When to display lights, which lights per vessel type, exact arc degrees, and day shapes — the most tested block in Part C of the OUPV exam.
Rules 20–31 govern every display requirement for navigation lights and day shapes. The OUPV exam draws heavily from this block — expect questions on arc degrees, which vessel type shows what, and the specific stacking order of NUC, RAM, and fishing lights. Rule 20 sets the framework; Rules 21–31 define the specifics by vessel type.
Rules 20–31
Complete scope of Part C — Lights & Shapes
High
Exam frequency — tested repeatedly across all sections
5 arcs
Light arc values you must know cold for the exam
Sunset to Sunrise
All vessels must display the appropriate navigation lights from sunset to sunrise, regardless of weather or visibility conditions.
Restricted Visibility — Any Time
Lights must be displayed during periods of restricted visibility (fog, rain, haze) at any time of day, not just at night.
Daylight Hours — Clear Conditions
A vessel may display navigation lights during daylight hours if the master chooses — it is not prohibited. However, the exam tests the required conditions, not optional display.
Five light types, five arc values. These numbers appear repeatedly on the exam — know them exactly.
| Light | Color | Arc | Direction & Notes |
|---|---|---|---|
| Masthead (steaming) | White | 225° | Forward-facing — ahead to 22.5° abaft the beam on each side |
| Sidelight — Port | Red | 112.5° | From dead ahead to 22.5° abaft the beam, port side |
| Sidelight — Starboard | Green | 112.5° | From dead ahead to 22.5° abaft the beam, starboard side |
| Sternlight | White | 135° | Aft-facing — centered astern, 67.5° on each side |
| All-around light | White / Red / Green | 360° | Full horizon — used for anchor, fishing, NUC, RAM signals |
| Towing light | Yellow | 135° | Same arc as sternlight — shown instead of sternlight when towing |
Memory anchor: 225 + 135 = 360
Masthead (225°) + Sternlight (135°) = a full 360° of visibility around the vessel. Sidelights cover 112.5° each — together 225°, matching the masthead arc exactly.
May substitute a single all-around white light for the full masthead + sidelights + sternlight combination. If practicable, sidelights should also be shown.
May use a combined tricolor lantern at the masthead combining red, green, and white sectors — replacing separate sidelights and sternlight. Cannot use this combined lantern with any other masthead light.
A sailing vessel may additionally exhibit two all-around lights at the masthead — red over green — to signal sailing status. These may not be used with the combined tricolor lantern.
If it is impracticable to show the required lights, an all-around white light must be kept ready for immediate use to prevent collision.
A vessel aground shows all required anchor lights for its size, PLUS two additional all-around red lights in a vertical line.
Day shapes replace lights during daylight. Shapes are black, displayed where they can best be seen, typically forward.
| Vessel Status | Shape | Details |
|---|---|---|
| At Anchor | 1 black ball | Displayed forward, where best seen |
| Aground | 3 black balls — vertical | Three balls in a vertical line |
| Not Under Command (NUC) | 2 black balls — vertical | Two balls in a vertical line |
| Restricted in Ability to Maneuver (RAM) | Ball – Diamond – Ball (vertical) | Black ball top, black diamond middle, black ball bottom |
| Constrained by Draft (CBD) | Cylinder | Black cylinder — International Rules only |
| Fishing (not trawling) | Cone apex-down | Single cone, point facing downward |
| Trawling | 2 cones apex-to-apex (bicone) | Two cones joined at their points — hourglass shape |
Exam tip: RAM vs NUC shapes
NUC = 2 balls (simple). RAM = ball-diamond-ball (3 items, note the diamond in the middle). The exam tests this distinction regularly.
Memory: G over W — trawling. "Green tops the trawl."
Memory: R over W — fishing (not trawling). Red = not trawling.
An additional all-around white light is displayed in the direction the outlying gear (net) extends. This applies to vessels fishing (not trawling) only.
NUC cannot maneuver — engine failure, steering failure, etc. Red-red at masthead, plus running lights if moving.
RAM can maneuver, but is limited by its operation — dredging, cable-laying, etc. Red-white-red in a vertical stack.
NUC vs. RAM — exam distinction
NUC = red-red (2 lights, no white). RAM = red-white-red (3 lights, white in middle). The number and color pattern distinguishes them. Both add running lights only when making way — not when stopped or anchored.
A sailing vessel underway displays sidelights (red port, green starboard) and a sternlight (white, 135°) — but no masthead (steaming) light. Vessels under 20m may combine sidelights and sternlight into a single tricolor lantern at the masthead. A sailing vessel may also display an optional red-over-green all-around light at the masthead to signal sailing status. Vessels under 7m may show only an all-around white light if it is impracticable to show the required lights.
A masthead (steaming) light is white and covers an arc of 225°, facing forward. It is visible from dead ahead around to 22.5° abaft the beam on both sides. Masthead lights are required on power-driven vessels underway but are NOT shown by sailing vessels under sail alone.
Day shapes replace lights during daylight: a vessel at anchor displays one black ball forward; a vessel aground shows three black balls in a vertical line; a Not Under Command (NUC) vessel displays two black balls vertically; a RAM vessel shows ball-diamond-ball vertically; a vessel Constrained by Draft (CBD) shows a cylinder; a fishing vessel (not trawling) shows a cone apex-down; a trawling vessel shows two cones apex-to-apex (bicone). Shapes are typically black and displayed where they can best be seen.
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