Signaling at Sea

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From at least the time of the ancient Athenians, admirals have hoisted various flags as signals to other members of the fleet to take various actions.  For most of that time up until the mid-1700s, however, the number of possible signals were few and inflexible.  Originally, the flags were intended to convey a single message to all the ships of the fleet, often "come aboard the flagship for a conference" where the admiral could convey more detailed tactical directions.  The Anglo-Dutch naval wars of the 17th century provided considerable impetus on both sides for more and more elaborate signals, culminating in the Royal Navy's Permanent Fighting Instructions.  The edition issued in 1691, which remained basically intact for the next century, provided 45 fixed messages using a combination of eleven flags that could be displayed at nine different points on the flagship.  For example, a red flag hoisted on the backstay of the fore-topmast directed the vice admiral of the red squadron to tack and try to get to windward of the enemy.  The same flag on a flagstaff atop the mizzen-topmast directed the same maneuver of the rear admiral of the red squadron, but at the cap of the mizzen topmast ordered the ships on the starboard quarter to come to the starboard tack, or, at the mizzen peak (gaff), all the flagships of the fleet to come into the admiral's wake.  Although the Fighting Instructions did permit the admiral to issue commands to specific portions of his fleet, it was still a one-way communication that provided for only a limited repertory of maneuvers.  Moreover, as tactics came to require more and more signals--three were added in 1740, for example--the old system became increasingly unwieldy.

In 1738, a French officer named Mahé de la Bourdonnais devised the first numerical flag code, the basis on which all later development of flaghoist signaling was based.  Bourdonnais assigned a different flag to each number, 0 through 9.  With three sets of flags, a ship could make 1,000 different combinations of three-flag signals.  Coupled with a dictionary assigning a meaning to each combination, the Bourdonnais system would have permitted a marked advance in the sophistication of naval communications.  Unfortunately, his idea was never adopted by the French Navy. However, it was followed up a quarter century later by another Frenchman, Captain Sébastien François de Bigot, Vicomte de Morogues, founder of the French Marine Academy at Brest, who published his Tactique Navale ou Traite des Evolutions et des Signaux in 1763.  While Bigot de Morogues' signal code for the most part followed in the older tradition of assigning meanings to particular flags hoisted at specified locations, he also included a provision in his code for ten numerary flags, hoisted in 336 combinations of up to three flags each, and added both a preparatory flag to signal that a coded message would be transmitted and a requirement that the receiving ship acknowledge the signal.

Signals development in the Royal Navy lagged behind that of the French going into the last quarter of the 18th century, something of which a select group of British officers were well aware. One tactical innovator was Admiral Richard Kempenfelt, who was apparently one of the driving forces in having Bigot de Morogues' Tactique Navale translated into English in 1767. Kempenfelt pressed hard for the adoption of numerary signaling and, in 1781, received permission to experiment with such a system in his squadron operating in the North Sea. Tragically, Kempenfelt died with the Channel Fleet in 1782, but one of his fellow innovators was able to carry the new approach to fruition.  This was Richard, Earl Howe, who became First Lord of the Admiralty in 1783 and introduced his own numerary signal system throughout the fleet in 1790.  Howe's innovations included the use of substitute or repeater pennants, so that only one set of flags was necessary instead of three, and the introduction of additional control flags.  Howe also introduced the concept of assigning a number to each ship so that signals could be addressed individually instead of collectively.  Howe's numerary system was revised and expanded over the last decade of the 18th century, culminating in the Admiralty's 1799 Signal Book for Ships of War, which promulgated the individual flag designs used by the Royal Navy throughout the Napoleonic Wars and the War of 1812, although the number assigned to each flag changed periodically or whenever a signal book was compromised.

In 1800, Rear Admiral Sir Home Popham expanded on the Howe system with his Telegraphic Signals or Marine Vocabulary.  Working from the ten existing numeral flags of the Admiralty Signal Book, Popham developed the world's first alphabetic flag signal system.  Flags one through nine were assigned to letters A through J (I and J counting as a single letter).  Two-flag hoists accounted for the rest of the alphabet.  Popham's code included a numbered dictionary of 3,000 predefined words and sentences, plus the capability to spell out words not included in the dictionary--such as the word "duty" in Lord Nelson's immortal signal at Trafalgar in 1805:  "England expects that every man will do his D-U-T-Y," the word "duty" being the only one not included in the predefined hoists.  The Admiralty adopted the Popham system to augment that of the 1799 Signal Book, and by 1813 an expansion and revision of the Popham code contained 6,000 set phrases and some 60,000 words.

