Every vessel type, every required light, exact arc degrees, and the exam traps that sink candidates — organized by COLREGS rule so you can drill it cold.
Rules 23–30
The exact COLREGS rules governing navigation lights by vessel type
5 arcs
Arc values you must know: 225°, 135°, 112.5°, 360°, and 135° towing
High
Exam frequency — navigation lights appear across every section of the exam
Five arc values, five light types. These numbers appear on every exam — commit them to memory before everything else.
| Light | Color | Arc | Direction & Notes |
|---|---|---|---|
| Masthead (steaming) | White | 225° | Forward — ahead to 22.5° abaft the beam on each side. Power vessels only. |
| Sidelight — Port | Red | 112.5° | Dead ahead to 22.5° abaft the beam, port side |
| Sidelight — Starboard | Green | 112.5° | Dead ahead to 22.5° abaft the beam, starboard side |
| Sternlight | White | 135° | Aft — centered astern, 67.5° on each side of centerline |
| All-round light | White / Red / Green | 360° | Full horizon — anchor, NUC, RAM, fishing, pilot signals |
| Towing light | Yellow | 135° | Same arc as sternlight — replaces sternlight when towing |
Memory anchor: 225 + 135 = 360
Masthead (225°) + Sternlight (135°) = 360° — a complete circle around the vessel. Each sidelight covers 112.5°; two sidelights together equal 225°, matching the masthead arc exactly.
Length is the controlling variable. Know all three length categories and their exact requirements.
May substitute a single all-round white light for the entire light combination. Sidelights should also be shown if practicable. This is the smallest, slowest power vessel exception.
The most-tested distinction: sailing vessels show NO masthead light. The masthead light signals engine use.
May use a combined tricolor lantern at or near the masthead that incorporates the red, green, and white sectors — replacing the separate sidelights and sternlight. Cannot be used simultaneously with any other masthead light.
Any sailing vessel may additionally show two all-round lights in a vertical line at or near the masthead — red over green — to signal sailing status. These may NOT be used when the tricolor lantern is in use.
If it is impracticable to show the required lights, an all-round white light must be kept ready for immediate use and shown in sufficient time to prevent collision.
A sailing vessel proceeding under sail and engine simultaneously is treated as a power-driven vessel and must show a masthead light. During daylight, it must display a black cone, apex downward, in the forward part of the vessel.
These lights override normal underway lights. Know the vertical stack patterns and which vessels add running lights when making way.
Engine failure, steering casualty, disabled
Pattern: red-red. No white. No masthead light.
Dredging, cable-laying, mine clearance, replenishment underway
Pattern: red-white-red. White in the middle. Three lights total.
NUC vs. RAM — exam distinction
NUC = red-red (2 red lights, no white). RAM = red-white-red (3 lights, white sandwiched in middle). Both add running lights only when making way through the water. Neither shows a masthead light.
International Rules only — deep-draft vessel unable to deviate from course
Three vertical reds. CBD shows standard power lights AND the red stack — unlike NUC/RAM which replace the masthead light.
Dragging a net along the bottom
Memory: Green over White — trawling. “Green tops the trawl.”
Lines, pots, nets not dragged along bottom
Memory: Red over White — not trawling.
Vessel engaged in pilotage duty
Unique pattern: white over red. Only vessel type with this specific combination.
All anchor lights required for the vessel's length, PLUS two additional all-round red lights in a vertical line where best seen.
Note: the two red lights for aground vessels are distinct from NUC. Aground vessels also show their anchor white light(s); NUC vessels do not show anchor lights.
Sailing vessel with engine = power-driven vessel
A sailing vessel using its engine — even with sails raised — is legally a power-driven vessel under COLREGS. It must show a masthead light and display a black cone (apex down) in the forward part during daylight. Candidates frequently miss the masthead light requirement when sails are also set.
NUC is red-red, not red-white-red
Not Under Command (NUC) shows two all-round red lights — no white. RAM shows three lights: red-white-red. The exam will present similar vertical stack descriptions and ask you to identify the vessel. Count the lights and note whether white appears.
CBD shows standard power lights PLUS three reds
A vessel Constrained by Draft (CBD) does not replace its normal power-driven lights — it adds three all-round red lights in a vertical stack on top of the full masthead + sidelights + sternlight combination. NUC and RAM vessels do not show a masthead light; CBD vessels do.
Tricolor lantern cannot be used with other masthead lights
The combined tricolor lantern (sailing vessels under 20m) replaces sidelights and sternlight entirely. It cannot be used simultaneously with the optional red-over-green sailing signal or any other masthead light. Exam questions will present combinations that include both — those combinations are invalid.
