Buoyage & Aids to Navigation
Master the IALA-B buoyage system for the USCG OUPV exam: lateral marks, light characteristics, special marks, fog signals, daybeacons, ranges, and how to read the Light List.
IALA-B Buoyage System — The US Standard
The United States uses the IALA-B (International Association of Lighthouse Authorities — Region B) system. IALA-B covers North America, South America, Japan, South Korea, and the Philippines. The competing system, IALA-A, covers Europe, Africa, India, and Australia — and is the mirror-opposite for lateral marks (red = port, green = starboard in IALA-A). Knowing this distinction matters: an OUPV exam question may test whether you understand that rules differ by region.
The IALA-B system has five categories of marks. Each category has a unique combination of color, shape, topmark, light characteristic, and number or letter convention. No two categories share all of these attributes, so a well-prepared candidate can identify any mark from any single distinguishing feature.
Red Right Returning — The Core Rule
When returning from sea (heading toward a harbor, heading upstream on a river, or heading north and east on the Gulf Coast ICW) — keep red marks on your starboard side and green marks on your port side.
"Returning" means heading landward — toward a harbor, marina, or anchorage — or upstream on any river or waterway that flows toward the sea. On the Atlantic Intracoastal Waterway, the conventional direction is southbound (heading south = returning). On the Gulf Coast ICW, heading west is the conventional direction. The USCG chart and all ATON publications define the conventional direction for every waterway.
When heading seaward (outbound), simply reverse the rule: red marks go to port, green marks go to starboard. Many mariners say "red, right, return" and then just remember to flip it going out.
| Side | Port (left) returning from sea |
| Shape | Can (flat top) |
| Numbers | Odd (1, 3, 5...) |
| Light | Green — any characteristic |
| Topmark | Single green square |
| Side | Starboard (right) returning from sea |
| Shape | Nun (conical top) |
| Numbers | Even (2, 4, 6...) |
| Light | Red — any characteristic |
| Topmark | Single red triangle |
Light Characteristics — Complete Reference
A light characteristic describes the pattern of a navigational light: how it flashes, how long it is on and off, and at what interval. The characteristic is always noted on charts and in the Light List with an abbreviation, a period (total cycle time in seconds), and sometimes a group number. Example: Fl(2) R 6s = Group Flashing, 2 flashes, red, period of 6 seconds.
The period is the time for one complete cycle — from the start of a flash to the start of the next identical flash. On a chart, a light labeled "Fl R 4s 15ft 4M" means: Flashing red, every 4 seconds, 15 feet above mean high water, nominal range 4 nautical miles.
| Abbr | Name |
|---|---|
| F | Fixed |
| Fl | Flashing |
| Fl(2) | Group Flashing |
| Fl(2+1) | Composite Group Flashing |
| Oc | Occulting |
| Iso | Isophase |
| Mo(A) | Morse Code A |
| Q | Quick Flashing |
| VQ | Very Quick Flashing |
| LFl | Long Flashing |
Fl(2+1) R 6s breaks down as: Composite Group Flashing, group of 2 then 1 flash, red color, 6-second period. The number in parentheses always tells you how many flashes in the group.
"Fl 4s" does NOT mean the light is on for 4 seconds. It means the total cycle repeats every 4 seconds. A typical flash lasts about 0.5 seconds; the remaining 3.5 seconds are darkness.
Light Colors — What Each Means
The color of a navigational light is not decoration — it encodes meaning. In the IALA-B system, only four light colors are used on ATON: red, green, white, and yellow. Each color has a defined meaning and no two mark categories share the same color-to-purpose assignment.
Starboard lateral marks (even numbers) — keep to starboard when returning
Also used for danger sectors on sector lights
Port lateral marks (odd numbers) — keep to port when returning
Also used for safe sector on sector lights
Safe water marks, isolated danger marks, ranges, major lights, safe sectors
Default color for major navigational lights
Special marks — non-navigational purposes (surveys, military, fish farms)
Also used on some Intracoastal Waterway marks
Special Mark Types — Full Reference
Beyond port and starboard lateral marks, the IALA-B system includes three additional mark categories plus the special-purpose yellow marks. Each has a unique identifier that allows a mariner to determine mark type even if only one attribute is visible.
Red and white vertical stripes
Spherical or pillar with red sphere topmark
White — Morse A (· —)
Black with red horizontal bands
Pillar or spar; two black spheres topmark (vertical)
White — Group Flashing 2 — Fl(2)
Red body with green band(s)
Nun or can — inherits lateral shape
Red — Composite Group Fl(2+1)
Green body with red band(s)
Nun or can — inherits lateral shape
Green — Composite Group Fl(2+1)
Yellow
Any — X-shaped topmark
Yellow — any characteristic
Safe Water Marks — Fairway Buoys
Safe water marks (commonly called fairway buoys) indicate that navigable water exists on all sides of the mark. They are placed at the entrance to a channel, at the mid-channel position, or at a point where vessels should pass close aboard to enter a fairway.
Identification Checklist
- · Red and white vertical stripes — not horizontal bands
- · Spherical shape or pillar with red spherical topmark
- · White Morse code A light — short-long flash pattern
- · No number — may carry a letter (e.g., "M" for main entrance)
- · May be red-topped — the topmark is red, not the stripes
What to Do When You See One
- · Pass it on either side — safe water all around
- · It marks the beginning of a lateral buoyage system
- · Numbers start from here — next buoy will be "2" or "1"
- · Often a landfall buoy for offshore passages
- · May be the "sea buoy" referenced in a port guide
Isolated Danger Marks
An isolated danger mark is placed directly on top of, or very close to, an isolated hazard that has navigable water all around it. The hazard might be a submerged rock, a shoal pinnacle, a wreck, or a reef surrounded by deep water. The mark warns you that the danger is right here — not in a general area, but at this specific spot.
Preferred Channel Buoys — Junction Marks
When a channel divides into two channels (a bifurcation or junction), a preferred channel buoy marks the spot. Both channels are navigable, but one is considered the preferred or primary channel — typically the deeper, wider, or more commonly used route. The buoy tells you which way to go for the preferred channel and which side to pass it on.
- Color: Red body with green band(s)
- Light: Red Fl(2+1)
- Action: Keep on starboard — preferred channel is to starboard
- Memory: Top color = treat like that color lateral mark
- Color: Green body with red band(s)
- Light: Green Fl(2+1)
- Action: Keep on port — preferred channel is to port
- Memory: Top color = treat like that color lateral mark
The composite group flashing light — Fl(2+1) — is the only characteristic assigned exclusively to preferred channel marks. The pattern is two quick flashes followed by a single flash, then a period of darkness before the sequence repeats. This three-element pattern is immediately distinguishable from a plain Fl(2) or Fl(3), and the exam will test whether you know what Fl(2+1) means.
Special Marks — Yellow ATON
Special marks are yellow and mark areas or features that are not themselves navigational hazards but are important to know about. They do not carry the same lateral meaning as red and green marks. Any shape can be used; when a topmark is present, it is an X (cross shape). Any light characteristic may be used, but the light must be yellow.
Daybeacons and Range Lights
Daybeacons are fixed structures (usually a pile or dolphin) with a dayboard — a colored board that conveys the same information as a buoy. They do not float; they are anchored to the bottom. In shoal-water channels, daybeacons are often preferred over buoys because they cannot drag anchor or go adrift.
| Dayboard Type | Meaning |
|---|---|
| Square Green Dayboard | Port-hand — keep to port when returning from sea |
| Triangular Red Dayboard | Starboard-hand — keep to starboard when returning from sea |
| Octagonal White Dayboard | Safe water or mid-channel |
| Red-White Striped Diamond | Safe water — occasionally used for ICW approach |
Range Lights
A range is a pair of lights (or daymarks) positioned so that when aligned, they indicate the centerline of a channel. The front range light is always lower and closer to the water than the rear range light. When you steer so that the front and rear lights are vertically aligned (one above the other), you are on the range line — on the safe centerline of the channel.
