Dumaresq

The Dumaresq is a mechanical calculating device invented around 1902 by Lieutenant John Dumaresq of the Royal Navy.

The dumaresq is an analog computer that relates vital variables of the fire control problem to the movement of one's own ship and that of a target ship. It was often used with other devices, such as a Vickers range clock, to generate range and deflection data so the gun sights of the ship could be continuously set.

A number of versions of the Dumaresq were produced of increasing complexity as development proceeded.

The dumaresq relies on sliding and rotating bars and dials to calculate the relative motion of the enemy ship and to convert this into a "range rate" (the component of motion along the line of bearing) and "dumaresq deflection" (or "speed across" .. the component perpendicular to the range rate). Because the dumaresq is an analog or model of the relative motion of the two ships, it does not intrinsically favor which of its settings is an input and which is an output. This allows it to be used "backwards" -- a process called a "cross cut" -- to take sequential estimates of the range and bearing of an enemy vessel and discover its speed and heading that would be consistent.

The design of the dumaresq consists of a circular dial with a cross-bar passing over the centre which is oriented to match the heading of one's own ship. A sliding assembly can be moved backwards along a scale etched on this bar to indicate the ship's speed in knots. Suspended below the slider is a second bar, which recorded the speed and heading of the enemy ship by rotating and sliding against a similar scale to that on the main cross-bar. The result of these two settings are such that the tip of the enemy bar records enemy movement minus own movement as a vector sum. This is equivalent to the relative motion of the target ship.

The base disc of the Dumaresq features a graph which can be rotated along the line of bearing. When so aligned, the axis along the line of bearing indicates the range rate and the perpendicular axis indicates speed across. A pointer stem dangling from the enemy ship bar allows the values to be easily read off in convenient units (in 1902, range rate was expressed as the number of seconds required for the range to alter 50 yards, but was soon standardised on yards per minute)

The mark I Dumaresq was manufactured by Elliott Brothers, who paid for and obtained a patent on the device in the name of its inventor, John Dumaresq, in August 1904. By 1906 the device had been amended to add a rifle-like sight for directly obtaining a bearing to the target ship. By 1913 approximately 1000 devices of various versions had been purchased by the Royal Navy at a cost of £10,000.

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