International Maritime Signal Flags

At sea, communication is not always as simple as speaking over a radio. Noise, distance, equipment failure and language barriers can make it difficult for vessels to understand one another.

Maritime signal flags provide a standardised visual language. Each flag in the International Code of Signals has an assigned meaning, allowing vessels to communicate important information without relying entirely on spoken language.

Some flags convey a complete message on their own. Others are combined with additional flags to form more specific signals.

This guide explains how the system works, introduces the most useful flags to recognise and shows how signal flags fit into modern boating safety.

1. What is the International Code of Signals?

The International Code of Signals is an internationally recognised system for communication between ships and, where applicable, between ships and shore stations.

It was developed to help mariners communicate clearly, particularly when language differences or other difficulties make ordinary communication unreliable. The system includes flags, radio procedures, visual signals and other signalling methods.

The flag alphabet consists of 26 letter flags, along with numeral pennants and other signal pennants. A flag can have a specific meaning when displayed alone, while groups of flags can communicate more detailed messages.

For example:

• Alpha (A): A diver is down; keep well clear and proceed slowly.
• Bravo (B): The vessel is taking in, discharging or carrying dangerous goods.
• Oscar (O): Man overboard.

These meanings are part of the international code rather than informal interpretations of the flags’ colours or shapes.

2. The most important maritime signal flags

The following flags are useful for recreational boaters to recognise. Their single-flag meanings come from the International Code of Signals.

Flag	Name	Meaning when displayed singly
A	Alpha	I have a diver down; keep well clear at slow speed.
B	Bravo	I am taking in, discharging or carrying dangerous goods.
C	Charlie	Yes, affirmative.
D	Delta	Keep clear of me; I am manoeuvring with difficulty.
E	Echo	I am altering my course to starboard.
F	Foxtrot	I am disabled; communicate with me.
G	Golf	I require a pilot.
H	Hotel	I have a pilot on board.
I	India	I am altering my course to port.
J	Juliett	I am on fire and have dangerous cargo on board, or I am leaking dangerous cargo.
K	Kilo	I wish to communicate with you.
L	Lima	You should stop your vessel instantly.
M	Mike	My vessel is stopped and making no way through the water.
N	November	No, negative.
O	Oscar	Man overboard.
P	Papa	In harbour: all persons should report on board, as the vessel is about to proceed to sea. At sea, it may have a different prescribed meaning in the relevant context.
Q	Quebec	My vessel is healthy and I request free pratique.
R	Romeo	Has no general standalone operational meaning in the same way as the flags above; consult the Code for its use in combinations and prescribed contexts.
S	Sierra	I am operating astern propulsion.
T	Tango	Keep clear of me; I am engaged in pair trawling.
U	Uniform	You are running into danger.
V	Victor	I require assistance.
W	Whiskey	I require medical assistance.
X	X-ray	Stop carrying out your intentions and watch for my signals.
Y	Yankee	I am dragging my anchor.
Z	Zulu	I require a tug.

Important: Some signals have additional or context-specific meanings, and the use of a flag may depend on the signalling method and circumstances. The table is a learning aid, not a replacement for the complete International Code of Signals. When a message has safety implications, consult the official code and use the appropriate radio or emergency procedure as well.

3. How to recognise signal flags

Signal flags use distinct combinations of colours, stripes, crosses and geometric patterns. Learning to recognise the most common flags can help you interpret messages at a glance.

Here are several particularly useful examples.

Alpha: diver down

The Alpha flag is white and blue, divided vertically.

Its meaning is: “I have a diver down; keep well clear at slow speed.”

If you see this flag displayed by a vessel or at a diving location, proceed cautiously and maintain a safe distance. Divers may be underwater and difficult or impossible to see from the surface.

Do not assume the absence of an Alpha flag guarantees that no divers are present. Follow local diving rules and pay attention to other warnings.

Bravo: dangerous goods

The Bravo flag is red.

It indicates that the vessel is taking in, discharging or carrying dangerous goods, according to the International Code of Signals.

The flag is especially relevant around commercial vessels, ports and cargo operations. Do not interpret it simply as a generic warning that a vessel is dangerous; its prescribed meaning relates to dangerous goods.