The development of marine signaling in the United States Navy during this period paralleled that in Great Britain.  Commodore Thomas Truxtun published a set of Instructions, Signals and Explanations Ordered for the United States Fleet in 1797, which was basically a numerary system along the lines of Howe's 1790 code.  In 1809, however, Commodore David Porter issued a signal book to his squadron that went a small step beyond the Popham system.  Once again, the basic signals were numerical, but rather than simply numbering the letters of the alphabet, Porter's letters involved hoisting the numeric flags inverted or in combinations that did not match their position in the alphabet.  For example, the number 1 was signified by a solid red flag, but the letter A was divided horizontally blue over white.  The letter K was not a combination of the 1 and zero flags, but of A and I.  The randomness of the Porter code would have added considerable difficulty to an enemy's efforts to decipher a message.  Like their British counterparts, however, Truxtun and Porter's systems were basically numeric--the numbers keyed to words and phrases in the code book--although from Porter's code onward there was also the capability to send "telegraphic" messages by spelling out individual words letter-by-letter.  As in the Royal Navy, successive editions of the signal books contained the same basic flag designs, but with the numbers assigned to them rearranged.  The purpose of these changes was clear--to guard against compromise of this sensitive tactical communications information.  For the same reason, signal books were bound with heavy lead plates bolted to the covers so that they would sink to the bottom if a captain had them thrown overboard to prevent their capture.

In 1817, Captain Frederick Marryat of the Royal Navy published his Code of Signals for the Merchant Service.  Based on the 1799 naval code, it was also a numeric system, but with a vocabulary oriented more toward commercial and less toward naval needs.  Marryat's code was widely accepted, and by 1854 was known as the "Universal Code of Signals."  In the United States, a number of codes were developed for the merchant marine, including James M. Elford's Universal Signal Book (1818), Richard Berrian's American Telegraphic and Signal Book (1823), and J. R. Parker's American Signal Book (1832).  Parker's system was updated and improved in a number of successive editions and was eventually succeeded in 1847 by Henry J. Rogers' Rogers and Black's American Semaphoric Signal Book for the Use of Vessels Employed in the United States Naval, Revenue and Merchant Service.  As the title suggests, this volume was the first commercial signaling system adopted for use by the government for non-tactical communications.  Rogers continued to publish revised editions of this work under various titles until 1856.  A year later, a breakthrough in the science of marine communications made Rogers' system obsolete.

That breakthrough was midwifed by the British Board of Trade, the government agency responsible for the merchant marine.  In 1855, the Board had appointed a committee to develop an improved code, which was promulgated in 1857 as the Commercial Code of Signals.   The Board of Trade's code had not only the previous numeric flags but flags specifically for letters as well, although for only 18 of them.  The breathtaking increase in flexibility and order of magnitude leap in the number of signals that could be constructed made the new system a rapid success:  18 letters and 10 numerals could be hoisted in over 20,000 three-flag combinations, compared to only 1,000 using numbers alone.  Over the years, the Commercial Code was expanded, revised, and updated.  In 1870, it was renamed the International Code of Signals  and before the turn of the century all the major maritime powers were publishing their own editions of the code.  In 1965, management of the ICS was taken over by the United Nations' International Maritime Organization.  Each major seafaring country still publishes the ICS for the use of its mariners.

