters in length the sidelights when combined in one lantern shall be placed as nearly as practicable to the fore and aft centerline of the vessel.
(c) "Sternlight" means a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of 135 degrees and so fixed as to show the light 67.5 degrees from right aft on each side of the vessel.
(d) "Towing light" means a yellow light having the same characteristics as the "sternlight" defined in Rule 21(c).
(e) "All-round light" means a light showing an unbroken light over an arc of the horizon of 360 degrees.
(f) "Flashing light" means a light flashing at regular intervals at a frequency of 120 flashes or more per minute.
| Inland |
|---|
| (g) "Special flashing light" means a yellow light flashing at regular intervals at a frequency of 50 to 70 flashes per minute, placed as far forward and as nearly as practicable on the fore and aft centerline of the tow and showing an unbroken light over an arc of the horizon of not less than 180 degrees nor more than 225 degrees and so fixed as to show the light from right ahead to abeam and no more than 22.5 degrees abaft the beam on either side of the vessel. |
Color note: black text applies generally; blue is International Waters; green is Inland Waters.
MyCOLREGS explanation
Summary
Rule 21 defines the light vocabulary the rest of Part C builds on. A masthead light is a forward white light over a 225-degree arc (as nearly as practicable on the centerline, with an inland-only allowance for small boats where exact centerline placement isn't practicable). Sidelights are the familiar red-port/green-starboard pair, each over 112.5 degrees, and may be combined into one bow lantern on vessels under 20 meters. A sternlight is a white light aft over 135 degrees. A towing light is a yellow light built to the same spec as a sternlight. An all-round light shows over the full 360 degrees, and a flashing light flashes 120 or more times a minute. Inland waters add one more term power-driven vessels don't usually need to worry about: a "special flashing light," a yellow light used on tows, flashing 50-70 times a minute.
Real World Example
These arcs are built to overlap on purpose in the front: the masthead light's 225-degree arc spans the same forward sector as both sidelights, so a vessel approaching you nearly head-on shows three lights at once, white-red-green, which is what lets you recognize 'coming toward me' at a glance before you could ever make out her hull. Sidelights and sternlight, by contrast, are sized so together they cover the full circle around her with no dark arc where a light should be but isn't.
Practice Questions
Q1. You're approaching another vessel nearly head-on at night and see white, red, and green lights all at once. Based purely on Rule 21's definitions, why would all three show together like that?
A. The masthead light's 225° forward arc overlaps the same forward sector covered by both sidelights, so a vessel roughly bow-on to you sits inside all three arcs at once.
B. It doesn't - Rule 21 defines these arcs so they never overlap, so seeing all three at once means her lights are wired wrong.
C. The masthead light only shows once both sidelights go dark, so all three together means you're catching her mid-transition between two different light configurations.
D. Rule 21 combines the masthead light and sidelights into a single all-round light for any vessel under 20 meters, so seeing three separate colors means she's over 20 meters and non-compliant.
Q2. Rule 21(c) defines a sternlight and Rule 21(d) defines a towing light. Based purely on these two definitions, what's the one difference between them?
A. Color - a sternlight is white and a towing light is yellow; both otherwise cover the same 135-degree arc from right aft on each side.
B. Nothing does - Rule 21 defines them completely identically, so no distinction is actually made in this rule.
C. A towing light flashes at regular intervals while a sternlight burns with a continuous, unbroken beam.
D. A towing light covers a full 360-degree arc while a sternlight is limited to 135 degrees.
Practice Question Answers
Q1.
A. The masthead light's 225° forward arc overlaps the same forward sector covered by both sidelights, so a vessel roughly bow-on to you sits inside all three arcs at once. Rule 21(a)'s masthead light shows over a 225° arc centered on the bow; Rule 21(b)'s two sidelights each cover 112.5° from dead ahead back to 22.5° abaft the beam on their own side - all three arcs include dead-ahead, so a vessel roughly bow-on to you sits inside all three at once and shows white-red-green together. B invents a rule that isn't there - the arcs are designed to overlap forward, not avoid it. C describes light behavior Rule 21 never establishes - masthead/sidelights aren't sequenced on/off like that. D misapplies the under-20m sidelight-combining exception (Rule 21(b)), which combines just the two sidelights into one lantern, not the masthead light too, and has nothing to do with vessel length triggering non-compliance.
Q2.
A. Color - a sternlight is white and a towing light is yellow; both otherwise cover the same 135-degree arc from right aft on each side. Rule 21(c) defines the sternlight as a white light aft over 135°; Rule 21(d) defines the towing light as a yellow light with the same characteristics as the sternlight - same arc, differing only in color. B denies a real distinction Rule 21 does make (color). C invents a flashing characteristic not part of either definition. D misstates the towing light's arc - Rule 21(d) explicitly says it shares the sternlight's 135-degree characteristics, not a 360-degree all-round light.