Oscar: man overboard

The Oscar flag is divided diagonally into red and yellow sections.

It means “Man overboard.”

Recognising this flag is useful, but a person-overboard emergency requires immediate action. If you witness someone falling into the water, raise the alarm, maintain visual contact with the person, alert the skipper and follow the vessel’s man-overboard procedure.

For a practical response guide, read Man Overboard: What to Do.

Hotel: pilot on board

The Hotel flag is divided vertically into white and red sections.

It means “I have a pilot on board.”

It can be seen on vessels carrying a maritime pilot, particularly in commercial shipping and port approaches.

Quebec: request for free pratique

The Quebec flag is yellow.

Under the International Code of Signals, it indicates that a vessel is healthy and requests free pratique, which is permission from the relevant authority to enter port or begin certain activities after applicable health formalities.

Do not assume that displaying the flag alone completes all required port, customs, immigration or health procedures.

4. Single flags versus combinations of flags

A key feature of the International Code of Signals is that a flag’s meaning depends on how it is used.

Single-flag signals

Some flags convey a complete message when displayed individually.

Examples include:

• A — Alpha: diver down; keep well clear at slow speed.
• B — Bravo: taking in, discharging or carrying dangerous goods.
• O — Oscar: man overboard.
• W — Whiskey: medical assistance required.

These are among the meanings worth learning first because they may communicate important safety information.

Groups of flags

Two or more flags can be combined to convey more specific messages, including messages concerning navigation, medical situations and other operational needs.

The meaning of a combination must be read using the official code. It should not be guessed by combining the standalone meanings of the individual flags.

The Code includes prescribed procedures for hoisting and interpreting groups, including how they are separated and acknowledged. If you need to send or interpret a complex signal, consult the official publication rather than relying on a short reference table.

5. Signal flags and the phonetic alphabet

The international flag alphabet is closely associated with the standard phonetic alphabet used in radio communications.

For example:

Letter	Phonetic name
A	Alfa
B	Bravo
C	Charlie
D	Delta
E	Echo
F	Foxtrot
G	Golf
H	Hotel
I	India
J	Juliett
K	Kilo
L	Lima
M	Mike
N	November
O	Oscar
P	Papa
Q	Quebec
R	Romeo
S	Sierra
T	Tango
U	Uniform
V	Victor
W	Whiskey
X	X-ray
Y	Yankee
Z	Zulu

The phonetic alphabet helps radio operators distinguish letters that can sound similar, especially when reception is poor.

For example, spelling a vessel’s name or call sign using “Alfa, Bravo, Charlie” is generally clearer than pronouncing the letters individually.

The phonetic alphabet and the International Code of Signals are related, but they are not interchangeable. Saying “Oscar” over the radio to spell a name does not automatically mean that you are transmitting the standalone Oscar flag signal.

For radio procedures, see VHF Marine Radio: Channels and Procedures.

6. When are maritime signal flags used?

Signal flags remain useful even though VHF radios, satellite communications and electronic navigation systems are widespread.

Common uses include:

Safety warnings. A flag can communicate a recognised hazard or a vessel’s operational status to others nearby.

Diving operations. The Alpha flag warns other vessels to keep clear and proceed slowly.

Commercial and port operations. Flags can communicate information about cargo, pilotage and vessel movements.

Communication without a shared language. Standardised signals can convey important information even when crews do not speak the same language.

Backup communication. Visual signalling can provide an additional way to communicate when radio contact is difficult or unavailable.

Signal flags are not a substitute for all other communication methods. Their visibility depends on distance, lighting, weather, orientation and whether another vessel is looking in the right direction.

7. How signal flags differ from other nautical flags

Not every flag flown on a boat is an International Code of Signals message.

National flags

A national flag identifies a vessel’s nationality or is displayed according to applicable flag-state and maritime requirements.

Courtesy flags

A courtesy flag is commonly flown to show respect for the country whose waters a vessel is visiting. The rules and conventions governing its use depend on local practice.

