COLREGS Rule 20 — Application of Lights

Rule Text

What it means on the water

• →Lights are required from SUNSET to SUNRISE in all weathers.
• →Lights must also be shown during RESTRICTED VISIBILITY during the day.
• →Lights may be shown at any time when deemed necessary — this is a permissive provision.
• →Shapes are for DAYLIGHT hours only.
• →No other lights that could cause confusion may be shown during the hours when navigation lights are required.
• →Part C applies in all weathers — no exception for clear days.

Common exam mistakes

• ✗Thinking lights are only required in poor visibility or at night — restricted visibility during daylight also triggers the requirement.
• ✗Confusing when shapes vs. lights are used — shapes by day, lights by night and in restricted visibility.
• ✗Missing that certain other lights (searchlights, deck lights) are permitted if they cannot be mistaken for navigation lights.

USCG exam questions — Rule 20

These questions are drawn from the same pool used in real USCG licensing exams. Correct answers and explanations are shown.

1. 1. According to the COLREGS, what is the minimum required visibility range for a masthead light on a vessel 50 meters or more in length?

   • A.2 miles
   • B.3 miles
   • C.5 miles
   • D.6 miles✓

   Why: Rule 22(a) requires masthead lights on vessels 50 meters or more in length to be visible at a minimum range of 6 miles. Sidelights on the same vessels require 3 miles, and the sternlight requires 3 miles — masthead lights have the greatest range requirement.

2. 2. What is the minimum visibility range required for sidelights on a vessel of 12 meters or more but less than 50 meters in length?

   • A.1 mile
   • B.3 miles
   • C.5 miles
   • D.2 miles✓

   Why: Rule 22(b) requires sidelights on vessels 12 meters or more but less than 50 meters in length to be visible at least 2 miles. Sternlights and towing lights on the same size vessels also require 2 miles.

3. 3. What arc does a sternlight cover?

   • A.112.5 degrees centered on dead astern
   • B.225 degrees from dead astern
   • C.135 degrees centered on dead astern✓
   • D.180 degrees from dead astern

   Why: Rule 21(c) defines the sternlight as showing a white light over an arc of 135 degrees (67.5 degrees on each side of dead astern). Together with the sidelights' 112.5-degree arcs each, these three lights collectively cover 360 degrees.

4. 4. What color is a towing light under the COLREGS and where is it positioned?

   • A.Yellow; in a vertical line above the sternlight showing over the same arc as the sternlight✓
   • B.White; at the stern above the sternlight
   • C.Yellow; at the masthead showing all-round
   • D.Yellow; on the port side of the stern

   Why: Rule 21(f) defines the towing light as a yellow light having the same characteristics as the sternlight (135-degree arc, centered on dead astern), positioned in a vertical line above the sternlight. Yellow over white at the stern identifies an astern tow.

5. 5. Under the Inland Rules, a vessel being pushed ahead, and not part of a composite unit, must exhibit which light not required by 72 COLREGS?

   • A.An all-round green light
   • B.A special flashing light✓
   • C.Two all-round white lights in a vertical line
   • D.A yellow towing light above the sternlight

   Why: Inland Rule 24(i) requires a vessel being pushed ahead (that is not part of a composite unit) to exhibit a special flashing light (a yellow light flashing at 50-70 flashes per minute) at the forward end. This light has no counterpart in 72 COLREGS.

6. 6. Under the Inland Rules, a power-driven vessel less than 12 meters in length may, in lieu of the prescribed lights, exhibit:

   • A.An all-round white light and sidelights✓
   • B.A single all-round white light only
   • C.An all-round white light and a special flashing yellow light
   • D.Sidelights and a sternlight only

   Why: Inland Rule 23(d) permits a power-driven vessel of less than 12 meters in length to show an all-round white light and, where practicable, sidelights, in lieu of the masthead, stern, and sidelights otherwise required. This matches 72 COLREGS Rule 23(d).

7. 7. Under the Inland Rules, what light arrangement must a power-driven vessel towing astern display when the tow exceeds 200 meters in length?

   • A.Two masthead lights in a vertical line and a diamond shape
   • B.Three masthead lights in a vertical line forward
   • C.Two masthead lights, sidelights, sternlight, and three all-round white lights in a vertical line
   • D.Three masthead lights in a vertical line and a diamond shape by day✓

   Why: Under Inland Rule 24(a), when the length of the tow exceeds 200 meters, the towing vessel shows three masthead lights in a vertical line. By day, a diamond shape is exhibited where it can best be seen. This matches the 72 COLREGS requirement in Rule 24(a).

8. 8. A vessel pushing ahead on the Western Rivers at night shows a special flashing yellow light at the bow. Under 72 COLREGS, such a vessel would show:

   • A.The same special flashing yellow light
   • B.Two all-round white lights in a vertical line
   • C.No special forward-facing light — COLREGS has no equivalent requirement✓
   • D.A fixed amber light

   Why: The special flashing yellow light at the bow of a pushed vessel is unique to the U.S. Inland Navigation Rules (Rule 24(i)). There is no equivalent light required by 72 COLREGS for a vessel being pushed ahead.

Frequently asked questions

When must navigation lights be displayed?

Navigation lights must be displayed from sunset to sunrise in all weather conditions. They must also be displayed during restricted visibility regardless of time of day, and may be displayed at any other time when deemed necessary.

When are shapes displayed instead of lights?

Shapes are displayed during daylight hours. They are the daytime equivalent of the lights used at night. In restricted visibility during the day, you display both the appropriate lights and any required shapes.

All COLREGS rules