The signal bridge was the center for visual signals to other ships steaming in company with the SLATER. Signals originated with the Captain or Officer of the Deck and were relayed by signalmen to other ships. Lookouts were constantly on the alert for other ships that might be signaling the SLATER. These signals were relayed in one of three ways.
In semaphore, a signalman holding two flags stood on a high platform and extended his arms to different positions that represented letters of the alphabet. This was the most rapid way of communication, well suited to plain language, but usable only in daylight.
Another daylight signaling system was flag hoists, colored flags raised to the yardarm on the ship's halyards or pulley lines. Each flag represented a letter, number or tactical term. Code books were used to encipher messages because the naval ships had special codes that were secret. The International Code of Signals was known to all nations and used when communicating with merchant ships. Click here for more information on the meaning of international maritime signal flags.
Flags were hung alphabetically in the large canvas covered flagbags on fingers. These fingers allowed air to circulate and keep them from mildewing. When making up a message, the signalman would clip the required flags together to make a message, then hoist the signal.
Signal flags were often used for indicating course and speed changes. The flagship of the group would hoist a command. The other ships in the group would hoist the same signal flag to indicate they received the command. When the command was to be performed, the flagship would drop their hoist and surrounding ships would execute the prescribed maneuver.
Finally, ships used a day or night system of sending Morse Code signals via flashing light, using 12" and 24" signal lamps. This was a rapid way of communicating but was rarely used at night for fear that the light would reveal one's position to the enemy. Infrared hooded lenses were used to mask the bright lights of the signal lamps; to limit their visibility.