Club burgees

A burgee identifies a yacht club or sailing organisation. It is usually a membership or affiliation symbol rather than an operational signal.

Decorative flags

Decorative nautical flags may be used for appearance, celebrations or personal expression. Unless they are displayed and interpreted according to the applicable signalling system, they should not be assumed to convey an official message.

The key distinction: An International Code of Signals flag has a standardised meaning when used according to the Code. A national flag, courtesy flag or club burgee serves a different purpose.

8. What to do when you see a signal flag

If you encounter a signal flag while underway, follow a simple process.

1. Identify the flag. Look for its colours and pattern, and establish whether it is likely to be an official signal flag.
2. Determine the context. Consider the vessel’s activity, location and surrounding conditions.
3. Consult the official code if needed. Do not guess the meaning of an unfamiliar flag or combination.
4. Respond appropriately. If the signal indicates a hazard or safety issue, take the precautions required by the circumstances and applicable regulations.
5. Use other communication methods when necessary. A VHF call or contact with the relevant authority may be appropriate, particularly when a safety message needs clarification.

For example, seeing an Alpha flag should prompt you to keep clear and proceed slowly. Seeing an Oscar flag should be treated as a serious indication of a man-overboard situation, although the appropriate response will depend on what you observe and whether assistance is required.

Never let the process of identifying a flag distract you from maintaining a proper lookout or navigating safely.

9. A practical signal-flag learning checklist

If you want to become more familiar with maritime flags, start with the signals most likely to be relevant to recreational boating.

• Learn to recognise the Alpha flag and its diver-down warning.
• Learn the Bravo flag and its relationship to dangerous goods.
• Memorise Oscar for man overboard.
• Recognise Whiskey for medical assistance.
• Learn the phonetic alphabet for clear radio communication.
• Understand the difference between a standalone flag and a group of flags.
• Know where to find the official International Code of Signals.
• Remember that signal flags do not replace radio distress procedures or required safety equipment.

Keep an official reference available if your activities require you to interpret more complex signals.

10. Record significant events in your boat logbook

A logbook helps you preserve the context of a boating trip, including unusual situations or safety-related events.

If a signal flag plays a significant role during a passage, consider recording:

• The date, time and position.
• The vessel or location displaying the signal, if known.
• The flag or combination observed.
• The surrounding conditions and visibility.
• Any communication or action that followed.
• Any safety issue or lesson worth reviewing later.

Record what you actually observed, and distinguish it from your interpretation if the meaning was uncertain. For a significant incident, follow the applicable reporting and documentation requirements.

With Skipper, you can document your trips using GPS tracks, locations, notes and photos, keeping useful details together for later review.

Conclusion

International maritime signal flags provide a standardised way for vessels to communicate important information visually. Learning a handful of common flags, especially Alpha, Bravo, Oscar and Whiskey, gives recreational boaters a useful starting point.

The most important principle is to interpret signals in their proper context. Standalone flags have prescribed meanings, combinations must be read using the official code, and national or decorative flags should not be confused with operational signals.

Use the International Code of Signals as your reference, maintain a proper lookout and use the appropriate communication and emergency procedures whenever safety is at stake.

Frequently asked questions

What are maritime signal flags used for?

Maritime signal flags communicate messages between vessels and shore stations. Under the International Code of Signals, individual flags and combinations can convey information about safety, navigation, vessel status and other operational matters.

What does the Alpha flag mean?

The Alpha flag means: “I have a diver down; keep well clear at slow speed.” It warns other vessels to stay clear and proceed slowly.

What does the Bravo flag mean?

The Bravo flag means that a vessel is taking in, discharging or carrying dangerous goods, as defined by the International Code of Signals.

What does the Oscar flag mean?

The Oscar flag means “Man overboard.” It signals a person-overboard situation.

Are maritime signal flags the same as sailing club or national flags?

No. International signal flags have standardised meanings under the International Code of Signals. National flags, courtesy flags, club burgees and decorative nautical flags serve different purposes.

Do signal flags replace a VHF radio in an emergency?

No. Signal flags can provide important visual information, but they do not replace appropriate distress procedures, a marine VHF radio or other required safety equipment.