Master every light arc, vessel-type requirement, and day shape under COLREGS Rules 20-31. This is one of the highest-frequency topics on the USCG captain exam — know it cold.
Rules 20-31
Full scope of COLREGS Part C
5 Arcs
Light arc values you must know exactly
High
Exam frequency — appears on every sitting
6 nm
Max visibility range — masthead light large vessel
COLREGS Part C (Rules 20-31) exists for one reason: collision avoidance in darkness and restricted visibility. Every vessel officer must be able to identify any vessel by its lights alone — from any angle of approach — and immediately understand that vessel's status, limitations, and rights-of-way. Day shapes carry the same information when lights are not required.
Rule 20 establishes the threshold: lights are required from sunset to sunrise AND during any period of restricted visibility, regardless of time of day. A vessel in fog at noon must show lights. A vessel in bright sun may also display lights — it is not prohibited. The exam tests when lights are required, not when they are optional.
Sunset to Sunrise
All vessels must display prescribed lights from sunset to sunrise. No exceptions based on weather or anchoring status.
Restricted Visibility — Any Time of Day
Lights must be shown during fog, rain, haze, smoke, or any condition reducing visibility — day or night. This is Rule 20(b) and a common exam trap.
Daylight with Good Visibility
Vessels may show lights during daylight in clear conditions, but are not required to. Day shapes are the daytime equivalent of lights and carry the same information.
Rule 21 defines every light type. Every question about arcs traces back to these five definitions. Know them exactly — the exam will give you slight variations and expect you to catch errors.
| Light Name | Color | Arc | Notes |
|---|---|---|---|
| Masthead (Steaming) | White | 225° | Forward-facing. From dead ahead to 22.5° abaft beam on each side. Power-driven vessels underway only. |
| Sidelight (Port) | Red | 112.5° | Port side. Dead ahead to 22.5° abaft beam on port side. Screen required to prevent cross-bow visibility. |
| Sidelight (Starboard) | Green | 112.5° | Starboard side. Dead ahead to 22.5° abaft beam on starboard side. |
| Sternlight | White | 135° | Aft-facing. 67.5° either side of dead astern. Minimum range 2 nm for vessels under 50m, 3 nm for 50m+. |
| All-Around Light | White, Red, or Yellow | 360° | Visible from any direction. Used for anchor lights, NUC, RAM, fishing, and special signals. |
| Towing Light | Yellow | 135° | Same arc as sternlight. Replaces white sternlight on towing vessels in some configurations. |
| Flashing Light | Yellow | 360° | Flashing at 120+ flashes per minute. Used by WIG craft and hovercraft in certain situations. |
Memory Trick — Arc Math
The arcs are designed to add up cleanly. Two sidelights at 112.5° each cover 225° — exactly matching the masthead light arc. The sternlight at 135° covers what the sidelights miss. Together: 225° + 135° = 360°. If you know 112.5 and 135, you can derive everything else.
Mnemonic: "Side Eleven-Two-Point-Five, Stern One-Thirty-Five" — say it three times.
Rule 22 specifies the minimum range in nautical miles at which each light must be visible. Range requirements scale with vessel size. The exam frequently tests masthead and sternlight ranges.
| Vessel Size | Masthead | Sidelights | Sternlight | All-Around |
|---|---|---|---|---|
| 50m or more | 6 nm | 3 nm | 3 nm | 3 nm |
| 12m to 50m | 5 nm | 2 nm | 2 nm | 2 nm |
| Under 12m | 2 nm | 1 nm | 2 nm | 2 nm |
| Under 7m (sail/oar) | 2 nm | 1 nm | 2 nm | 2 nm |
Exam tip: A 6 nm masthead light means the vessel is 50m or more in length. A 5 nm masthead light means 12-50m. The sternlight on vessels under 12m is still 2 nm — same as larger vessels — a detail that catches candidates off guard.
Rule 23 covers power-driven vessels underway. This is the baseline — all other vessel types are modifications of or exceptions to this configuration. A power-driven vessel making way shows the full set: masthead forward, sidelights, and sternlight.
Vessel 50m or More
The second masthead light must be aft of and higher than the forward light by at least 4.5m on vessels 50m+ in length.
Vessel Under 50m
Vessels under 12m may combine sidelights into a bicolor lantern. Under 7m in some conditions may show only an all-around white light.
