Signaling at Sea

Knowledge of signal communications can be very valuable if radio equipment is inoperative, or there is no common frequency between you and the station you wish to communicate with, or where there is a language difference. Vessels within sight or hearing of each other may communicate using code flags, flashing lights, or sound signals. (See Fig. 1.)

The International Code of Signals

The International Code of Signals is published by the National Imagery and Mapping Agency as Publication No. 102. It is published in the nine most commonly used languages and it's well worthwhile to have a copy on board most small vessels.

Pub. 102 contains signals that can be sent by flag hoists, flashing light and sound, and other means. The International Morse code is used for flashing light and sound signals, as well as radiotelegraphy.

Signals basically consist of single letters and two-letter combinations; three-letter combinations, all starting with "M," are used solely for medical messages. Many signals can be expanded or made more specific with complements. These may express a variation in the meaning of the basic signal; questions and answers related to the meaning of the basic signal; or supplementary, specific, or more detailed meanings of the basic signal. Most complements consist of a single numeral, but there may be two, three, four, five, or six numerals when signaling such values as latitude and longitude, time, or date.

Single-Letter Signals

Signals consisting solely of one letter are used for the most basic messages. Typical ones are listed below:

There is also another set of single-letter signals that is used only with numerical complements to communicate azimuth or bearing, course, speed, latitude and longitude, or time and date.

Yet another set of single-letter signals, usually made by sound or radiotelephony, is used between icebreakers and assisted vessels.

Two-Letter Signals

The General Signal Code consisting of signals of two letters, many with a single numerical complement, is used for many types of messages. Typical signals are:

Flag Hoist Signaling

The most common use of the International Code of Signals is visual signaling using code flags. A set of flags consists of 26 flags for the letters of the alphabet, ten number pennants, three substitute pennants, and the answering pennant. (The U.S. Navy uses a fourth substitute and calls them all "repeaters.") The substitutes are necessary because a set contains only one flag for each letter and some flag hoists require repetitions of one or more letters.

Five standard colors are used — red, white, blue, yellow, and black. Most of the flags are of two colors, selected and arranged for maximum contrast. Two flags are of a single color only, three use three colors, and one uses four colors.

Flag hoists are read from the top flag or pennant downward; if there is more than one hoist from a single spreader or yardarm, each hoist is read in turn from outermost inward. Additional details on flag hoist signaling can be found in Pub. 102.

The International Morse Code

Letters, numerals, and punctuation marks are signaled in the Morse code by combinations of dots and dashes. Letters have from one to four dots or dashes, numerals have five, and punctuation marks have six. The dots and dashes, and the spaces between them, are defined in terms of units. A dot is one unit in length and a dash is three units. The space between the dots and/or dashes within a character is one unit; the space between characters is three units; and the space between words is five units. The length of any unit is set by the method of signaling — much longer in sound or flashing light than radiotelegraphy.

Regardless of the method of signaling, you should not try to send at a speed greater than what you can do smoothly. You should also not send at a speed greater than that at which you can receive, lest you find yourself unable to keep up with the other operator.

Flashing Light Signaling

Signaling by flashing light is carried out by using Morse Code. It is necessary to learn the dot-dash sequences and their meanings, and to practice sending and receiving, in order to be able to communicate with another vessel, up to several miles distant, either day or night. Proficiency at Morse Code is also required for most amateur (ham) radio licenses. Practice tapes and flash cards designed to improve your learning curve are available from several sources.

A signaling lamp (also known as an Aldis lamp) is needed for proper Morse Code signalling. It is portable and designed to operate on 12 volts. Few recreational boats carry these items today, so you may have to rely on your boat's searchlight, or even a flashlight. Lights are produced with the capability of sending a Mayday signal (SOS) automatically. These may be used to satisfy the Coast Guard distress signal requirement.

Signaling lamps must be pointed directly in the direction of the other vessel. Sights are attached to ensure you are pointing the light directly at the target. The usual range is several miles, both day and night. These lamps are very useful for rescue purposes, and even as an ordinary spotlight.

Sound Signaling

Although signaling between vessels may be carried out by whistle using the Morse code for some of the signals from Pub. 102, this is a slow method and, unless in open waters, should never be resorted to. Confusion as to any sound signals given, or their misinterpretation, can have disastrous results. Sound signals are best left to those specified in the Navigation Rules.

International Diving Flag

The International Code Flag A means:
"I have a diver down; keep well clear at slow speed"

In the United States, divers commonly show a red flag with a diagonal white stripe. The meaning of this flag is the same as International Code Flag A. The diving flag is frequently seen attached to a float, which in turn is attached to the diver by a tether. As the diver moves below the surface, the warning flag follows along. Unfortunately, this can give the diver a false sense of security — showing the diving flag does not relieve the diver of the responsibility of staying clear of channels and navigation aids. The dive flag should not be flown on a permanent basis, as is seen aboard some dive boats. The flag should only be used when divers are actually in the water.

With the increasing number of underwater swimmers and diving parties operating along the coasts and in the harbors, boaters are urged to keep well clear whenever they see the diving flag. Give any vessels flying this flag a very wide berth and proceed at slow speed. Keep in mind the possibility of floating tethers, lifelines, and other temporary obstructions in the vicinity of any divers. Divers near channels should have an observer stationed aboard the dive boat. Mariners are urged to contact the observer if in any doubt as to the location of dive operations.

VHF Marine Channels and Their Uses