Despite this age of high technology when satellites and radios transmit communications at incredible speed, the oldest form of communication can still play a vital role in the Navy. Visual communication has a distinct advantage over other forms. For all its advances, science has yet to produce a silent form of communication, one that cannot be detected by advanced technological equipment. Visual communication fills the need for a reliable, silent, and relatively secure means of communication at ranges up to 15 miles.
The three main types of visual signal are flashing light, semaphore, and flaghoist.
Letters and numbers are broken down into short and long flashes of light known as Morse code. A transmitting signalman sends messages one letter at a time with a slight pause between each letter. The receiving signalman flashes a light for each word received until the message is complete.
Flashing-light signaling is accomplished by two methods, directional and nondirectional. With the directional method, the sender aims his light directly at the receiving ship or installation. The ship’s standard signal-searchlight is most often used but the blinker tube (or blinker gun) and the multipurpose lamp may also be used. The signal-searchlight is mounted to the ship’s rail or to a special stand and is worked with levers attached to a blinking screen, which allows the light to be quickly shown and blocked (to form the short and long flashes required for Morse code). The other two are portable, battery-operated lights with trigger switches to control flashes.
The nondirectional method is also called all-around signaling. Most of it is done by yardarm blinkers, lights mounted near the ends of the port and starboard yardarms on the mainmast and controlled by a signal key, similar to an old-fashioned telegraph, located on the signal bridge. This method is best for sending messages to several ships at once.
Lights used at night can be seen by an enemy, so an alternate system called “Nancy” uses invisible infrared light. Messages sent by this system can be seen only by those who have a special Nancy receiver, which gathers infrared rays and converts them to visible light. Nancy, with a range of from 10,000 to 15,000 yards, can be used only at night and is a more secure method of communication.
Semaphore requires little in the way of equipment and is relatively simple once the user becomes accustomed to it. Words are transmitted by holding the arms in a specific position to represent individual letters. When sender and receiver are close, as when their ships are alongside one another for UNREP, no special equipment is necessary. The semaphore characters are made simply by moving the hands to the proper positions. At greater distances, flags attached to short staffs held by the sender will make the signals much more visible. Standard semaphore flags are usually 15 to 18 square inches, and each staff is just long enough to grasp firmly. Most semaphore flags issued to the fleet today are fluorescent and made of sharkskin. For night semaphore, flashlights with special light-diffusing cones attached are held in the same manner as semaphore flags.
A good signalman can send or receive about 25 five-letter groups a minute. Only 30 positions need to be learned to be able to communicate by this method.
Semaphore is much faster than flashing light for short-distance transmissions. It may be used to send messages to several ships at once if they are in suitable positions but works best when used one on one. Because of its speed, semaphore is better adapted than the other visual methods for long messages.
Although semaphore’s usefulness is limited somewhat by its short range, it is more secure than flashing light or radio because there is less chance of interception by an enemy or unauthorized persons. Speed and security, therefore, are the two factors favoring the use of semaphore.
This is an effective system of visual signaling, but it can be used only in daytime. It is mostly used for transmitting tactical orders but has some administrative uses as well. The meanings of each signal must be looked up in a signal book. There is a signal flag for each letter of the alphabet, a set of flags for each numeral 0 through 9, a set of pennants for each numeral 0 through 9, and other flags and pennants with special meanings. A compete set of signal flags will have sixty-eight flags and pennants. [See Appendix F for a chart of all flags and pennants.]
Most ships carry only two or three complete sets of flags, but special substitute pennants may be employed to repeat flags that are already flying. The first substitute repeats the first flag or pennant in the same hoist, the second substitute repeats the second flag or pennant, and so on. With this system, there is virtually no limit to the combinations of flags and pennants that may be displayed (except for halyard space) and thousands of different signals can be sent.
Some signal flags have special meanings when used alone:
Bravo Ship is handling explosives or fuel oil.
Five Ship is broken down; cannot maneuver on its own.
Oscar Man overboard.
Papa Personnel recall. All hands return to ship.
Quebec Boat recall. All boats return to ship.