IMO signal flags

The International Code of Signals (ICS) is the IMO standard for visual, sound, and radio signalling between vessels and between vessels and shore. Its core component — the alphabet flags A to Z — replaced an earlier proliferation of national flag codes when the first international Code came into force in 1857; the present version is the 1969 revision, published by the IMO and updated by Maritime Safety Committee resolutions.

Flown singly, each flag has a defined standalone meaning (the table below). Flown in two-letter groups they form coded messages from the four-volume Code (e.g. NC = distress, YU = I am going to communicate by ICS). Flown in three-letter groups they reference vessel-, geographic-, or medical-signal-section codes. Numerical pennants (0–9) and three substitutes (used to repeat a letter without a second physical flag) complete the set.

Alphabet flags — standalone meanings

AAlpha
I have a diver down; keep well clear at slow speed.
BBravo
I am taking in, discharging, or carrying dangerous goods.
CCharlie
Affirmative — yes.
DDelta
Keep clear — I am manoeuvring with difficulty.
EEcho
I am altering my course to starboard.
FFoxtrot
I am disabled; communicate with me.
GGolf
I require a pilot. By fishing vessel: I am hauling nets.
HHotel
I have a pilot on board.
IIndia
I am altering my course to port.
JJuliet
I am on fire and have dangerous cargo — keep well clear.
KKilo
I wish to communicate with you.
LLima
You should stop your vessel instantly.
MMike
My vessel is stopped and making no way through the water.
NNovember
Negative — no.
OOscar
Man overboard.
PPapa
In harbour: vessel is about to proceed to sea. At sea: your lights are out or burning badly.
QQuebec
My vessel is healthy and I request free pratique.
RRomeo
(No standalone meaning in the 1969 Code revision.)
SSierra
My engines are going astern.
TTango
Keep clear of me — I am engaged in pair trawling.
UUniform
You are running into danger.
VVictor
I require assistance.
WWhiskey
I require medical assistance.
XX-ray
Stop carrying out your intentions and watch for my signals.
YYankee
I am dragging my anchor.
ZZulu
I require a tug. By fishing vessel: I am shooting nets.

Common two-letter groups

NC — I am in distress and require immediate assistance. (One of the recognised distress signals under COLREG Annex IV.)
AC — I am abandoning my vessel.
AE — I must abandon my vessel.
CB — I require immediate assistance.
JL — You are running the risk of going aground.
PP — Keep well clear of me.
RU — Keep clear of me; I am manoeuvring with difficulty.
UM — Harbour is closed to traffic.
YU — I am going to communicate with your station by means of the International Code of Signals.

When and where flags are flown

· Q (Quebec) — flown on entering port from a foreign voyage requesting free pratique (clearance to disembark / load); lowered when granted.
· H (Hotel) — flown when a pilot is aboard.
· G (Golf) — flown when a pilot is required.
· P (Papa) — "Blue Peter"; flown in harbour to indicate "all persons report on board, vessel is about to proceed to sea".
· B (Bravo) — flown when handling, taking in, or discharging dangerous goods (especially during bunkering).
· A (Alpha) — flown when a diver is in the water.
· L (Lima) — quarantine signal in some jurisdictions; flown when stop is required.

Numerical pennants & substitutes

Ten numerical pennants (0–9) and three substitute pennants. The substitute repeats whatever flag was flown in the corresponding position of the group above it — needed because a vessel typically carries one set of each flag. Without substitutes, signalling "LL" (or any letter twice) would be impossible.

Colours rendered per ICS standard: white, black, red, blue, yellow. Flags are geometric approximations for identification; for regulatory use always consult the official IMO publication of the International Code of Signals.