Range Rate (prior to around 1913, this was called rate of change of range. Postwar, it was sometimes called speed-along, as in "relative target speed along the line of bearing") is the mathematical derivative of the range to the target. Just as knowing the range to a target can help you hit it at that instant, knowing how the range is changing over time can be essential to ensure sustained hitting beyond that instant.
Range to a target ship tends to change over time because one or both of them is moving as the battle progresses. A dumaresq or similar device would often be used to convert the ships' movements and the target bearing to a range rate. The calculated range rate would then be used along with a range estimate to set a range clock which would then generate a succession of ranges over time. A range clock set to the correct range and the range rate would often offer its operators a fairly accurate range for several minutes.
Units of Measurement
The Royal Navy started out applying their calculated range rates by a using a man holding a stopwatch to call out intervals, at which time the range would be adjusted upward or downward by 50 yards on all sights. The units they chose to express range rate, then, were the number of seconds required for the range to change by 50 yards. This awkward convention of expressing what is essentially a speed in the vernacular of a period of time was very much a hallmark of the lack of a reasonable fire control process to integrate a present range by applying its derivative continually over time. The convention existed just slightly longer than the creation of a better apparatus for calculating a changing range; the first Vickers range clock still had their range rate set in these terms.
However, the natural rethinking occurred and the Royal Navy soon started thinking of range rate in yards per minute, opening or closing, usually rounding to the nearest 100. For example, some rates might be "opening 100", "closing 400" or "no rate". This was the practice followed during all World War I naval actions where range rate was used in fire control. The US Navy also used yards per minute.
The German Navy measured range in hectometres (hundreds of metres). Presumably, they measured range rate in hectometres per minute.
- Pollen Aim Corrector System, Part I. Technical History and Technical Comparison with Commander F. C. Dreyer's Fire Control System, p. 40.
- Dreyer, Frederic; Usborne, Cecil through Gunnery Branch, Admiralty. (1913). Pollen Aim Corrector System, Part I. Technical History and Technical Comparison with Commander F. C. Dreyer's Fire Control System. P. 1024. in Admiralty Library, Portsmouth.