Aids to Navigation — IALA-B (US) Buoyage

TL;DR — In the US (IALA-B) lateral system, red marks are kept to starboard when returning from sea ("red right returning"), carry even numbers, and include nun buoys; green marks are kept to port, carry odd numbers. This rule reverses in IALA-A regions outside the Americas.

---

What the Rule Says

The United States follows the IALA-B buoyage system, which governs the color, shape, numbering, and light characteristics of lateral aids to navigation. Bowditch Ch. 5 §501

Color Assignment — IALA-B

Red is assigned to port-hand lateral aids in IALA-B. Light List — light-characteristic (R) These are the marks a mariner keeps to port when proceeding outbound (toward sea), or equivalently, keeps to starboard when returning from sea. Bowditch Ch. 5 §501

Green is assigned to starboard-hand lateral aids in IALA-B. Light List — light-characteristic (G) These are kept to starboard when outbound, or to port when returning from sea. Bowditch Ch. 5 §501

Numbering Convention

• Even numbers appear on red (port-hand) aids. Bowditch Ch. 5 §501
• Odd numbers appear on green (starboard-hand) aids. Bowditch Ch. 5 §501
• Numbers increase as you proceed from seaward toward the head of navigation (i.e., upriver or into port).

Buoy Shape — The Nun

The nun buoy is a conical, pointed-top buoy. It is red, carries even numbers, and is maintained on the starboard side of the channel when returning from sea. Light List — Nun Buoy The pointed top is the distinguishing physical characteristic that differentiates a nun from a can buoy (flat-top, green, odd-numbered, port side).

Light Colors

When a lateral aid is lighted:

• A red light confirms a port-hand mark (red, even-numbered) in IALA-B. Light List — light-characteristic (R)
• A green light confirms a starboard-hand mark (green, odd-numbered) in IALA-B. Light List — light-characteristic (G)

Light characteristics (flashing, occulting, quick, etc.) further distinguish individual aids, but color alone is sufficient to determine which side of the channel the aid marks.

The IALA-A Contrast — Critical for Near-Coastal Candidates

Outside the Americas — including Europe, Africa, Australia, and most of Asia — the IALA-A system applies. In IALA-A:

• Port-hand marks are RED, the same color as IALA-B port-hand marks, but the lateral meaning is reversed relative to the channel. Light List — IALA-A Port-Hand Mark
• Starboard-hand marks are GREEN, the same color as IALA-B starboard-hand marks, but again reversed in channel position. Light List — IALA-A Starboard-Hand Mark

The practical consequence: a mariner trained on "red right returning" who enters an IALA-A port without recognizing the system change will navigate on the wrong side of every lateral mark. The IALA region is identified in the chart border; always verify before relying on lateral marks in foreign waters. Light List — IALA-A Starboard-Hand Mark

---

Why It Matters on the Exam

OUPV and Master 100 GT written exams test IALA-B buoyage heavily and from multiple angles. Expect questions that:

1. Test color-to-side association directly. "A red buoy with an even number should be kept on which side of your vessel when entering port?" The answer is starboard. Bowditch Ch. 5 §501 Light List — light-characteristic (R)

2. Test shape identification. "You observe a conical, pointed-top buoy painted red. What type of buoy is this, and on which side do you pass it when returning from sea?" Answer: nun buoy, keep to starboard. Light List — Nun Buoy

3. Test the numbering rule. "Which of the following buoy numbers would you expect to find on a green buoy?" Answer: an odd number. Bowditch Ch. 5 §501

4. Test light color. "A lighted buoy displays a green flashing light. In the US system, this buoy marks which side of the channel?" Answer: the starboard side of the channel (keep it to port when returning from sea). Light List — light-characteristic (G)

5. Test IALA-A vs. IALA-B awareness. "You are transiting a European port. A red lateral buoy is to your starboard. Are you in the correct channel position?" In IALA-A, a red mark is a port-hand mark, meaning it should be to your port when entering — so no, you are on the wrong side. Light List — IALA-A Port-Hand Mark

6. Test the mnemonic under pressure. The phrase "red right returning" encapsulates the IALA-B rule: red aids are kept to the right (starboard) when returning (proceeding from sea toward port or upriver). Bowditch Ch. 5 §501 Examiners sometimes rephrase the scenario as "proceeding upstream" or "heading toward the harbor" to confirm the candidate understands "returning from sea" means any direction away from open water and toward the head of navigation.

