Before the time of the internet, telephones and fax machines, ships would use a coded signal flag system. In 1799 Admiral Home Popham published his own telegraphic signal codes book.
Popham's 3rd edition codebook 1801, assigned the digits 0 to 9 to ten signal flags, which were used in combination. Code numbers 1–25 represented letters of the alphabet (omitting J and with V=20 before U=21) - as the diagram below, illustrates.
Higher numbers were assigned meanings by a code book, contact us to find out more. The code numbers typically would have been hoisted on the mizzenmast, one after another, preceded by the "telegraphic flag" (a red over white diagonally-split flag, see below) to show that the following signals would use the Popham code. As well as digit flags, the code used "repeat" flags so that only one set of digits was needed; thus the word do, coded as "220", used a "2" flag, a "first repeat" flag here serving as a second 2, and a "0" flag. The end of the message would be indicated by an "end of code" flag (blue over yellow diagonally split).
Popham's code was famously used for the "England expects that every man will do his duty" signal at Trafalgar by Nelson: for this, a team of four to six men would have prepared and hoisted the flags onboard Lord Nelson's flagship HMS Victory, the whole process taking about 4 minutes. The message shows one of the shortcomings of Popham's code — even the two-letter "do" required three flags hoisted for the signal.
Here is how you would setup your own signal message.
1. Prepartory flag consisting of a red and white diagonal flag which is flown at the start of a signal to show that it was a telegraphic signal.
2. The message finished flag consisted of blue and yellow diagonal.
3. The message was understood the affirmative signal or a repeat of the signal that was sent.
4. If the message was not understood then the affirmative signal with a white flag was flown.
5. If the message was to be answered a further flag was flown.
6. If a number was to be sent then a numeral pennant was flown.