For many years preceding the invention of the telegraph, some type of semaphore signaling from high places or towers was used to send messages between distant points. Claude Chappe, a Frenchman, developed one such system in 1794. Chappe employed a set of pivoting arms mounted on towers spaced five to 10 miles apart, with the arms conveying semaphore messages that were read with telescopes. Later, more modern semaphore machines included movable arms or rows of lights that simulated arms, and these were adopted by the railroads.
About the time Chappe was working on his ideas, a comparable development of signaling was going on at sea. Early signaling between vessels was conducted via prearranged messages transmitted by flags, lights, and even the movement of sails. Codes were developed in the sixteenth century based on the number and position of signal flags, lights, and or number of cannon shots fired. In the seventeenth century, British Admiral Sir William Penn and others developed regular codes for naval communication; and toward the close of the eighteenth century, Admiral Kempenfelt developed a method of flag signaling that was very close to what we now use. Sir Home Popham later increased the effectiveness of ship-to-ship communication by improving methods of flag signaling.
The final visual flag signaling code between ships was called semaphore and was accomplished by sailors who held a small flag in each hand, and with their arms extended, moved them to different angles to indicate letters or numbers. The art of semaphore signaling lost its luster with the adoption of a newer technology called Morse Code and has now been almost entirely abandoned. Even the Morse Code SOS, in its turn, officially went out of use on February 1, 1999, for most ships in distress at sea. The International Maritime Organization replaced it with newer satellite technology, the Global Maritime Distress and Safety System that can pinpoint the location of a ship signaling for help.
But still today, flags provide one of the most compelling forms of signaling a distress message to ships and shore alike. Most sailors would recognize other visual distress signals, such as a flame, a red flare, orange smoke, as well as sound signals, such as a gun fired at regular intervals or the continuous sounding of a foghorn. Many might notice a US ensign flown upside down—a universally recognized distress signal. But how many know that the square orange flag with a black square above a black ball is an official and internationally accepted distress flag signal. The orange flag was established as a device to be laid horizontally so that aircraft could easily see the distress signal. In addition, a large orange cloth with a square shape and a round shape could be made large enough so as to be seen from the air. This signal could be placed on a beach, or even towed behind a boat. We can imagine how many sailors might miss the International Code of Signals two-flag code NC (November-Charlie) meaning, "I am in distress and require immediate assistance."
In fact international code flags are still used to signal between two ships or between ship and shore. But unless you look carefully at commercial shipping, you may never see them displayed except at fleet parades and around naval installations. These flags usually come in a set of 40 with distinctly different colors, shapes, and marking patterns that can rarely be confused even in conditions of low light or poor visibility. The flags include 26 square or swallowtail flags that depict the letters of the alphabet, 10 numeral pendants, one answering pendant, and three substitutes or repeaters. Each is specifically designed so that it can be identified either by color or pattern. The International Code of Signals specifies a meaning for every one-flag and two-flag hoist.
Signals using just one flag carry urgent or very common messages. Most cruising sailors would recognize the all-yellow Q (Quebec) flag flying from the spreaders in a foreign country to mean, "I am requesting free practique." This common one-flag hoist today means that the vessel has just entered port and needs medical, agricultural, customs, or immigration clearance before going ashore.
There are a number of other one-flag hoists with vitally important meanings. While recreational boaters and divers use the poppy-red flag with one diagonal white stripe to indicate that a diver is in the water, the official International Code of Signals message for "diver down" is the blue-and-white swallowtail A (Alpha). Another code flag that is commonly used is the all-red swallowtail B (Bravo), which means "I am taking on or discharging explosives or hazardous cargo." The diagonally divided red-and-yellow O flag (Oscar) is internationally recognized as "Man overboard," while the red-white-and-blue vertical tricolor T (Tango) flag means "Do not pass ahead of me."
In fact, every one of the 26 letter flags in the set has a meaning, and usually it is an urgent message. In some cases the signal relates to some activity on the ship flying the flag, such as the red-white-and-blue rectangles of the W (Whiskey) flag, which means, "I require medical assistance." But sometimes the message is for you, like the red-and-white rectangles on the "U" (Uniform) flag indicating that "You are standing into danger."
Of course it isn't possible (or necessary) to memorize all the flag hoist messages unless you are the signalman on a commercial ship. But every sailor should know the basics to keep him or herself out of danger, and they should perhaps carry a book with the message explanations for ready reference in case they come across one with which they aren't familiar. There are different interpretations of these flag meanings. SailNet has elected to use The International Code of Signals, US Edition, 1969 Edition, (Revised 1999).
Ever wonder what all that alpha-numeric babble is about? Here's a quick rundown on the international code flags with their meanings and phonetic names.
Two Flag Signals
AC - I am abandoning my vessel.
LO - I am not in my correct position; used by a light vessel.
RU - Keep clear of me; I am maneuvering with difficulty.
AN - I need a doctor.
NC - I am in distress and require immediate assistance.
SO - You should stop your vessel instantly.
BR - I require a helicopter.
PD - Your navigation lights are not visible.
UM - The harbor is closed to traffic.
CD - I require assistance in the nature of ...
PP - Keep well clear of me.
UP - Permission to enter harbor is urgently requested. I have an emergency.
DV - I am drifting.
QD - I am going ahead.
YU - I am going to communicate with your station by means of the International Code of Signals.
EF - SOS/MAYDAY has been canceled.
QT - You should not anchor. You are going to foul my anchor.
FA - Will you give me my position?
QQ - I require health clearance.
GW - Man overboard. Please take action to pick him up.
QU - Anchoring is prohibited.
ZL - Your signal has been received but not understood.
JL - You are running the risk of going aground.
QX - I request permission to anchor.
Three Flag Signals
ZD1 - Please report me to the Coast Guard, New York
ZD2 - Please report me to Lloyds, London.