Exam Trap — Second Masthead Light
Vessels under 50m MAY show a second masthead light but are not required to. Vessels 50m or more MUST show a second masthead light. The exam will test this distinction. If you see two masthead lights, the vessel is at least 50m, or is a towing vessel (which also shows multiple mastheads).
Rule 25 covers sailing vessels underway. The critical rule: a sailing vessel under sail alone does NOT show a masthead (steaming) light. The masthead light is exclusively for power-driven vessels. A sailing vessel running its engine — even under sail — must show power-driven vessel lights.
Sailing Under Sail Only
Under 20m Optional — Tricolor
Optional — Red Over Green
Sailing Vessel Under Engine (Motorsailing)
When a sailing vessel uses its engine — even if sails are also set — it is considered a power-driven vessel and must display power-driven vessel lights: masthead light, sidelights, and sternlight. During daylight hours, a motorsailing vessel must also display a cone shape, apex downward, forward where it can best be seen.
Exam note: The cone apex-down = sailing vessel also using engine. This is tested frequently.
Memory Trick — Sailing Lights
"Sail has no steaming light." A sailing vessel under sail alone has no masthead (steaming) light. If you see a white masthead light, the vessel is using an engine. Red over green = sailing vessel showing off her colors (optional). Tricolor = compact, under 20m.
A vessel Not Under Command (NUC) cannot maneuver as required by the rules due to exceptional circumstances — engine failure, steering failure, or similar casualty. Rule 27(a) prescribes the signals. NUC status grants priority: all other vessels must keep clear of a NUC vessel.
Night Lights
No masthead light. Two red all-around lights replace it.
Day Shapes
Two black balls in a vertical line, displayed where they can best be seen.
Rule 27(a) — Two red lights at night, two black balls by day.
Memory Trick — NUC
"Two Reds for the Dead" — a NUC vessel is effectively 'dead in the water' operationally. Two red all-around lights stacked vertically. By day, two black balls. If she's moving, she also shows sidelights and stern, but still no masthead.
A RAM vessel is one whose work restricts its ability to maneuver as required by the rules. Examples: dredging, cable-laying, pipeline work, mine clearance, underway replenishment, and launching or recovering aircraft. Unlike NUC, the restriction is predictable and related to work being performed.
Night Lights — RAM Underway
Day Shapes — RAM
Ball-diamond-ball vertically. Unmistakable pattern distinct from NUC (ball-ball).
Memory Trick — RAM vs NUC Lights
NUC = Red-Red (two reds). RAM = Red-White-Red (stoplight minus green).Think of RAM as 'restricted but still alive' — she has a white in the middle. Day shapes: NUC = ball-ball (two black circles), RAM = ball-diamond-ball (three shapes, diamond in middle).
Rule 28 applies only under International Rules (COLREGS), not Inland Rules. A vessel constrained by draft is one whose draft in relation to the available water depth significantly restricts its ability to deviate from its current course. This category does NOT exist in the Inland Rules — a frequent exam distinction.
Night Lights — CBD
Three red all-around lights in a vertical line.
Day Shape — CBD
A cylinder — tall, straight, like a vessel that must stay on course.
International Rules only — no CBD category in Inland Rules.
Memory Trick — CBD
"Three Reds — Deep Draft Ahead."Three red all-around lights stacked = CBD. The cylinder day shape looks like a deep-draft tanker hull — straight and unyielding. And remember: CBD is International only. The exam loves to ask which vessel categories exist only under one ruleset.
Rule 30 governs vessels at anchor and aground. Anchor lights are the most common lights seen in anchorages and are tested regularly. The requirement varies by vessel length, and the exam will test both the light count and the day shape.
Under 50m at Anchor
Day shape: one black ball forward
50m or More at Anchor
Forward light must be higher than aft light
Vessel Aground
Day shape: three black balls in vertical line
Rule 30(e) — Small Vessel Exemption
A vessel under 7m at anchor, if not in or near a narrow channel, fairway, or anchorage, or where other vessels normally navigate, need not exhibit the prescribed anchor lights. However, if the master chooses not to light the vessel, they accept the risk. The exam may test this exemption.
Rule 26 covers vessels engaged in fishing — defined as fishing with nets, lines, trawls, or other apparatus that restricts maneuverability. A vessel trolling with a fishing rod does NOT qualify as 'engaged in fishing' under the rules and must show power-driven vessel lights.
Trawling Vessel
Day shape: two cones apex-to-apex (bicone/hourglass)
Fishing (Not Trawling)
Day shape: cone apex-down (like a dunce cap)
Memory Trick — Fishing vs Trawling
Trawling: Green over White = 'Go While Trawling.'Fishing (not trawling): Red over White = 'Stop and fish.' Day shapes: Trawling = bicone (hourglass, two cones apex-to-apex). Fishing = one cone apex-down. Remember: trawling has TWO cones, fishing has ONE.
Rule 24 is one of the more complex rules because the towing vessel's lights change based on the tow configuration: towing astern, towing alongside, or pushing ahead. The length of the tow (whether it exceeds 200m) also changes the requirements.
Towing Astern — Tow 200m or Less
Towing Vessel
Tow
Towing Astern — Tow Exceeds 200m
Towing Vessel
Tow
Pushing Ahead or Towing Alongside
Pushing Vessel
Vessel Being Pushed
Memory Trick — Towing Light Count
2 mastheads = tow 200m or less. 3 mastheads = tow over 200m.The yellow towing light sits above the white sternlight — yellow on top means it's towing something behind. Pushing ahead gets two yellow all-around lights aft instead of a sternlight.
Rule 29 covers pilot vessels on pilotage duty. The key signal is unmistakable: white over red all-around lights at the masthead. When underway, the vessel also shows sidelights and sternlight. At anchor, the pilot vessel shows the white-over-red plus anchor lights.
Pilot Vessel Underway on Duty
Identification Notes
Memory Trick — Pilot Light
"White Over Red — Pilot Ahead."White top, red bottom. Think of the pilot's white uniform hat on top and a red warning signal below — important vessel coming aboard, give way.
Day shapes are the daytime equivalent of navigation lights. They carry exactly the same information as the corresponding night lights. Shapes are black, geometric, and displayed where they can best be seen. The exam will test your ability to identify vessel status from shapes alone.
| Vessel Status | Shape(s) | Rule | Notes |
|---|---|---|---|
| At Anchor | 1 black ball (forward) | Rule 30 | Under 50m. Displayed forward. |
| At Anchor (50m+) | 1 black ball (forward) | Rule 30 | Same as under 50m, but also illuminated at night with two lights. |
| Aground | 3 black balls (vertical) | Rule 30 | Three balls in vertical line. Plus anchor lights at night. |
| Not Under Command | 2 black balls (vertical) | Rule 27 | Two balls. Night equivalent: two red all-around lights. |
| Restricted in Ability to Maneuver | Ball - Diamond - Ball (vertical) | Rule 27 | Three shapes: ball on top, diamond in middle, ball on bottom. |
| Constrained by Draft | Cylinder | Rule 28 | International Rules only. Black cylinder. |
| Fishing (not trawling) | Cone apex-down | Rule 26 | Single cone, apex pointing down. |
| Trawling | 2 cones apex-to-apex | Rule 26 | Bicone (hourglass shape). Two cones meeting at tips. |
| Tow exceeds 200m | Diamond (on both tug and tow) | Rule 24 | Diamond on towing vessel AND on the tow. |
| Motorsailing (sail + engine) | Cone apex-down (forward) | Rule 25 | Sailing vessel using its engine. Cone displayed forward. |
Day Shape Memory System
Count the shapes:
Shape identifies function:
While lights and shapes are governed by Part C (Rules 20-31), they work in conjunction with sound signals in Part D (Rules 32-37) and restricted visibility signals in Rule 35. Understanding how lights and sound signals relate helps you answer combination questions on the exam.
NUC — Restricted Visibility Sound Signal
1 prolonged + 2 short blasts every 2 minutes
Same as sailing vessel, fishing vessel, towing vessel, RAM, and CBD in restricted visibility. All non-power-driven or restricted vessels use the same sound signal.
Power-Driven Vessel Making Way — Restricted Visibility
1 prolonged blast every 2 minutes
Only power-driven vessels making way use one prolonged blast. Stopped vessels (not making way) use two prolonged blasts.
Power-Driven Vessel Not Making Way — Restricted Visibility
2 prolonged blasts every 2 minutes
Underway but stopped. No headway. Still showing navigation lights — just not moving through water.
