International maritime signal flags

The set of signal flags on the bridge of the Liberty ship SS Jeremiah O'Brien

The system of international maritime signal flags is one system of flag signals representing individual letters of the alphabet in signals to or from ships. It is a component of the International Code of Signals (INTERCO).

Overview

There are various methods by which the flags can be used as signals:

• each flag spells an alphabetic message, letter by letter.
• individual flags have specific and standard meanings; for example, diving support vessels raise the "A flag" indicating their inability to move from their current location because they have a diver underwater.
• one or more flags form a code word whose meaning can be looked up in a code book held by both parties. An example is the Popham numeric code used at the Battle of Trafalgar.
• in yacht racing and dinghy racing, flags have other meanings; for example, the P flag is used as the "preparatory" flag to indicate an imminent start, and the S flag means "shortened course" (for more details see Race Signals).

NATO uses the same flags, with a few unique to warships, alone or in short sets to communicate various unclassified messages. The NATO usage generally differs from the International meanings, and therefore warships will fly the Code/Answer flag above the signal to indicate it should be read using the International meaning.

During the allied occupations of Axis countries after World War II, use and display of those nations' national flags were banned. In order to comply with the international legal requirement that a ship identify its registry by displaying the appropriate national ensign, swallow-tailed versions of the C, D, and E signal flags were designated as, respectively, provisional German, Okinawan, and Japanese civil ensigns. Being swallowtails, they are commonly referred to as the "C-Pennant" (C-Doppelstander), "D-Pennant", and "E-Pennant".

Letter flags (with ICS meaning)

• 

  A (Alfa)

  "I have a diver down; keep well clear at slow speed."
  With three numerals, azimuth or bearing.
• 

  B (Bravo)

  "I am taking in, or discharging, or carrying dangerous goods." (Originally used by the Royal Navy specifically for military explosives.)
• 

  C (Charlie)

  "Affirmative." * **
  With three numerals, course in degrees magnetic.
• 

  D (Delta)

  "Keep clear of me; I am maneuvering with difficulty."
  With two, four, or six numerals, date.
• 

  E (Echo)

  "I am altering my course to starboard."** Crew at meals
• 

  F (Foxtrot)

  "I am disabled; communicate with me." (When flown from an aircraft carrier; "Warning; flight operations underway.")
• 

  G (Golf)

  "I require a pilot."
  When made by fishing vessels operating in proximity of the fishing grounds it means: "I am hauling nets."
  With four or five numerals, longitude. (The last two numerals denote minutes and the rest degrees.)
• 

  H (Hotel)

  "I have a pilot on board."
• 

  I (India)

  "I am altering my course to port."**
• 

  J (Juliet)

  "I am on fire and have dangerous cargo on board: keep well clear of me, or I am leaking dangerous cargo."
• 

  K (Kilo)

  "I wish to communicate with you." With one numeral, "I wish to communicate with you by..."; 1) Morse signalling by hand-flags or arms; 2) Loud hailer (megaphone); 3) Morse signalling lamp; 4) Sound signals.
• 

  L (Lima)

  In harbour: "The ship is quarantined."
  At sea: "You should stop your vessel instantly."
  With four numerals, latitude. (The first two denote degrees and the rest minutes.)
• 

  M (Mike)

  "My vessel is stopped and making no way through the water."**
• 

  N (November)

  "Negative."*
• 

  O (Oscar)

  "Man overboard."** (often attached to the man overboard pole on boats).
  With a sinister hoist, the semaphore flag.
• 

  P (Papa)

  The Blue Peter.
  In harbour: All persons should report on board as the vessel is about to proceed to sea.
  At sea: It may be used by fishing vessels to mean: "My nets have come fast upon an obstruction."
• 

  Q (Quebec)

  "My vessel is 'healthy' and I request free pratique."
• 

  R (Romeo)

  "The way is off my ship."
  With one or more numerals, distance (range) in nautical miles.
• 

  S (Sierra)

  "I am operating astern propulsion." **
  With one or more numerals, speed in knots.
• 

  T (Tango)

  "Keep clear of me; I am engaged in pair trawling."
  With four numerals, local time. (The first two denote hours and the rest minutes.)
• 

  U (Uniform)

  "You are running into danger."
• 

  V (Victor)

  "I require assistance."
  With one or more numerals, speed in kilometres per hour.
• 

  W (Whiskey)

  "I require medical assistance."
• 

  X (Xray)

  "Stop carrying out your intentions and watch for my signals."
• 

  Y (Yankee)

  "I am dragging my anchor."
• 

  Z (Zulu)

  "I require a tug."
  When made by fishing vessels operating in close proximity on the fishing grounds it means: "I am shooting nets."
  With one or more numerals, time (UTC). (The first two denote hours and the rest minutes.)

* ^  N and C together (No and Yes) is used as a distress signal.

** Also signallable on a ship's whistle using Morse code. See International Code of Signals.

Substitute

Substitute or repeater flags allow messages with duplicate characters to be signaled without the need for multiple sets of flags.

The four NATO substitute flags are as follows:

First substitute	Second substitute	Third substitute	Fourth substitute

To illustrate their use, here are some messages and the way they would be encoded:

"N"
"O"
"NO"
"NON"
"NOO"
"NOON"
"NONO"
"NONON"
"NONNN"

See also

• Character encoding
• Day shapes
• Diver down flag
• England expects that every man will do his duty
• Flag of Germany after World War II (C-Pennant)
• Flag semaphore
• Heliograph
• International Code of Signals
• List of international common standards
• NATO phonetic alphabet
• Signal lamp

External links

• "How Ships Talk With Flags", October 1944, Popular Science
• John Savard's flag page. Collection of different flag systems.
• Freeware to aid memorizing the flags
• La flag-alfabeto - signal flags used for the Esperanto language - the flags for the Esperanto letters with diacritical marks have the lighter color in the normal flag replaced with light green, which is not used in any normal flag.