Navigation and Seamanship

4.2.5.6. Sound signaling

Sound producing apparatus such as whistle, siren, fog horn are used for sound signalling at sea. Sound signalling is required at the time of restricted visibility. Signals other than the single letter signals should be used only in extreme emergency and never in frequented navigational waters. The signals should be made slowly and clearly. They may be repeated, if necessary, but at sufficiently long intervals to ensure that no confusion can arise and that one letter signals cannot be mistaken as two-letter groups. The Morse symbols representing letters, numerals, etc., are expressed by dots and dashes which are signalled either singly or in combination. The dots and dashes and spaces between them should be made to bear the following ratio, one to another, as regards their duration:

(a) A dot is taken as the unit;

(b) A dash is equivalent to three units;

(c) The space of time between any two elements of a symbol is equivalent to one unit; between two complete symbols it is equivalent to three units; and between two words or groups it is equivalent to seven units.