Signaling at sea by the United States Navy is now governed by a set of NATO and U.S. directives as well as by the International Code of Signals, including the following:

Allied Tactical Publication 1, Volume II, Allied Maritime Tactical Signal and Maneuvering Book  (NATO CONFIDENTIAL)
National Imagery and Mapping Agency Publication 120, International Code of Signals.
Naval Telecommunications Publication 4(D), Fleet Communications

Semaphore Flags
One of the most commonly used methods of visual communications is semaphore.  The signalman sends messages letter by letter--generally using the same short codes contained in either the NATO or commercial signal books--by holding the two flags in various positions similar to the hands of a clock.  Semaphore signals can be sent and received much more quickly than flaghoists or even flashing light using Morse code.  Flaghoist signals have the advantage, on the other hand, of being visible at a greater distance, of allowing several messages to be conveyed at once on multiple halyards, of allowing the captain or admiral to keep them flying for extended periods, and of providing a means of synchronization of action based on when they are hoisted, kept at the dip, closed full up (two-blocked), or hauled down.  The same flag used singly can also be used for "wig-wag" signaling, a form of Morse code transmission based on waving the flag to the right or left invented by the U.S. Army Signal Corps' Albert J. Myer in the mid-19th century.
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Some Illustrative Flaghoist Signals

Call Signs and Address Groups
When entering or leaving port during daylight hours, U.S. warships display their international call signs from the most inboard port signal halyard and the "address group" of an embarked flag officer or unit commander from the most inboard starboard halyard.  The outboard halyards are left free for hoisting emergency and tactical signals.  In U.S. waters, the call sign and address group are hoisted or hauled down when the ship enters or leaves international waters.  The call sign is normally a randomly assigned sequence of four letters, starting with "N," but on occasion the letters have a specific tie to the ship.  For instance, USS New Jersey was NJBB (New Jersey battleship), while the Second Fleet flagship Mount Whitney (JCC/LCC-20) is NOGB, reputedly in honor of her first commanding officer, Orley G. Baird.  Non-government U.S. ships have call signs beginning with "K" or "W."

Call Sign
Inboard Port Halyard
Address Group
Inboard Starboard Halyard
NOGB (USS Mount Whitney) Commander, Second Fleet

It is more common, however, for the call sign to have no connection with the name of the ship:

NAPJ (USS Constitution)

Visual Call Signs

For tactical use, to identify what ship a signal is addressed to, the sender hoists the addressee's "visual call sign."  This is composed by using a single-letter identifier for the type of ship, normally combined with the pennant for the last digit of its hull number.  For example:
Alfa 7 Charlie 9 Delta 3 Lima 4 Mike 1
Cimarron (AO 177) or
Rainier (AOE 7)
Yorktown (CG 49), 
Princeton (CG 59) or
Vicksburg (CG 69)
Spruance (DD 963), 
John Young (DD 973), 
John Paul Jones (DDG 53),
Stethem (DDG 63) or
Decatur (DDG 73)
Boxer (LHD 4),
Nassau (LHA 4),
Austin (LPD 4),
Trenton (LPD 14) or
Gunston Hall (LSD 44)
Avenger (MCM 1),
Gladiator (MCM 11),
Osprey (MHC 51) or
Raven (MHC 61)
As will be obvious, this system would allow the same call to apply to different ships, so where there could be confusion, one will be assigned a call sign composed of the last two numbers, or a different number altogether if necessary.

International Code of Signals

If hoisted by a warship, the CODE/ANSWERING pennant would be added as the first flag in the hoist.
Single Letter Signals
B - I am taking in, or discharging, or carrying dangerous goods I - I am altering my course to port X - Stop carrying out your intentions and watch for my signals

Two-Flag Signals
JI - Are you aground? OY- The port is mined PB - I am engaged in minesweeping operations; you should keep clear of me

Two-Flag Signals with Complements
JJ7M - My maximum draft when I went aground was 7 meters  (1st substitute in lieu of second "J") IT1 - I am on fire and have dangerous cargo aboard CG1 - I will stand by to assist you

Medical Signals
MMD - Rash consists of rose-colored spots that do not blench on pressure MQE18 - My probable diagnosis is chickenpox MTI38 - You should give by mouth seasick tablets.

