Maritime flag signalling

Maritime flag signalling, generally flaghoist signalling, is the principal means other than radio by which ships communicate to each other or to shore (distinguished from flags showing nationality, ownership, or (for naval vessels) organizational status). Virtually all signalling by non-naval vessels is now organized under the International Code of Signals (whether by flaghoist, semaphore, signal lamp, or other means), which specifies a standard set of flags and codes. Naval vessels generally use an extended set of flags and their own codes. This article will touch on the historical development of maritime flag signalling.

In the early days of sail, the use of signals to communicate between ships was primitive, as seen by one admiral's instructions to his fleet in 1530:

"Whensoever, and at all tymes the Admyrall doth shote of a pece of Ordnance, and set up his Banner of Council on Starrborde bottocke of his Shippe, everie shipps capten shall with spede go aborde the Admyrall to know his will."

By 1653, the Royal Navy had issued instructions by which an admiral could signal various orders by hoisting flags in various locations on his ship. Modern naval code signalling began with the invention of maritime signal flags in the mid-17th century by the Duke of York who was created Lord High Admiral after the Restoration. A ship's message had to be approved by the Officer of the watch, and his system was augmented and changed in various ways over the following century. In 1790, Admiral Lord Howe issued a new signal book for a numerary system using numeral flags to signal a number; the number indicated the message, not the mast from which the flags flew. Substitute flags were also instituted to indicate repeated numerals, and there was consideration of making the flags more distinct. In 1799, Captain Sir Home Popham published his first list of words and sentences which could be referenced by a number (or "code"); three subsequent editions added letter flags, with the 1801 edition numbering 2994 codes. Popham's code was famously used for the "England expects that every man will do his duty" signal at Trafalgar by Nelson.

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