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:

AI have a diver down; keep well clear at slow speed.
BI am taking in, or discharging, or carrying dangerous goods.
DKeep clear of me - I am maneuvering with difficulty.
HI have a pilot on board.
KI wish to communicate with you.
LYou should stop your vessel instantly.
OMan overboard.
QMy vessel is healthy and I request free pratique.
UYou are running into danger.
VI require assistance.
WI require medical assistance.
XStop carrying out your intentions and watch for my signals.
YI am dragging my anchor.

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:

ACI am abandoning my vessel.
ANI need a doctor.
AQI have injured/sick person
(or number of persons indicated) to be taken off urgently.
CBI require immediate assistance.
CKAssistance is not (or is no longer) required by me
(or vessel indicated).
CPI am (or vessel indicated is) proceeding to your assistance.
DVI am drifting.
DXI am sinking (lat...long...if necessary).
EDYour distress signals are understood.
ELRepeat the distress position.
FAWill you give me my position?
FOI will keep close to you.
GWMan overboard. Please take action to pick him up
(position to be indicated if necessary).
ILI can only proceed at slow speed.
ITI am on fire.
JGI am aground; I am in dangerous situation.
JHI am aground; I am not in danger.
JWI have sprung a leak.
KJI am towing a submerged object.
KMI can take you (or vessel indicated) in tow.
KQPrepare to be taken in tow.
KT1I am sending a towing hawser.
LBITowing hawser is fast to chain cable.
NCI am in distress and require immediate assistance.
NFYou are running into danger.
NGYou are in a dangerous position.
OQI am calibrating radio direction finder or adjusting compasses.
PNYou should keep to leeward of me (or vessel indicated).
PPKeep well clear of me.
QDI am going ahead.
QII am going astern.
QQI require health clearance.
RBI am dragging my anchor.
RUKeep clear of me. I am maneuvering with difficulty.
TPFishing gear has fouled my propeller.
UYI am carrying out exercises - keep clear of me.
YGYou appear not to be complying with the traffic separation scheme.
ZMYou should send (or speak) more slowly.
ZSMy vessel is healthy and I request free pratique.

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

International Mores Code
A. _N_ .
B_ . . .O_ _ _
C_ . _ .P. _ _ .
D_ . .Q_ _ . _
E.R. _ .
F. . _ .S. . .
G_ _ .T_
H. . . .U. . _
I. .V. . . _
J. _ _ _W. _ _
K_ . _X_ . . _
L. _ . .Y_ . _ _
M_ _Z_ _ . .
1. _ _ _ _6_ . . . .
2. . _ _ _7_ _ . . .
3. . . _ _8_ _ _ . .
4. . . . _9_ _ _ _ .
5. . . . .0_ _ _ _ _
Ä (German ) = AE (Danish) . _ . _
Á or Å (Spanish or Scandinavian) . _ _ . _
Ch (German or Scandinavian) _ _ _ _
É (French) .. _ ..
Ñ (Spanish) _ _ . _ _
Ö (German) = Ø (Danish) _ _ _ .
Ü (German) . . _ _