---

Common Pitfalls

Pitfall 1 — Confusing "returning" with compass direction

"Returning from sea" is not a compass bearing. It is a functional direction — toward the head of navigation, upriver, or into port. A vessel heading due west may be "returning from sea" if the port lies to the west. Always orient yourself relative to the direction of increasing buoy numbers, not a compass heading.

Pitfall 2 — Mixing up nun and can buoy shapes

Candidates sometimes associate the pointed top with green (perhaps thinking of a Christmas tree). The nun is red and pointed; the can is green and flat-topped. The shape of the nun buoy — conical, pointed top, red — is a tested fact. Light List — Nun Buoy

Pitfall 3 — Assuming IALA-B applies worldwide

The IALA-A system is used outside the Americas. In IALA-A, starboard-hand marks are green Light List — IALA-A Starboard-Hand Mark and port-hand marks are red Light List — IALA-A Port-Hand Mark — the same colors as IALA-B — but the lateral assignment is reversed. A candidate who memorizes only "red = keep to starboard" without understanding the IALA-A reversal will answer international-scenario questions incorrectly.

Pitfall 4 — Confusing light color with light characteristic

Light color (red or green) tells you which side of the channel the aid marks. Light characteristic (flashing, occulting, isophase, quick, etc.) identifies the specific aid on the chart. Exam questions may describe both; do not let an unusual characteristic distract you from the color-to-side rule. Light List — light-characteristic (R) Light List — light-characteristic (G)

Pitfall 5 — Even/odd number reversal

Even numbers go with red (port-hand) aids; odd numbers go with green (starboard-hand) aids. Bowditch Ch. 5 §501 Under exam pressure, candidates sometimes reverse this. A reliable cross-check: if the buoy is red and the number is odd, something is wrong with your recall — red is always even in IALA-B.

Pitfall 6 — Forgetting to verify IALA region on the chart

The Light List and Bowditch both emphasize checking the chart border to confirm the IALA region before interpreting lateral marks in foreign waters. Light List — IALA-A Starboard-Hand Mark This is not merely academic; it is a practical seamanship requirement for any Near-Coastal voyage.

---

Quick Check

Q1 — You are entering a US harbor. A red, conical buoy with the number "6" is 200 yards ahead. On which side do you pass it?

Starboard side. In IALA-B, red aids are kept to starboard when returning from sea ("red right returning"). The conical shape identifies it as a nun buoy. The even number "6" confirms it is a red, port-hand aid. Light List — Nun Buoy Bowditch Ch. 5 §501

Q2 — A lighted buoy displays a green flashing light. In the US system, what does this tell you about the buoy's lateral significance?

It is a starboard-hand mark — it marks the starboard side of the channel when returning from sea, meaning you keep it to port when inbound. Green lights are used on starboard-hand lateral aids in IALA-B. Light List — light-characteristic (G)

Q3 — Which buoy numbers would you expect to find on green aids in the US lateral system?

Odd numbers. In IALA-B, even numbers appear on red (port-hand) aids and odd numbers appear on green (starboard-hand) aids. Bowditch Ch. 5 §501

Q4 — You are transiting a channel in northern Europe. A red lateral buoy is to your port side. Are you correctly positioned in the channel?

Yes. In IALA-A (used outside the Americas), port-hand marks are red — the same as IALA-B — but the lateral assignment is reversed relative to the channel. In IALA-A, a red mark is a port-hand mark and should be kept to port when entering. Keeping it to port is correct. Light List — IALA-A Port-Hand Mark

Q5 — What physical shape distinguishes a nun buoy, and what color is it in the US system?

A nun buoy has a conical, pointed top and is red. It carries even numbers and is kept to starboard when returning from sea in IALA-B. Light List — Nun Buoy

Q6 — In IALA-A, what color are starboard-hand marks, and how does their channel position differ from IALA-B?

In IALA-A, starboard-hand marks are green — the same color as in IALA-B — but their position in the channel is reversed. In IALA-B, green marks are kept to port when returning from sea; in IALA-A, green marks are kept to starboard when entering. Always verify the IALA region in the chart border before interpreting lateral marks in foreign waters. Light List — IALA-A Starboard-Hand Mark