Rule 21 - Definitions

The Rule

(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 

Inland Only: 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

Inland Only: 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 Only: (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.

Annex V:

 (a) Law enforcement vessels may display a flashing blue light when engaged in direct law enforcement or public safety activities. This light must be located so that it does not interfere with the visibility of the vessel’s navigation lights.

 (b) The blue light described in this section may be displayed by law enforcement vessels of the United States and the States and their political subdivisions.

  (a) Vessels engaged in government sanctioned public safety activities, and commercial vessels performing similar functions, may display an alternately flashing red and yellow light signal. This identification light signal must be located so that it does not interfere with the visibility of the vessel’s navigation lights. The identification light signal may be used only as an identification signal and conveys no special privilege. Vessels using the identification light signal during public safety activities must abide by the inland navigation rules, and must not presume that the light or the exigency gives them precedence or right of way.

 (b) Public safety activities include but are not limited to patrolling marine parades, regattas, or special water celebrations; traffic control; salvage; firefighting; medical assistance; assisting disabled vessels; and search and rescue.

Discussion: This rule describes the various lights we will discuss in the next section of the Rules. There are a few key points in this Rule. First, there are significant inland differences regarding “Special Flashing Lights” as compared to the “Flashing Light,” and this is often a testable point on exams. Second, previously buried rules in Annex V have been recently published in the inland rules, regarding law enforcement and public safety vessels. See the description of Annex V for more information. Finally, Inland Rules have a couple of provisions for vessels less than 12 meters that don’t appear in the International Rules.

A diagram is helpful when memorizing the arc of visibility of lights described in this Rule. 

Test Strategy: There are a surprising number of questions from this Rule and you can expect to see 1-3 questions on your exam from this Rule. Most questions regard definitions of lights, the differences between Inland and International Rules, or the arc of visibility of a light. 

21 Visibility of Lights.pdf