The use of signal flags goes back a long way and was made famous by Nelson shortly before the battle of Trafalgar. Although not so extensively used today, flags as a limited means of communication has managed to survive. The merchant navy make use of a total of 40 individual flags, 26 alphabetical flags, 10 numeral pennants, 3 substitute pennants and what is termed an answering pennant.

A flag is rectangular in shape, whilst a pennant is tapered. See the images at the end of this page indicating the shape and colours of the flags/pennants. Each of the alphabetical flags has a single meaning but used together with other flags and/or pennants in hoists of two or more can have a multitude of meanings. Used with a publication called the International Code of Signals (INTERCO), it affords ships of varying nationalities the ability to converse in a common “language”.

How are flags used?

Flag signalling.

The flags are hoisted on halyards, special lines running through sheaves on a vessel’s masts or yards. There are basically three positions where flags may be hoisted, ie at the gaff at the rear of the main mast, and at the port and starboard yardarms. Although it is very unlikely that multiple flag hoists will ever again be used, it may be of interest to note the order in which the flag hoists are read. The sequence is:

1. the hoist on the gaff;
2. the outboard hoist on the starboard yardarm;
3. the inner hoist on the starboard yardarm;
4. the outer hoist on the port yardarm; and finally;
5. the inner hoist on the port yardarm.

The alphabetic flags are either used singly (each has a specific single meaning) or they can be used in groups (code) with various meanings, or they can be used to spell out words (plain language). The substitutes are used to copy a particular flag in a hoist when you don’t have sufficient flags.
As an example, say you want to make up the following hoist: M, B, M, B, but you only have one set of flags.
You would hoist the flag M, followed by flag B, followed by the first substitute, followed by the second substitute.

• M
• B
• !st sub (means copy the first flag in the hoist)
• 2nd sub (means copy the second flag in the hoist)

The answering pennant is used as follows:
You are communicating with another vessel and have hoisted the following signal from the International code of signals: C,M,T. On seeing the hoist, the other ship will hoist the answering pennant “at the dip” (two thirds of the way up the halyard). By doing this the other ship is saying “I see your hoist”. Once the officer of the watch has decoded the hoist, he will hoist the answering pennant “close up” (all the way to the top of the halyard). This indicates that he understands the meaning of the hoist. When he has the reply, he will take down the answering pennant and hoist his response to the signal.

See the list at the end of this page for the single meanings of the alphabetic flags.

Advantages of using flags. The advantages of using flags as a means of communication are:

1. It is silent. Unless you are within range, you will not be able to make out any message.
2. It does not require a power source to operate.
3. It does not require a skill to operate.

Disadvantages of flags. The shortcomings of using flags is the following:

1. The range that flags can be read is extremely limited.
2. It is very slow.
3. The amount of traffic that can be sent is very limited.
4. It cannot be seen at night or in restricted visibility.

Signalling flags.

Signalling pennants.

Single meanings of flags:
Flag & Meaning

A = I have divers down. Keep well clear of me.
B = I am loading, discharging or carrying dangerous cargo or substances.
C = Yes or affirmative.
D = Keep clear of me, I am manoeuvring with difficulty.
E = I am altering my course to starboard.
F = I am disabled, communicate with me.
G = I require a pilot. (fishing boats – I am hauling nets).
H = I have a pilot onboard.
I = I am altering my course to port.
J = I am on fire and have dangerous cargo onboard, keep well clear of me.
K = I wish to communicate with you.
L = You should stop your vessel immediately.
M = My vessel is stopped and making no way through the water.
N = No or negative.
O = Man overboard.
P = The ship is about to sail, all personnel to report onboard. (fishing vessels – My nets have come fast on an obstruction).
Q = My vessel is healthy, request free pratique.
R = (with one or more numerals, it indicates distance in miles.
S = My engines are going astern.
T = Fishing vessels – Keep clear of me, I am engaged in pair trawling.
U = You are running into danger.
V = I require assistance.
W = I require medical assistance.
X = Stop carrying out your intentions and watch for my signals.
Y = I am dragging my anchor.
Z = I require a tug. (fishing vessels – I am shooting nets).