Navigation Lights Cheat Sheet: COLREGS Rules 20 to 31

Navigation lights are one of the most tested parts of the Rules of the Road, and honestly one of the most satisfying to learn. Once the patterns click, you can look at a cluster of lights on a dark horizon and know what the vessel is doing. This cheat sheet walks through Rules 20 to 31 in order, with the rule number for every item so you can check the official text.

Rule 20: When lights and shapes apply

Lights are shown from sunset to sunrise and in restricted visibility, and may be shown any other time it's necessary. Shapes are shown by day. During those hours, don't show other lights that could be confused with the required ones or impair their visibility.

Rule 21: The light definitions and arcs

Learn these arcs and half of the lights section falls into place.

Notice that 225 plus 135 equals 360. The masthead light and sternlight together cover the whole horizon, and that's why you see one or the other but never both from a single angle (except right at the 22.5-degree line). A flashing light flashes 120 or more times per minute. Inland Rule 21 adds the special flashing light: yellow, 50 to 70 flashes per minute, showing an arc of not less than 180 and not more than 225 degrees.

Rule 22: Minimum visibility ranges

Study trick: on big ships, only the masthead light reaches 6 miles. On small boats, only the sidelights drop to 1 mile.

Rule 23: Power-driven vessels underway

A masthead light forward, a second masthead light abaft and higher (optional under 50 meters), sidelights, and a sternlight. An air-cushion vessel in non-displacement mode adds an all-round flashing yellow light. A power-driven vessel under 12 meters may show an all-round white light and sidelights instead. Under International Rule 23(d)(ii), a power-driven vessel under 7 meters whose maximum speed doesn't exceed 7 knots may show just an all-round white light, with sidelights if practicable. Inland Rule 23(e) lets Great Lakes vessels carry an all-round white light in place of the second masthead light and sternlight.

Rule 24: Towing and pushing

• Towing astern: two masthead lights in a vertical line (three if the tow is longer than 200 meters, measured from the tug's stern to the end of the tow), sidelights, sternlight, and a yellow towing light above the sternlight. By day, a diamond shape when the tow exceeds 200 meters.
• Composite unit: a pusher rigidly connected to the vessel ahead is lit as one power-driven vessel under Rule 23.
• Pushing ahead or towing alongside: two masthead lights in a vertical line and sidelights. International adds a sternlight. Inland uses two towing lights in a vertical line instead. On the Western Rivers (except below the Huey P. Long Bridge at mile 106.1 Above Head of Passes on the Mississippi River) and waters specified by the Secretary, Inland Rule 24(j) changes this again: a vessel pushing ahead or towing alongside shows sidelights and two towing lights in a vertical line, with no masthead lights.
• Vessel being towed: sidelights, sternlight, and a diamond if the tow exceeds 200 meters. Inland Rule 24(f) adds a special flashing light at the forward end of a vessel pushed ahead or towed alongside.

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Rule 25: Sailing vessels

Sidelights and a sternlight. A sailing vessel under 20 meters may combine them in one tricolor lantern at or near the top of the mast. A sailing vessel may also show two all-round lights at or near the masthead, red over green, but not together with the tricolor lantern. Under 7 meters, she should show the lights if practicable, or have a white light ready to show in time. Under oars, the same option applies. By day, a vessel motorsailing shows a cone, apex downward. Inland Rule 25(e) says a vessel under 12 meters isn't required to show the cone, but may.

Rule 26: Fishing vessels

• Trawling: all-round green over white (study trick: "green over white, trawling tonight"), plus a masthead light abaft and higher than the green (not required under 50 meters). By day, two cones apexes together.
• Fishing other than trawling: all-round red over white (study trick: "red over white, fishing at night"). If outlying gear extends more than 150 meters horizontally, an all-round white light or a cone apex upward in the direction of the gear.
• When making way, both add sidelights and a sternlight.

Rule 27: Not under command and restricted in ability to maneuver

• Not under command (NUC): two all-round red lights in a vertical line, two balls by day. Add sidelights and sternlight when making way. (Study trick: "red over red, the captain is dead.")
• Restricted in ability to maneuver (RAM): all-round red, white, red in a vertical line; ball, diamond, ball by day. Add masthead lights, sidelights, and sternlight when making way, or anchor lights when at anchor.
• Dredging or underwater operations with an obstruction: RAM lights plus two all-round reds (or two balls) on the side with the obstruction, and two all-round greens (or two diamonds) on the side where it's safe to pass.
• Mine clearance: three all-round green lights or three balls, meaning it's dangerous to come within 1,000 meters.
• Small diving operations: a rigid replica of International Code flag "A," at least 1 meter high.

Rule 28: Constrained by her draft (International only)

Three all-round red lights in a vertical line, or a cylinder by day. Inland Rule 28 is reserved.

Rule 29: Pilot vessels

All-round white over red at or near the masthead (study trick: "white over red, pilot ahead"), plus sidelights and sternlight underway, or anchor lights at anchor.

Rule 30: Anchored and aground

• At anchor: an all-round white light (or one ball by day) forward, and a lower all-round white light at or near the stern. Under 50 meters, one all-round white light is enough. Vessels 100 meters or more must also light their decks.
• Aground: the anchor lights plus two all-round red lights in a vertical line, or three balls by day.
• Small-vessel exemptions (Rule 30(e) and (f), both sets): a vessel under 7 meters at anchor, not in or near a narrow channel, fairway, or anchorage or where other vessels normally navigate, doesn't need anchor lights or the ball. A vessel under 12 meters aground doesn't need the two red lights or three balls.
• Inland Rule 30(g) exempts vessels under 20 meters in a Coast Guard special anchorage area from anchor lights and shapes.

Rule 31: Seaplanes and WIG craft

Where a seaplane or WIG craft can't show lights and shapes with the characteristics or positions in these rules, she shows ones as closely similar as possible.

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Frequently asked questions

What is the arc of a masthead light?

225 degrees, from right ahead to 22.5 degrees abaft the beam on each side (Rule 21(a)).

What lights does a vessel aground show?

Anchor lights plus two all-round red lights in a vertical line, or three balls by day (Rule 30(d)).

What does red over white mean?

A vessel engaged in fishing other than trawling (Rule 26(c)). Green over white means trawling.

Are navigation lights the same under Inland and International rules?

Mostly. The main differences are in Rule 23(e), Rule 24 (pushing ahead and the special flashing light), and Rule 28, which only exists in International.

Mini quiz: test yourself

1. What is the minimum range of a masthead light on a vessel 50 meters or longer?
2. You see red over white all-round lights with no sidelights. What is it?
3. What day shape shows a vessel restricted in her ability to maneuver?
4. A tug's tow is 250 meters long. How many masthead lights in a vertical line?
5. A vessel shows an all-round white over red. What is she?

Answers: 1) 6 miles (Rule 22(a)). 2) A vessel fishing other than trawling, not making way (Rule 26(c)). 3) Ball, diamond, ball in a vertical line (Rule 27(b)). 4) Three (Rule 24(a)). 5) A pilot vessel on pilotage duty (Rule 29).

Keep studying

• USCG Rules of the Road study guide
• Kayak navigation lights
• COLREGS day shapes
• Inland vs International navigation rules
• Sound signals and whistle signals (Rules 32 to 37)