Range Light Rules
- · Front (near) range = lower, closer to the water
- · Rear (far) range = higher, farther from the water
- · When aligned, you are on the range bearing
- · If rear appears to the right of front, steer left
- · If rear appears to the left of front, steer right
- · Ranges often use fixed white or isophase lights
Using Ranges in Practice
- · Range bearing is published on NOAA charts
- · Set your compass to the range course before entry
- · The range line may extend beyond the channel — do not follow it past the turn
- · Day ranges have rectangular dayboards — sometimes orange-and-white striped
- · Range lights often have no color — just intensity difference
Fog Signals on Aids to Navigation
Fog signals on ATON help mariners locate or identify a mark when visibility is reduced. Not all buoys have fog signals — the Light List entry for each aid will state if a fog signal is present, its type, and its characteristic (e.g., "Bell" or "Horn: 1 blast ev. 30s"). Buoy fog signals that depend on wave action may be silent in calm conditions, which is a critical operational consideration.
| Signal Type | Chart Symbol |
|---|---|
| Bell | B |
| Gong | G |
| Whistle | W |
| Horn | Ho |
| Diaphone | Dia |
| RACON | Racon |
Bell, gong, and whistle buoys require wave action to produce sound. In flat calm conditions — exactly the conditions where fog is most common — these signals may be completely silent. Never rely solely on a buoy fog signal for navigation in reduced visibility. Use radar, GPS, and depth sounder as primary tools.
On NOAA charts, fog signals are noted next to the aid symbol with an abbreviation: "BELL" or "B" for bell, "GONG" or "G" for gong, "WHISTLE" or "W" for whistle, "HORN" or "Ho" for horn. The Light List entry will have the exact characteristic and period.
Intracoastal Waterway (ICW) Marks
The ICW uses the same red-green lateral system as any other US waterway, but ICW marks have a distinctive yellow marking — either a yellow triangle, yellow square, or yellow border — to distinguish them from marks that belong to a crossing waterway. When a cross-waterway buoy could be confused with an ICW mark (because both share the same body of water), the yellow identifier tells you which system the mark belongs to.
Chart Symbols for ATON (from NOAA Chart 1)
NOAA Chart 1 is the official key to all chart symbols used on US nautical charts. Every practicing mariner should be familiar with the ATON symbols in Chart 1 Section Q. On charts, buoy symbols show position, type, color, and sometimes the light characteristic in a condensed abbreviation immediately adjacent to the symbol.
| Symbol | Meaning |
|---|---|
| Circle with dot | Buoy — generic; the dot marks the exact position |
| Triangle pointing up | Nun buoy (starboard/red) |
| Square / flat top | Can buoy (port/green) |
| Diamond | Safe water mark or special mark buoy |
| Solid circle | Fixed light (lighthouse or light structure) |
| Magenta flash mark | Lighted buoy or light — magenta sector indicates light |
| Double circle | Lightship or lanby (large automated navigation buoy) |
| Upright post symbol | Daybeacon (fixed unlit or lighted structure on pile) |
| Letter + number beside symbol | Light characteristic and period (e.g., Fl R 4s 5ft 4M) |
A label like Fl R 4s 15ft 4M "6" reads as:
- Fl R — Flashing red light
- 4s — Period of 4 seconds (one flash every 4 seconds)
- 15ft — Light is 15 feet above mean high water
- 4M — Nominal range of 4 nautical miles
- "6" — The buoy is numbered 6 (red, even number, starboard side returning)
Light List and Local Notice to Mariners
The USCG publishes seven volumes of the Light List covering all US waters, including the Great Lakes and US territories. The Light List is the official record of every ATON maintained by USCG and is the source to check before any unfamiliar passage. Light Lists are published annually but may be corrected by the Local Notice to Mariners.
| Column | Information |
|---|---|
| No. | District sequential number for the aid |
| Name and Location | Descriptive name and geographic reference |
| Position | Latitude and longitude to nearest second |
| Characteristic | Light characteristic abbreviation and period (e.g., Fl R 4s) |
| Range (nm) | Nominal range in nautical miles at standard meteorological visibility |
| Structure | Color, material, height, and type of structure |
| Remarks | Fog signal, RACON, seasonal dates, radio beacon, or other notes |
Local Notice to Mariners (LNM)
The Local Notice to Mariners (LNM) is published weekly by each USCG district and is the primary tool for correcting nautical charts and Light List entries. The LNM reports: lights that are extinguished or changed, buoys that have dragged off station, new hazards or wrecks, temporary changes for events or construction, and changes to published regulations. Mariners may subscribe to LNM email alerts or download current issues from the USCG Navigation Center website.
