A brief history of ships sharing statements by hoisting and waving signal flags
Even though we like to romantically think of the ocean as a serene, relaxing place, it’s actually pretty noisy. Between the wind and waves, anyone talking on the deck of a ship is competing with around 85 decibels of splashing and spraying, which is close to the equivalent of a standing 100 feet away from a 45-mile-per-hour diesel locomotive. You could probably raise your voice enough to talk to someone next to you, but since Ancient Greece people have been trying to come up with ways to communicate between different ships at sea. This was often to coordinate military action, but today includes a variety of statements that might need sharing over moderate distances when speaking isn’t an option.
Hoisting preset signals
Sound can travel pretty far through the air, but from the deck of a ship it can’t really compete with light. To compromise, ancient naval commanders would hoist special flags above their ships that signaled to others that it was time to come over and speak face to face. This obviously has some scheduling limitations, and so people tried putting more information into the flags themselves. Different colors, shapes and positioning was used to build up a vocabulary of commands that could be posted so that other ships could view them from larger distances, whenever their view was clear.
Eventually the details of reading these flags evolved into more elaborate codes, complete with numbers, letters and ways to make substitutions if your available flags didn’t include two copies of a certain signal. Using what amounted to three sets of flags, a ship could basically broadcast around 1,000 different signals. In many systems, any message that didn’t have a dedicated flag combination could just be spelled out, letter by letter. There have been many revisions of these systems, some peaking at 70,000 possible signals, but today things have been simplified a bit. The current International Maritime signal-flag vocabulary can still use combinations of flags, but many of the most commonly used messages can be shared as a single flag for convenience. These include important messages like “I have a doctor on board,” or “I am taking on or discharging explosives.”
Sharing with semaphore
Like all forms of technology, there’s never been a single standard for flag-based communication. In the seventeenth century, Robert Hooke started designing the precursor to semaphore flags, which didn’t actually involve the ocean at all. Semaphore towers were tall buildings set as far as 150 miles apart, each with a pivoting crossbar on top. Each end of the crossbar was fitted with an extension that could be arranged to make the entire shape resemble, a line, an “S”, an “L”, or more. Arranging the angles of these bars allowed for 196 distinct positions, each corresponding to a character that could be seen from a great distance. The system allowed for thousands of signals to be sent faster than a horse could deliver a message.
The towers were successful for many years, even delaying the initial adoption of telegraph wires in France. However, they found a more permanent home on board ships, where the crossbars and beams could be replaced by a human holding two red and yellow flags. By adopting different poses with one’s arms, a message could be sent fairly quickly, although it needed to be seen at the right moment, since nobody wanted to be standing on deck waving flags all day long.
Flags, lights and voices
Flags still have their place on ships today, but they do face a lot of competition. Flashing lights signaling Morse code were one earlier alternative to flags, with the big advantage of being visible at night, and maybe even in poor weather conditions like rain. From a military standpoint, radio transmissions are superior to all of the above, since flags and light beacons are not only visible to your own crews, but to nearby enemies as well. Codes could be used to make things more opaque, but being able to communicate directly with your voice is now a much faster, detailed way to say “hi” when out on the water.
Source: Signaling at Sea by Joseph McMillan, Sea Flags