Why Navigation Lights Matter — On the Water and on the Exam
Navigation lights exist for one reason: to prevent collisions at night and in restricted visibility. When you cannot see another vessel's hull, her lights tell you what type of vessel she is, which direction she's heading, and whether you need to give way. Getting this wrong at sea is dangerous. Getting it wrong on the exam is an easily avoidable mistake.
In my years as a working captain, I've stood many night watches where accurate light identification was the difference between a comfortable transit and a close call. The light system is elegant once you understand it — every configuration tells a story about what that vessel is doing and how much freedom of movement she has.
Navigation lights questions appear reliably on every USCG exam. On the OUPV, expect 3-5 lights questions. On the 100-Ton, expect 6-8. They come in two forms: "What vessel displays these lights?" and "What lights does this vessel display?" Both require the same underlying knowledge.
The 5 Types of Navigation Lights
Before memorizing configurations, understand the five fundamental light types:
1. Masthead Light
A white light showing over an unbroken arc of 225 degrees. It covers from dead ahead to 22.5 degrees abaft the beam on each side. Masthead lights are shown by power-driven vessels underway. On vessels over 50 meters, two masthead lights are required — the aft light higher than the forward light.
Key fact: sailing vessels do NOT carry masthead lights (unless using an optional combined tri-color masthead light instead of separate sidelights and sternlight). This distinction is exam gold.
2. Sternlight
A white light showing over an unbroken arc of 135 degrees, centered on dead astern. In other words, it covers 67.5 degrees on each side of dead astern. The sternlight is what you see when you're overtaking another vessel — it means you're behind them.
Remember: masthead (225°) + sternlight (135°) = 360°. Together they cover the full circle. This is by design.
3. Sidelights
Red on the port (left) side, green on the starboard (right) side. Each sidelight shows over an unbroken arc of 112.5 degrees — from dead ahead to 22.5 degrees abaft the beam on each side. On small vessels (under 20m), these can be combined in a single bicolor lantern on the bow.
The memory trick: "Red right returning" tells you the right-of-way rule for navigating channels (red buoys on your right returning from sea), but for lights remember the mnemonic: port wine is red, and port is on the left.
4. All-Round Light
A light showing an unbroken arc of 360 degrees. All-round lights indicate special status or are used for anchored vessels. Color matters: white for anchors, red and/or green for specific vessel types (NUC, RAM, fishing), yellow for towing astern.
5. Towing Light
A yellow light with the same arc as the sternlight (135 degrees), shown by a vessel towing astern. If you see a yellow sternlight, the vessel ahead of you has something on a line behind it.
Arc Degrees: The Numbers You Must Know
The exam tests arc degrees directly. Memorize these:
- Masthead light: 225 degrees
- Sternlight: 135 degrees
- Sidelight (each): 112.5 degrees
- All-round: 360 degrees
- Towing light: 135 degrees (same as sternlight)
Notice that 225 + 135 = 360. And 112.5 + 112.5 + 135 = 360. The geometry is intentional — lights are designed so that from any position around the vessel, you can see some combination of lights that tells you what's happening.
Vessel Configurations That Are Tested
These are the specific vessel types that appear on virtually every USCG exam:
Power-Driven Vessel Underway, Less Than 50m
The standard pleasure boat or small charter vessel:
- One forward masthead light (white, 225°)
- Sidelights (red port, green starboard, 112.5° each)
- Sternlight (white, 135°)
Power-Driven Vessel Underway, 50m or More
The same as above, plus:
- A second masthead light aft, mounted higher than the forward masthead light
The aft masthead light being higher than the forward one is an exam detail. From the side, you can determine a large vessel's rough heading by which masthead light appears higher.
Sailing Vessel Underway
Sailing vessels underway carry:
- Sidelights (red port, green starboard)
- Sternlight (white, 135°)
- No masthead light
Optional: instead of separate sidelights and sternlight, a sailing vessel may use a combined tri-color lantern at the masthead. But if using the tri-color masthead lantern, she may NOT also display the separate sidelights and sternlight. One or the other — not both.
Sailing vessels under power (motorsailing) must display power-driven vessel lights and a cone shape point-down by day. This trips up many students who think "sailing vessel" means any vessel with sails.