- · Discrepancies in aids to navigation
- · New or changed aids
- · Wrecks and obstructions
- · Marine events (regattas, fireworks)
- · Bridge construction and restrictions
- · Regulatory zone changes
The National Notice to Mariners (NtM) is published by NGA (National Geospatial-Intelligence Agency) and covers international and offshore corrections. The Local Notice to Mariners (LNM) covers US coastal and inland waters. For the OUPV exam, LNM is the relevant publication.
Practice Problems with Solutions
Work through each question before expanding the answer. These questions are representative of the type and difficulty found on the USCG OUPV deck general examination.
Q1. You are entering a harbor from the sea. You see a red nun buoy numbered '4' to your left and a green can buoy numbered '3' to your right. Are you on the correct side of the channel?
No — you are on the wrong side. Red right returning means red buoys go on your starboard (right) side when entering (returning from sea). If the red nun '4' is to your left (port), you are in the wrong part of the channel. Steer to put the red nun to your right and the green can to your left. The fact that the red buoy is even-numbered (4) and the green is odd-numbered (3) confirms these are properly numbered lateral marks.
Q2. A buoy has black and red horizontal bands, two black spheres arranged vertically as a topmark, and displays a white group flashing 2 light. What type of buoy is this and what does it mean?
This is an isolated danger mark. The black-over-red bands, two-sphere vertical topmark, and white Fl(2) light are the complete identifier set for isolated danger marks. It means there is a hazard (shoal, rock, wreck) directly beneath or immediately adjacent to this buoy. Navigable water exists all around the hazard, but you must not pass close to the buoy itself. Give it a wide berth on all sides.
Q3. A buoy has red and white vertical stripes, a red spherical topmark, no number but the letter 'A', and its light is white with a short flash followed by a long flash. What type is it?
This is a safe water mark (fairway buoy). The red-and-white vertical stripes (not horizontal bands), red spherical topmark, letter designation with no number, and white Morse A light (short-long = dit-dah) are the complete safe water mark identifier set. You may pass this buoy on either side — safe water exists all around it. It typically marks the entrance to a channel or the offshore approach.
Q4. You see a buoy with a red body, a green band, and a light that flashes: flash, flash — pause — flash — long pause — repeat. What type of buoy is this and which side is the preferred channel on?
This is a preferred channel buoy (junction buoy) with a red top band. The composite group flashing pattern Fl(2+1) — two flashes, pause, one flash, long pause — is the exclusive identifier for preferred channel marks. The top band is red (red body means the top color is red). To follow the preferred channel, keep this buoy on your starboard side — treat it like a red lateral mark. The light will be red Fl(2+1).
Q5. A Light List entry reads: 'Fl G 2.5s 8ft 4M "7"'. What does each element tell you?
Fl G 2.5s = Flashing green light, one flash every 2.5 seconds. 8ft = the light is 8 feet above mean high water (used for range of visibility calculation). 4M = nominal range of 4 nautical miles under standard visibility conditions. '7' = the buoy is numbered 7, which is an odd number, confirming this is a port-hand (green, left side returning from sea) lateral mark. Keep this buoy to your port side when returning from sea.
Q6. You are crossing to a harbor in dense fog. You hear a moaning, low-pitched sound from a buoy ahead. What type of fog signal is this, and can you rely on it being audible in all conditions?
The moaning sound describes a whistle buoy, which produces sound by compressed air being forced through a whistle chamber as the buoy heaves in the swell. You cannot rely on it being audible in all conditions — whistle buoys require wave action to sound. In calm conditions (which frequently accompany dense coastal fog), the buoy may be completely silent. Use radar, GPS, and depth sounder as primary navigation tools in fog. Fog signals are supplementary, not primary.
Q7. On a NOAA chart you see two vertical dashes with a line between them labeled '267° — 087°' in a channel. What does this symbol represent and how do you use it?
This is a range — a pair of structures (either lights or daymarks) aligned on a bearing of 267° (westbound) or 087° (eastbound). When you steer so that the front (lower, nearer) range mark is directly below the rear (higher, farther) range mark — both vertically aligned — you are on the centerline of the channel on that bearing. If the rear mark appears to the right of the front mark, steer right; if it appears to the left, steer left. Follow the range line only within the charted safe channel limits.
Q8. You are southbound on the Atlantic ICW and see a red nun buoy with a yellow triangle. A nearby green can buoy does not have any yellow marking. Which buoy belongs to the ICW system and which side do you keep it on?
