Signal flags

Signal flags are a common sight at sea – and even on land. They are often difficult to recognise as such. Signal flags are used for the exchange of messages and are subject to strict rules, as set out in the 'International Signal Book' containing the 'International Signal Flags'. The first Signal Book was published in 1817 by Captain Marryat. Since then, it has been revised several times and adapted to the changing needs of shipping. In the course of this work, the 'International Signal Book' has been very carefully translated into the languages of the major seafaring nations involved, so that messages can be exchanged without difficulty or misunderstanding even between signalling partners who do not understand each other’s national language. The 'International Signal Book' consists of two volumes. The first volume covers signalling using international signal flags, light Morse code and hand signals. It governs the entire exchange of messages by means of visual signals. The second volume is the radio communications manual. This is used to encode radio telegraphic messages using groups of five letters. A set of signal flags consists of 26 letter flags, 10 number pennants, 4 auxiliary staffs (substitutes) and 1 reply pennant (code/answer). The flags are arranged in sequence one below the other and read from top to bottom. Such a flag arrangement is called a signal. The number pennants can be used to easily form any number. The reply pennant is used as a comma to indicate decimal places. Despite its considerable length and the variety of signals, the internal structure of the 'International Code of Signals' is kept clear and simple, so that once you are familiar with its layout, you can find the signals and their meanings without difficulty and without having to search for long time.

Pilot signals:

Generally, a pilot was requested by hoisting the national flag on a white background. Some countries also had special flags for this purpose, known as pilot flags or pilot call flags. However, according to the 'International Code of Signals', this could also be done using signal flags G or PT. The use of signal flag G has become the standard practice internationally for this purpose; pilot flags or pilot call flags are no longer in use. At night, a blue light or a white light held immediately above the railing at short intervals is displayed every 15 minutes. This light should be visible for one minute each time.

The call:

If a vessel wishes to send a message to another, it flies its national flag and the distinguishing signal of the vessel being called. The distinguishing signal consists of four flags corresponding to the radio call sign. If the vessel being called is unknown, it is first requested, by means of the VH signal, to display its distinguishing signal. See: Vessel flagging Coastal signal stations are called using flag Z. In the following example, a vessel, A, wishes to enquire at a coastal signal station whether, with a draught of 26 feet, it can cross the shoal.

Example:

The example depicts a kind of dialogue between Participant A (ship) and Participant B (signalling station).

Vessel A signals the coastal signal station by using flag Z. The signal station then raises the reply pennant half-mast (half-high). Vessel A sends the signal: BTK = Can I pass over the shoal? The signal station raises the reply pennant fully when it has understood the signal. Vessel A lowers the BTK signal, and Signal Station B returns the reply pennant to half-mast. Vessel A now displays: ML = My draught is … feet. The signal station raises the reply pennant to full mast again when the signal is acknowledged, i.e. understood. Vessel A lowers the signal ML, and the signal station returns the reply pennant to the half-mast position. Vessel A now displays the number pennants 2 and 6. The signal station raises the reply pennant when the signal has been understood. Vessel A indicates the end of its transmission by hoisting the reply pennant fully. The signal station lowers the reply pennant and sets the signal XK = 'Wait until high tide'. Vessel A hoists the reply pennant fully when it has understood the signal, and the signal station terminates the communication by hoisting the reply pennant.