International Nautical Signal Flags Blue

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

• A series of flags can spell out a message, each flag representing a letter.
• Individual flags have specific and standard meanings; for example, diving support vessels raise the “A” (Alfa) flag indicating their inability to move from their current location because they have a diver underwater and to warn other vessels to keep clear to avoid endangering the diver(s) with their propellers.
• One or more flags can form a code word whose meaning differs from the standard code, such as when used by various navies or private organizations. An example is the Popham numeric code used at the Battle of Trafalgar.
  • U.S. Navy and Coast Guard vessels typically fly their maritime call sign or other identifying flags from their port signal halyards, and starboard signal halyards for outgoing messages.
  • NATO uses the same flags and protocols, 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 a signal to indicate it should be read using the international meaning, as seen in the image of USS Blue Ridge above.
• In yacht and dinghy racing, flags have other meanings; for example, the P (Papa) flag is used as the “preparatory” flag to indicate an imminent start, and the S (Sierra) flag means “shortened course” (for more details see Race signals).