Anchor lights: size threshold is 50m, not 100m
The two-anchor-light requirement (forward + aft) applies to vessels 50 meters and over. Candidates frequently confuse this with fog signal thresholds (100m for bell-and-gong) or other size categories. For Rule 30 anchor lights, the dividing line is 50m.
Towing light is yellow, same arc as sternlight
When a vessel is towing, it shows a yellow towing light in place of its white sternlight. The towing light has the same 135° arc as the sternlight. Candidates confuse the color (yellow vs. white) or the arc (135°, not 225° or 360°). The towing light signals to vessels astern that a tow is in progress.
The three primary navigation lights are: (1) the masthead (steaming) light — white, 225°, facing forward from dead ahead to 22.5° abaft the beam on each side; (2) sidelights — red on port and green on starboard, each covering 112.5° from dead ahead to 22.5° abaft the beam; and (3) the sternlight — white, 135°, centered aft. Together, masthead (225°) plus sternlight (135°) cover a full 360°, and each sidelight's 112.5° exactly matches half the masthead arc.
A sailing vessel underway under sail alone shows sidelights (red port, green starboard, 112.5° each) and a sternlight (white, 135°) — but NO masthead light. Vessels under 20m may substitute a combined tricolor lantern at the masthead. An optional red-over-green all-round light at the masthead signals sailing status but cannot be used with the tricolor lantern. Critically, when a sailing vessel uses its engine — even with sails up — it becomes a power-driven vessel and must show a masthead light.
A Not Under Command (NUC) vessel shows two all-round red lights in a vertical line — no white light. A vessel Restricted in Ability to Maneuver (RAM) shows three all-round lights in a vertical line: red over white over red. The key distinction: NUC is red-red (2 lights, no white); RAM is red-white-red (3 lights, white in the middle). Both add sidelights and sternlight only when making way through the water, not when stopped or at anchor.
A power-driven vessel underway shows: one masthead light (white, 225°) for vessels under 50m, or two masthead lights (forward lower, after higher) for vessels 50m and over; plus sidelights (red port, green starboard) and a sternlight (white, 135°). Vessels under 12m may use a single all-round white light plus sidelights if the full combination is impracticable. Vessels under 7m making no more than 7 knots may show only an all-round white light.
A vessel at anchor under 50m shows one all-round white light forward. A vessel at anchor 50m and over shows two all-round white lights — one forward and one at or near the stern, the aft light lower than the forward light. Vessels may also illuminate their decks. A vessel aground shows all required anchor lights for its length PLUS two additional all-round red lights in a vertical line.
A trawling vessel shows a green all-round light over a white all-round light (vertical), plus sidelights and sternlight when making way. A fishing vessel not trawling shows a red all-round light over a white all-round light (vertical), plus sidelights and sternlight when making way. Memory aid: 'Green tops the trawl.' If nets extend more than 150 meters, an additional all-round white light is shown in the direction of the outlying gear (non-trawling vessels only).
A combined lantern (tricolor lantern) combines the red, green, and white sectors of sidelights and sternlight into a single fitting at the masthead. It may only be used on sailing vessels under 20 meters in length. When the tricolor lantern is in use, it replaces the separate sidelights and sternlight entirely — the vessel may not simultaneously use other masthead lights. The tricolor lantern cannot be used with the optional red-over-green sailing signal.
A vessel engaged in pilotage duty shows a white all-round light over a red all-round light in a vertical line at or near the masthead. When underway, it also shows sidelights and a sternlight. When at anchor, it shows the white-over-red pilot lights in addition to the required anchor lights for its length. The white-over-red combination is unique to pilot vessels — no other vessel type uses this specific pattern.
Complete Part C coverage including when to display lights, day shapes, and all vessel-type requirements under Rules 20–31.
Part B — right-of-way hierarchy, overtaking, crossing, head-on situations, and Inland vs. International rule differences.
Full COLREGS overview — all five parts, structure, and key rule summaries for the captain's license exam.
Rules 32–37: maneuvering signals, fog signals by vessel type, distress signals, and the danger/doubt signal.
Key differences between Inland and International Rules — whistle signals, right-of-way on rivers, and Inland-only requirements.
Anchorage procedures, anchor watch requirements, scope calculation, and anchor light rules in depth.
1,628+ USCG exam questions — full coverage of Rules 23–30, spaced repetition flashcards, and instant answer explanations. Free to start.
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