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

Frequencies (MHz)Channel Usage
Channel NumberShip TransmitShip ReceiveIntended Use
01A156.050156.050PORT OPERATIONS and COMMERCIAL only within the USCG designated Vessel Traffic Services (VTS) Area at New Orleans and lower Mississippi River.
05A156.250156.250PORT OPERATIONS only within the VTS radio protection areas of New Orleans and Houston.
06156.300156.300INTERSHIP SAFETY. Required on all VHF-FM equipped vessels. For intership safety purposes, and search and rescue (SAR) communications with ships and aircraft of the U.S. Coast Guard. Must not be used for nonsafety communications.
07A156.350156.350COMMERCIAL (Intership and Ship-to-Coast). A working channel for commercial vessels to fulfill a wide scope of business and operational needs.
08156.400156.400COMMERCIAL (Intership). Same as Channel 7A, except limited to intership communications.
09156.450156.450COMMERCIAL and NONCOMMERCIAL. (Intership and Ship-to-Coast). Recommended as calling channel for noncommercial stations. Used for bridge tenders in some areas.
10156.500156.500COMMERCIAL (Intership and Ship- to -Coast). Same as Channel 7A.
11156.550156.550COMMERCIAL. Same as Channel 7A. VTS in designated areas.
12156.600156.600PORT OPERATIONS. Restricted to the operational handling, movement, and safety of ships and, in emergency, to the safety of persons. VTS in designated areas.
13156.650156.650NAVIGATIONAL (Ship's) Bridge-To-(Ship's) Bridge. This channel is available to all vessels and is required on large passenger and commercial vessels (including many tugs). Use is limited to navigational communications such as in meeting and passing situations. Abbreviated operating procedures (call signs omitted) and one watt maximum power (except in certain special instances) are used on this channel for both calling and working. For recreational vessels, this channel should be used for listening to determine the intentions of large vessels. This is also the primary channel used at locks and some bridges.
14156.700156.700PORT OPERATIONS (Intership and Ship-to-Coast). Same as channel 12. Used for VTS in designated areas.
15---156.750ENVIRONMENTAL (Receive Only). A channel used to broadcast environmental information to ships such as weather, sea conditions, time signals for navigation, notices to mariners, etc. Most of this information is also broadcast on the weather (WX) channels.
16156.800156.800DISTRESS SAFETY and CALLING (Intership and Ship-to-Coast). Required channel for all VHF-FM equipped vessels. Must be monitored at all times station is in operation (except when actually communicating on another channel). This channel is also monitored by the Coast Guard, public coast stations, and many limited coast stations. Calls to other vessels are normally initiated on this channel. Then, except in an emergency, you must switch to a working channel.
17156.850156.850STATE CONTROL. Available to all vessels to communicate with ships and coast stations operated by state or local governments. Messages are restricted to regulation and control, or rendering assistance. Use of low power (one watt) setting is required by international treaty.
18A156.900156.900COMMERCIAL (Intership and Ship-to-Coast). Same as Channel 7A.
19A156.950156.950COMMERCIAL (Intership and Ship-to-Coast). Same as Channel 7a.
20A157.000157.000PORT OPERATIONS (Intership only). Available to all vessels This is a traffic advisory channel for use by agencies directing the movement of vessels in or near ports, locks, or waterways. Messages are restricted to the operational handling, movement and safety of ships and, in emergency, to the safety of persons.
21A157.050157.050U.S. GOVERNMENT ONLY.
22A157.100157.100U.S. GOVERNMENT. Used as a working channel with USCG.
23A157.150157.150U.S. GOVERNMENT ONLY.
24157.200161.800PUBLIC CORRESPONDENCE (Ship-to-Coast). Available to all vessels to communicate with public coast stations
25157.250161.850PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
26157.300161.900PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
27157.350161.950PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
28157.400162.000PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
63A156.175156.175Same as Channel 01A.
65A156.275156.275PORT OPERATIONS (Intership and Ship-to-Coast). Same as Channel 12.
66A156.325156.325PORT OPERATIONS (Intership and Ship-to-Coast). Same as Channel 12.
67156.375156.375COMMERCIAL (Intership). Same as Channel 7A, except limited to intership communications. In the New Orleans VTS radio protection area, use is limited to NAVIGATIONAL bridge-to-bridge intership purposes. Available to NONCOMMERCIAL in Puget Sound area only.
68156.425156.425NONCOMMERCIAL (Intership and Ship-to-Coast). A working channel for noncommercial vessels. May be used for obtaining supplies, scheduling repairs, berthing and accommodations, etc. from yacht clubs or marinas and intership operational communications such as piloting or arranging for rendezvous with other vessels. Channel 68 is the most popular noncommercial channel and therefore is usually heavily congested.
69156.475156.475NONCOMMERCIAL (Intership and Ship-to-Coast). Same as Channel 68.
71156.575156.575NONCOMMERCIAL (Intership and Ship-to-Coast). Same as Channel 68.
72156.625156.625NONCOMMERCIAL (Intership). Same as Channel 68, except limited to intership communications; available for COMMERCIAL communications in Puget Sound area only
73156.675156.675PORT OPERATIONS (Intership and Ship-to-Coast). Same as Channel 12.
74156.725156.725PORT OPERATIONS (Intership and Ship-to-Coast). Same as Channel 12.
77156.875156.875PORT OPERATIONS (Intership). Limited to intership communications to and from pilots concerning the docking of ships. Power limited to one watt.
78A156.925156.925NONCOMMERCIAL (Intership and Ship-to-Coast). Same as Channel 68.
79A156.975156.975COMMERCIAL (Intership and Ship-to-Coast). Same as Channel 7A.
80A157.025157.025COMMERCIAL (Intership and Ship-to-Coast). Same as Channel 7A.
81A157.075157.075U.S. GOVERNMENT ONLY.
82A157.125157.125U.S. GOVERNMENT ONLY.
83A157.175157.175U.S. GOVERNMENT ONLY.
84157.225161.825PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
85157.275161.875PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
86157.325161.925PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
87157.375161.975PUBLIC CORRESPONDENCE (Ship-to-Coast). Same as Channel 24.
88157.425162.025PUBLIC CORRESPONDENCE. In the areas of the Puget Sound and Strait of Juan de Fuca and approaches, same as channel 24.
88A157.425157.425COMMERCIAL (Intership). Beyond 120 km from Puget Sound area in the Great Lakes, and on the St. Lawrence Seaway, same as Channel 7A, except limited to intership use. Also available for communications between commercial fishing vessels and associated aircraft while engaged in commercial fishing.