The red nun with the yellow triangle is the ICW mark. The yellow triangle (on a red buoy) is the ICW starboard-hand identifier — keep it to your starboard side when heading southbound (the conventional direction on the Atlantic ICW). The green can without a yellow marking belongs to a crossing waterway, not the ICW. ICW marks always carry yellow identifiers precisely to prevent confusion where cross-waterway marks are present in the same body of water.
Frequently Asked Questions
What does red right returning mean in the IALA-B system?
Red right returning is the foundational rule of the US IALA-B buoyage system: when returning from sea toward a harbor or heading upstream on a river, keep red buoys on your starboard (right) side and green buoys on your port (left) side. Red buoys are even-numbered; green buoys are odd-numbered. On the opposite leg — heading seaward or downstream — you reverse the rule and keep red on your port side.
What is the shape of a port-hand lateral mark in the US system?
Port-hand lateral marks (left side when returning from sea) in the IALA-B system are green, can-shaped (flat-topped cylinder), carry odd numbers, and display green lights when lighted. The memory aid is: port = can, green, odd. Starboard marks are red, nun-shaped (conical top), carry even numbers, and display red lights. Memory: starboard = nun, red, even.
What light characteristic identifies a safe water mark?
Safe water marks (fairway buoys) display Morse code A — one short flash followed by one long flash — as their light characteristic, in white. The buoy has red and white vertical stripes, is spherical or has a red spherical topmark, and carries no number (only a letter if it has any marking). Safe water marks indicate navigable water all around; they mark the center of a channel or approach.
How do you identify a preferred channel buoy at a junction?
Preferred channel buoys mark channel junctions or bifurcations. They have red and green horizontal bands. The color of the top band tells you the preferred (main) channel: if the top band is red, keep the buoy on your starboard side to follow the preferred channel (treat it like a red lateral mark). If the top band is green, keep it on your port side. The light is composite group flashing: Fl(2+1) — two quick flashes then one, in the color of the top band.
What distinguishes an isolated danger mark from other buoys?
Isolated danger marks are placed directly over a hazard that has navigable water all around it — a shoal, rock, or wreck surrounded by safe water. They have black and red horizontal bands, display a group flashing 2 (Fl(2)) white light, and carry a topmark of two black spheres arranged vertically. They carry no number; they may have a name or letter. The key exam point: the light is group flashing 2, and the bands are black over red.
What fog signals are found on different types of aids to navigation?
Buoys with fog signals use one of four types: bell buoys have a clapper struck by wave action; gong buoys produce multiple tones from several gongs of different sizes; whistle buoys make a moaning sound from compressed air as the buoy heaves in swells; horn buoys use electric horns on a timed cycle. Lighthouses and major light structures may use diaphones, foghorns, or electronic sound signals on a fixed schedule. On chart symbols, W = whistle, B = bell, G = gong, Ho = horn.
How do you read an entry in the USCG Light List?
Each Light List entry contains: (1) the USCG light number for that district, (2) the name or location of the aid, (3) latitude and longitude, (4) the light characteristic abbreviation (e.g., Fl R 4s = flashing red every 4 seconds), (5) nominal range in nautical miles, (6) structure description (color, height, type), and (7) remarks such as fog signal type, RACON designation, or seasonal discontinuance. The Light List is published by USCG district and is corrected by Local Notice to Mariners (LNM) issued weekly.
Quick Reference — Mark Identifier Card
Use this card to identify any IALA-B mark from a single attribute. On the exam, you may be given only one or two attributes and asked to identify the mark type or the action to take.
| If You See | Mark Type |
|---|---|
| Red nun, even number, red light | Starboard lateral mark |
| Green can, odd number, green light | Port lateral mark |
| Red/white vertical stripes, Morse A white light | Safe water mark |
| Black/red bands, two spheres topmark, Fl(2) white | Isolated danger mark |
| Red/green bands, Fl(2+1) light | Preferred channel mark |
| Yellow buoy, X topmark, yellow light | Special mark |
| Green square dayboard, odd number | Port daybeacon |
| Red triangle dayboard, even number | Starboard daybeacon |
| Two structures aligned, lower and higher | Range |
| Moaning sound in fog | Whistle buoy |
| Multiple tones from a buoy in fog | Gong buoy |
| Single-tone clapper sound from buoy | Bell buoy |
Drill Buoyage Until You Can't Get It Wrong
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