Vessel at Anchor — Restricted Visibility
Rapid ringing of bell for 5 seconds every minute
Vessels 100m+ also sound a gong aft. The fog bell warns approaching vessels of a stationary anchor hazard.
Vessel Aground — Restricted Visibility
Three distinct strokes on bell, 5-second rapid ringing, three distinct strokes every minute
Signals: I am aground and immovable. In addition to bell, may sound appropriate whistle signal.
This table is the condensed version of Rules 23-30. Use it for rapid review before the exam. For each vessel type, know: does it show a masthead light, sidelights, sternlight, and what special lights replace or augment the normal configuration.
| Vessel Type | Masthead | Sidelights | Stern | Special Lights |
|---|---|---|---|---|
| Power-Driven Underway | Yes (white) | Yes | Yes (white) | 2nd masthead if 50m+ |
| Sailing Vessel — Sail Only | No | Yes | Yes (white) | Optional: red/green tricolor or red-over-green |
| Motorsailing | Yes (white) | Yes | Yes (white) | Cone apex-down day shape |
| Vessel Not Under Command | No | If making way | If making way | Red over red all-around (2 reds) |
| RAM Vessel | Yes (if power-driven) | If making way | If making way | Red-White-Red all-around vertically |
| Constrained by Draft | Yes (if power-driven) | Yes | Yes | 3 red all-around lights vertical; cylinder by day |
| Fishing (not trawling) | No | If making way | If making way | Red over white all-around; cone apex-down by day |
| Trawling | No (or aft if 50m+) | If making way | If making way | Green over white all-around; bicone by day |
| Towing Astern (tow =200m) | 2 vertical | Yes | Yellow towing light | Yellow towing light above white stern |
| Towing Astern (tow 200m+) | 3 vertical | Yes | Yellow towing light | Diamond shape; diamond on tow too |
| Pushing Ahead | 2 vertical | Yes | 2 yellow all-around | Two yellow lights aft replace white sternlight |
| At Anchor (under 50m) | No | No | No | 1 all-around white (forward); ball by day |
| At Anchor (50m+) | No | No | No | 2 all-around white (fore higher); ball by day |
| Aground | No | No | No | 2 red all-around + anchor lights; 3 balls by day |
| Pilot Vessel on Duty | No | If making way | If making way | White over red all-around at masthead |
The USCG exam on navigation lights follows predictable patterns. Knowing what traps the exam sets lets you answer faster and with more confidence. These are the most common question types.
Arc Degree Questions
Trap: The exam gives a close number — 110°, 115°, 130°, 140° — instead of the exact value.
Answer: Masthead: 225°. Each sidelight: 112.5°. Sternlight: 135°. All-around: 360°. Towing light: 135° (same as stern). Memorize exact values only.
NUC vs RAM Identification
Trap: Describes a vessel that cannot maneuver and asks whether it is NUC or RAM.
Answer: NUC = exceptional circumstances, unplanned (engine failure, casualty). RAM = nature of the work restricts it (dredging, cable laying). If it's doing a job, it's RAM. If it broke down, it's NUC.
CBD International vs Inland
Trap: Asks about a vessel constrained by draft on Inland waters.
Answer: CBD does not exist under Inland Rules. If a question involves CBD on inland waters, the answer involves a different category or vessel type.
Tow Length and Masthead Count
Trap: Shows two mastheads on a towing vessel and asks if the tow exceeds 200m.
Answer: Two mastheads = tow is 200m or less. Three mastheads = tow exceeds 200m. The masthead count directly encodes tow length.
Fishing vs Trolling
Trap: Describes a sport fishing vessel trolling with lines and asks what lights it shows.
Answer: Trolling (with rod and reel) does not qualify as 'engaged in fishing' under COLREGS. A trolling vessel shows power-driven vessel lights, NOT fishing vessel lights.
Sailing Vessel with Engine
Trap: Asks whether a sailing vessel using its engine shows a masthead light.
Answer: Yes. Any vessel using machinery for propulsion — even if sails are set — is a power-driven vessel and must show masthead light, sidelights, and sternlight. During day: cone apex-down forward.
Anchor Light Count by Size
Trap: Asks how many anchor lights a vessel shows without specifying if it is 50m+ or under 50m.
Answer: Under 50m: one all-around white light forward. 50m or more: two all-around white lights (forward and aft, forward must be higher). The size threshold is 50m, not 20m or 100m.