U.S.-Soviet Incidents at Sea Agreement
The INCSEA Agreement between the United States and the USSR was signed in 1972 as a means of reducing the chances that an incident between U.S. and Soviet naval units could escalate into hostilities between the two superpowers.  One of the elements of the agreement, which remains in effect between the United States Navy and the naval forces of the USSR's successor states, is a code of flag signals to convey mutually understandable statements of intent.  INCSEA signals are initiated by hoisting the ICS letter flags YANKEE VICTOR PENNANT ONE:

Operational Signals

ROMEO for Replenishment

Four sets of signals are used for the enormously complex task of conducting underway replenishment at sea.  Underway replenishment, or UNREP, is the process by which fuel, ammunition, and other supplies are transferred from one ship to another in the midst of operations far from port.  They may be conducted by helicopter (vertical replenishment) or by attaching lines and hoses between two ships steaming alongside each other (connected replenishment).  Normally this tricky maneuver is coordinated by radio, but in wartime conditions it would often be essential to refrain from transmissions that could reveal a battle group's position to the enemy.  The U.S. Navy therefore makes regular use of the capability to conduct UNREPs in complete radio silence by using flaghoists.
The first message hoisted sets 
the ROMEO CORPEN, the course and speed at which the replenishment is to take place.  It is prescribed by hoisting the letter flag ROMEO (R), 
and the CORPEN pennant, one of the special Navy signals,
followed by three Navy numeral flags indicating the compass course for the replenishment, in this case ZERO, 
FIVE, for a heading of 95 degrees, or just a little south of east. 
The course is followed by a "tack," a length of halyard separating the course from the speed, 

then one or two flags indicating the speed in knots, in this case ONE
and TWO, or 12 knots.
When the control ship--normally the ship providing the replenishment--is steady on the prescribed course and speed, it hoists the ROMEO flag "at the dip" at the outboard halyard on the side it wants the receiving ship to come along.  "At the dip" means that the flag is hoisted well below the top of the signal halyard.  A flag hoisted at the dip normally signifies preparation for an evolution that is to be carried out when the flag is hoisted "close up" or "two-blocked."

The receiving ship responds by hoisting ROMEO at the dip when it is ready to come alongside.  The control ship then two-blocks ROMEO close-up, signifying "I am ready for your approach," and the receiving ship does the same to say "I am commencing my approach."

Once the messenger line used to connect the refueling hoses and resupply lines  is in hand, ROMEO is hauled down and (if fuel or explosives are being handled) BRAVO is hoisted in its place.  If BRAVO is close-up, it means fuel or explosives are being transferred; if at the dip, that supply has been temporarily suspended.  When the fueling or resupply is complete, BRAVO is hauled down.

Meanwhile, 15 minutes before the two ships are to disengage, the control ship hoists the PREP pennant at the dip.  When the transfer is complete, it is closed up, signifying that the last replenishment station is disengaging, then, when all lines are clear, it is hauled down and the evolution is complete.

Foxtrot and Hotel for Flight Operations

Similar procedures are used for signaling the conduct of flight operations at sea.  The course and speed for launching and recovering aircraft, the FOXTROT CORPEN, can be signalled the same way as the ROMEO CORPEN for replenishment.  Then, as the carrier is setting flight stations, it hoists the FOXTROT flag at the dip, meaning that the ship will be ready for flight operations as soon as the wind is suitable.  When everything is ready, FOXTROT is two-blocked.  If operations are suspended, FOXTROT is dipped again, and when the carrier secures from flight stations FOXTROT is hauled down.

HOTEL is used the same as FOXTROT, but signals the conduct of helicopter operations only.

Amphibious Operations

A special flag code is used to control landing craft conducting amphibious operations.  As spelled out in the U.S. Army's Marine Crewman's Handbook (FM 55-501) these are:
MIKE - Attention ECHO - Execute
ALFA - Assemble in column FOXTROT - Forward
HOTEL - Cease firing KILO - Increase speed
X-RAY - Commence firing ROMEO - Line abreast
OSCAR - Man overboard UNIFORM - V-formation

Close V-formation

Open V-formation

In addition, a combination of letter, number, and solid color flags are used to control landing craft coming alongside various stations of the amphibious ship.

"Prep" for Sunrise and Colors

The PREP pennant is used as a warning to ships in port to prepare for execution of sunrise or morning colors.  It is hoisted by the senior officer present afloat or by a signal tower ashore and repeated by ships present five minutes before the evolution to be carried out.  Hauling it down is the signal to execute the evolution--to raise the ensign and jack at 8:00 a.m., for instance.

BRAVO ZULU, or BZ, is one of the most puzzling expressions heard by non-initiates when dealing with Navy people.  It means "well done," and comes from the two-flag signal with that meaning in the NATO signal book, ATP-1.

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Copyright 2000, 2001 by Joseph McMillan