Dumaresq Deflection
Dumaresq Deflection (or Speed-Across) is the angular difference between a gun's orientation in bearing and the line of sight to the target. It was sometimes referred to simply as Deflection, although that can be an ambiguous term used to refer to Gun Deflection or Dumaresq Deflection. Gun deflection's was intended to place the shell on target after factors that cause lateral deviation of shell or target during time-of-flight.
Naval gun sights were designed to permit the sighting telescopes to be angled in pitch and yaw so that proper elevation and deflection could be established to hit the target while it was centred in the telescopic sights. These angles were usually established by sightsetting equipment.
Related Instruments
A dumaresq or similar device would often be used to convert the relative motion of the firing and target ships and the target bearing to a range rate and a dumaresq deflection. The dumaresq deflection could then be converted to a Gun Deflection suitable for aiming by reference to a graphic chart that related the two variables at any given [[Gun Range]|gun range]].
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.
See Also
See Also
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