Aground vs Anchor Light Pattern
Trap: Describes an anchor light configuration and asks if the vessel is anchored or aground.
Answer: Anchored = one or two white all-around lights. Aground = anchor lights PLUS two red all-around lights. Three black balls by day = aground. One black ball = anchored. If you see red lights in an anchorage, the vessel is aground.
The height of masthead lights above the hull is specified in Annex I to COLREGS. These details appear less frequently on the exam but show up on advanced-level questions. The key rule: the forward masthead light must be in the forward quarter of the vessel, and on vessels 50m or more, it must be at least 6m above the hull.
Forward Masthead Light Height
Second Masthead Light Requirement
Know what each rule number covers. The exam sometimes asks which rule governs a specific vessel type.
Application — When lights are required (sunset/sunrise, restricted visibility)
Definitions — Masthead, sidelight, sternlight, towing light, all-around, flashing
Visibility of Lights — Minimum range by vessel size
Power-Driven Vessels Underway — Masthead, sidelights, sternlight
Towing and Pushing — Masthead counts, yellow towing light, pushed vessel
Sailing Vessels Underway and Vessels Under Oars
Fishing Vessels — Trawling (green-white) vs not trawling (red-white)
Vessels Not Under Command or Restricted in Ability to Maneuver
Vessels Constrained by Draft (International Rules only)
Pilot Vessels — White over red all-around lights
Anchored Vessels and Vessels Aground
Seaplanes and WIG Craft — Show closest practical lights
What is the arc of a masthead light?
225 degrees. Forward-facing, from dead ahead to 22.5 degrees abaft the beam on each side. White. Required on power-driven vessels underway, prohibited on sailing vessels under sail alone.
What does a vessel showing red over red (two red all-around lights) indicate?
Not Under Command. The vessel cannot maneuver as required by the COLREGS due to exceptional circumstances. All other vessels must keep clear.
What does a vessel showing red-white-red (three all-around lights) indicate?
Restricted in Ability to Maneuver. The nature of the vessel's work restricts its ability to maneuver. RAM vessels include dredgers, cable layers, mine clearance vessels, and vessels conducting underway replenishment.
A vessel shows three masthead lights in a vertical line while underway. What does this mean?
The vessel is towing astern and the tow exceeds 200m in length. Two mastheads = tow 200m or less. Three mastheads = tow over 200m.
What lights does a vessel at anchor show?
A vessel under 50m shows one all-around white light forward. A vessel 50m or more shows two all-around white lights: one forward (higher) and one aft (lower). No sidelights, no sternlight, no masthead light.
What is the difference between a trawling vessel and a fishing vessel under Rule 26?
Trawling: dragging a trawl through water. Shows green over white all-around lights. Day shape: bicone (two cones apex-to-apex). Fishing (not trawling): nets, lines, traps that restrict maneuverability. Shows red over white all-around lights. Day shape: one cone apex-down.
Does a Constrained by Draft vessel exist under Inland Rules?
No. The CBD category (Rule 28) exists only under International Rules (COLREGS). There is no equivalent vessel category under Inland Rules. This is a frequently tested distinction.
Start With Arc Values
Lock in the five arc numbers first: 225, 112.5, 135, 360, and the towing light 135. Once these are automatic, everything else hangs on them.
Learn by Vessel Type
Drill one vessel type per session. Power-driven first, then sailing, then NUC/RAM, then fishing, then towing, then anchor/aground. Each has a clear logic.
Always Pair Lights with Day Shapes
For every light configuration you learn, immediately learn the corresponding day shape. The exam tests both, and they reinforce each other as memory anchors.
Learn the Exceptions
CBD is International only. Tricolor is sailing vessels under 20m. Trolling is not fishing. Motorsailing shows power-driven lights. The exam targets exactly these edge cases.
Practice Identification Scenarios
Have someone describe a light pattern — you identify the vessel type and status. Then reverse it: name a vessel type — describe its lights. This two-way drill builds deep recall.
Use the Stacking Logic
NUC and RAM lights stack on top of the base vessel lights. A RAM vessel underway shows RAM lights PLUS power-driven vessel lights. Understanding stacking prevents confusion on combination questions.
NailTheTest has hundreds of USCG-style navigation lights questions with instant feedback, explanations, and exam-realistic pacing. Practice until lights and shapes are automatic.
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