International maritime signal flags are various flags used to communicate with ships. The principal system of flags and associated codes is the International Code of Signals. Various navies have flag systems with additional flags and codes, and other flags are used in special uses, or have historical significance.
There are various methods by which the flags can be used as 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 was 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" (German: C-Doppelstander), "D-pennant", and "E-pennant".
|Flag||ICS meaning as single flag||Meaning when used with numeric complements|
|"I have a diver down; keep well clear at slow speed."||Azimuth or bearing|
|"I am taking in or discharging or carrying dangerous goods." (Originally used by the Royal Navy specifically for military explosives.)|
|"Affirmative."[a][b]||Course in degrees magnetic|
|"Keep clear of me; I am maneuvering with difficulty."[b]||Date|
|"I am altering my course to starboard."[b]|
|"I am disabled; communicate with me."[c]|
|"I require a pilot."By fishing vessels near fishing grounds: "I am hauling nets."||Longitude (The first 2 or 3 digits denote degrees; the last 2 denote minutes.)|
|"I have a pilot on board."[b]|
|"I am altering my course to port."[b]|
|"I am on fire and have dangerous cargo on board: keep well clear of me."or"I am leaking dangerous cargo."|
|"I wish to communicate with you."||"I wish to communicate with you by...":1) Morse signaling by hand-flags or arms;2) Loud hailer (megaphone);3) Morse signaling lamp;4) Sound signals.|
|In harbour: "The ship is quarantined."At sea: "You should stop your vessel instantly."||Latitude (the first 2 digits denote degrees; the last 2 denote minutes.)|
|"My vessel is stopped and making no way through the water."[b]|
|"Man overboard."[b] (often attached to the man overboard pole on boats).With a sinister hoist, the semaphore flag.|
|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."|
|"My vessel is 'healthy' and I request free pratique."|
|No ICS meaning as single flag.||Distance (range) in nautical miles.|
|"I am operating astern propulsion."[b]||Speed (velocity) in knots|
|"Keep clear of me."[b]Fishing boats: "Keep clear of me; I am engaged in pair trawling."||Local time. (The first 2 digits denote hours; the last 2 denote minutes.)|
|"You are running into danger."|
|"I require assistance."||Speed in kilometres per hour.|
|"I require medical assistance."|
|"Stop carrying out your intentions and watch for my signals."|
|"I am dragging my anchor."|
|"I require a tug."By fishing vessels near fishing grounds: "I am shooting nets."[d]||Time (UTC). (The first 2 digits denote hours; the last 2 denote minutes.)|
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:
The International Code of Signals includes only the first three of these substitute flags. To illustrate their use, here are some messages and the way they would be encoded: