A number of visitors to the temporary exhibit “Viva Hoover! The 1928 Goodwill Tour”have asked about one of the large photographs of the battleship USS Maryland — what is that clock-like thing on the mast?
It’s a device called a “range clock,” and you will see it in pictures of battleships from World War I until about the beginning of World War II. Range clocks were invented by the Royal Navy (they called them “concentration dials”) during World War I to help squadrons of battleships aim at the same target. When the United States joined the war in 1917, the US Navy adopted the concept.
The standard practice of the battleship era was for the battleships to enter combat in a line-ahead formation. The lead ship would attempt to maneuver so that, ideally, the following ships could concentrate fire from their big guns on a specific enemy target or targets. By World War I, improvements in gun technology meant that big-gun battleships could fire at enemy ships at a distance of five miles or more. As the British Royal Navy focused on bottling up the German High Seas Fleet, the rainy, foggy weather of the North Sea, combined with copious smoke from the ships’ coal fired boilers and big guns, made it difficult to see enemy ships clearly at range.
In fact, it was fairly rare for all the ships in the line of battle to actually see the enemy at the same time, so large dials with black numbers resembling clock faces were installed on the front of the fore mast and on the rear of the main mast. Similar to a clock, there was a short hand and a long hand. Different numbering systems were used at various times, sometimes involving Roman numerals along with Arabic numerals, to indicate range in thousands and hundreds of yards. Once a ship found the range to the target, it would display the range on the dials, visible to the next ship in front and behind in the line of battle.
Hash marks, called bearing indicators, were painted on one turret fore and aft, indicating the angle of the turret and therefore the angle to target. With the range and bearing data from the ship in front or behind them, and proper adjustments made for the distance between the ships, gunnery officers on a battleship could target an enemy ship they could not see. They could then pass the information on to the next ship in line, allowing the entire battle line to concentrate fire on an unseen enemy.
Between the wars, improved radio communication made range clocks and bearing indicators obsolete. Photos of USS Maryland from the late 1920s show no evidence of bearing indicators, though the range clocks are still in place. Throughout the 1930s and into World War II, range clocks were removed whenever battleships were repaired or reconstructed.