Navigation Rules reference
Navigation lights and shapes
At night a vessel tells you what it is and what it's doing with lights. By day it says the same with black shapes. This is the set worth knowing cold.
The four basic lights
| Light | Color | Arc of the horizon | Rule |
|---|---|---|---|
| Masthead light | White | 225 degrees, right ahead to 22.5 degrees abaft the beam on either side | Rule 21(a) |
| Sidelights | Green to starboard, red to port | 112.5 degrees each, right ahead to 22.5 degrees abaft the beam on its side | Rule 21(b) |
| Sternlight | White | 135 degrees, 67.5 degrees from right aft on each side | Rule 21(c) |
| All-round light | Depends on the signal | 360 degrees | Rule 21(e) |
A power-driven vessel underway
A masthead light forward, sidelights and a sternlight. A second masthead light, abaft of and higher than the forward one, is required at 50 meters and over and optional below that (Rule 23(a)).
Under 12 meters, a power-driven vessel may show an all-round white light and sidelights instead (Rule 23(d)). On the Great Lakes, Inland Rule 23(e) lets a power-driven vessel carry an all-round white light in place of the second masthead light and sternlight.
Special situations, night and day
The left panel is what you see at night, the right is the day shape.
Restricted in ability to maneuver
Night: red over white over red. Day: ball, diamond, ball.
Red, white, red: limited by the work ahead.
Trawling
Night: green over white. Day: cone, point down, cone, point up.
Green over white, trawling tonight.
Fishing, not trawling
Night: red over white. Day: cone, point down, cone, point up.
Red over white, fishing tonight.
Aground
Night: anchor lights, plus red over red. Day: ball, ball, ball.
Anchor lights plus red over red.
Constrained by draft
Night: red over red over red. Day: cylinder.
International rules only. Optional.
Mine clearance
Night: three green, in a triangle. Day: ball, ball, ball.
One at the foremast, one at each yard end. Keep 1,000 meters off.
Sailing, optional masthead
Night: red over green. Day: no shape.
Red over green, sailing machine.
Common questions
When do navigation lights have to be on?
From sunset to sunrise, in all weathers, and also by day in restricted visibility if they are carried (Rule 20).
What lights does a small power boat show?
Under 12 meters, a power-driven vessel may show an all-round white light and sidelights instead of the usual masthead light, sidelights and sternlight (Rule 23(d)).
Which side is the red light on?
Port. The green sidelight is on the starboard side (Rule 21(b)).
When are day shapes shown?
By day. The rules on shapes apply in the daytime, and the rules on lights apply from sunset to sunrise (Rule 20).
Test yourself
4 questions. Pick an answer to see its rule.
A law enforcement vessel patrolling a marine regatta may exhibit a flashing blue light or which of the following alternates?
Show the answer
D. an alternately flashing red and yellow light
Patrol vessels at a marine event may use a flashing blue light or the alternately flashing red and yellow light. Remember red and yellow as the non-police alternative.
33 CFR Parts 88 and 100. Source: NMC sample exam.
A commercial vessel engaged in public safety activities may display a(n) identity light(s). Which of the following is appropriate for these activities?
Show the answer
A. alternately flashing red and yellow light
Blue is for law enforcement. A commercial vessel doing public safety work may use the alternately flashing red and yellow light instead, and it gets no right of way from it.
33 CFR 88.07. Source: NMC sample exam.
Which lights is a 45-meter power-driven vessel pushing ahead or towing alongside required to display?
Show the answer
B. Two masthead lights in a vertical line
Under International rules a pushing or alongside-towing power vessel under 50 meters shows two masthead lights in a vertical line, and sidelights and a stern light. Yellow towing lights are for towing astern.
Rule 24(c). Source: NMC sample exam.
You have sighted three red lights in a vertical line on another vessel dead ahead at night. Which vessel would display these lights?
Show the answer
D. A vessel constrained by its draft
Three all-round red lights in a vertical line mark a vessel constrained by draft, which exists only in the International rules.
Rule 28. Source: NMC sample exam.
The sound signals pair with these in fog, and the full Rules of the Road practice test has more.
Night scene, then day shape
Helm shows each vessel’s lights in a night scene and flips to its day shapes, with flashcards and spaced review.
Start freeKeep studying
- Free OUPV practice testReal sample questions, each with its rule. No sign-up.
- Sound signalsEvery horn, bell and fog signal, with audio and citations.
- What the exam coversThe modules, and what to confirm in your letter.
- Six-pack licenseWho needs one and what the regulations ask for.
- Rules of the roadWho gives way, and how Inland differs.
- BuoysRed right returning, and what each mark means.
Sources
- USCG NAVCEN, Amalgamated Navigation Rules (Rules 20 to 31)
- 33 CFR 83.20, Rule 20
- 33 CFR 83.21, Rule 21
- 33 CFR 83.23, Rule 23
Helm is an independent study aid, not affiliated with or endorsed by the U.S. Coast Guard or the National Maritime Center. This page is for study, not navigation guidance or legal advice. Check the current rules before you rely on anything here.