Vessel at Anchor
- Under 50m: one all-round white light where it can best be seen
- 50m and over: all-round white forward plus all-round white (may be less bright) aft, lower than the forward light
Vessel Not Under Command (NUC)
Two all-round red lights in a vertical line, plus sidelights and sternlight if making way. By day: two balls in a vertical line. NUC means the vessel cannot maneuver due to an exceptional circumstance — engine failure, steering casualty. These vessels get highest priority in the Rule 18 hierarchy.
Vessel Restricted in Ability to Maneuver (RAM)
Three all-round lights in a vertical line: red-white-red (top to bottom). Plus sidelights and sternlight if making way. By day: ball-diamond-ball shape. RAM vessels include dredges, cable-layers, and mine-clearance vessels — vessels doing work that restricts their ability to keep clear.
Vessel Constrained by Draft (CBD)
Three all-round red lights in a vertical line, plus normal underway lights. By day: a cylinder shape. International Rules only — this designation applies to a vessel whose draft relative to available water depth genuinely restricts her ability to deviate from the channel.
Vessel Engaged in Trawling
Green-over-white all-round lights (the "Christmas tree" pattern), plus sidelights and sternlight if making way over 2 knots. By day: two cones joined at their apices (a diamond-like shape).
Vessel Engaged in Other Fishing (Not Trawling)
Red-over-white all-round lights. If gear extends more than 150m horizontally, an additional all-round white light in the direction of the gear. Plus sidelights and sternlight if making way over 2 knots.
Pilot Vessel
White-over-red all-round lights at the masthead, plus normal underway lights when underway or an anchor light when at anchor. Pilot vessels are on station guiding commercial ships — they get the white-over-red so that commercial traffic can identify them and make radio contact.
How to Remember Them
After teaching this material to many students, these are the memory anchors that stick:
- NUC = "Not Under Command" = Red-Red. Two reds, no movement control. Think: red means stop, and they literally cannot maneuver.
- RAM = Red-White-Red. Three lights, middle one white. Remember it as "sandwich" — red bun, white filling, red bun.
- Fishing (trawling) = Green over White = "Christmas tree." Green on top (like a star), white below (like snow).
- Fishing (other) = Red over White. Red is higher status in the fishing light hierarchy — it means non-trawling gear, which often has less maneuverability than a trawl.
- Pilot vessel = White over Red. Opposite of non-trawling fishing (red-over-white). Pilots are important — they get to be on top (white above red).
- CBD = Three Reds. Most burdened power-driven vessel, most red lights.
The Most Common Exam Trick Questions
These are the questions that catch students who think they know the material:
- "A sailing vessel underway is showing a masthead light and sidelights. What is she doing?" — Motorsailing. A sailing vessel carrying engine power must display masthead lights. The exam uses this to test whether students know the motorsailing rule.
- "A vessel shows all-round red-white-red lights. What is she?" — RAM. Students confuse this with NUC (two all-round reds) or CBD (three all-round reds). Count the lights and note the colors.
- "A power-driven vessel shows a forward masthead light but no aft masthead light. How long is she?" — Less than 50 meters. This is a length-determination question. The exam tests whether you know when the second masthead light is required.
- "A vessel at anchor shows a single all-round white light. How long is she?" — Under 50 meters. Vessels 50m and over require two anchor lights.
- "A towing vessel shows a yellow sternlight. What does this indicate?" — She is towing astern. The yellow towing light replaces the normal white sternlight when towing. If the tow is more than 200m, she also displays an additional yellow towing light above the first.
Tying It Back to the Rules
Navigation lights questions aren't just trivia — they connect directly to the Rules of the Road. When you can correctly identify a vessel by her lights, you can apply the correct rule. If you see two all-round red lights ahead, you know you're looking at a vessel not under command — give way, stay well clear, don't expect it to maneuver for you.
Study lights and rules together, not separately. The light configuration tells you the vessel type; Rule 18 tells you where that vessel sits in the give-way hierarchy. Together, they tell you exactly what action to take.
The free COLREGS Quick Reference on The Chartroom includes a lights summary chart you can print. The Rules of the Road practice tool includes lights-specific question sets so you can drill this material separately from the rest of COLREGS until you're scoring consistently above 90%.