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.

See also

• · Morse code — for radio and Aldis-lamp signalling.
• · Sound signals — COLREG Rule 32-37 acoustic signalling.
• · Distress procedures — Annex IV signals including NC.
• · Navigation lights — visual companion at night.