Deflection Calculator

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Deflection Calculator Internals.[1]
Deflection Calculator showing Link Gear[2]
Dreyer Calculator, in use[3]

The Deflection Calculator was a British Fire Control Instrument that positioned a Dumaresq abreast a Wind Dumaresq with link gear to calculate Gun Deflection from both, passing these to a Deflection Totaliser arranged below with a transmitter to send the aggregate deflection onward.

Genesis and Nature

The Deflection Calculator was first mentioned around 1917, and was to be situated not in the T.S., but in gunnery control positions. It performed a good many of the deflection-related tasks tackled by Dreyer Fire Control Tables.

Inputs and Mechanical Design

A mechanical telltale in a slot on each dumaresq was worked by the handles for setting the Dumaresq and Wind deflections on the Totaliser. Curiously, the gun range for the two dumaresqs was set by separate handles.

Application and Use

The device was used as follows:[4]

  1. set the dumaresqs as usual
  2. set both range handles so that sliding pointers show gun range on the range scales.
  3. work the yellow dumaresq deflection handle on the totaliser until the dumaresq's telltale is aligned with its pointer. The dumaresq deflection pointer will then show dumaresq deflection corrected for range.
  4. work the purple drift deflection pointer to the required drift deflection.
  5. work the green wind deflection handle to place the wind dumaresq's telltale opposite its pointer; this establishes the proper wind deflection corrected for range on the totaliser's wind deflection column
  6. work the blue transmitting and spotting handle until the lower spotting pointer is opposite the total gun deflection pointer; this transmits the total gun deflection

Spotting corrections were applied by use of a third pointer on the final totaliser column which was frictionally connected. It would be zeroed and then the blue handle worked to apply the desired correction.

See Also


  1. Manual of Gunnery Volume III, 1920, Plate 18.
  2. Manual of Gunnery Volume III, 1920, Plate 19.
  3. Manual of Gunnery Volume III, 1920, Plate 21.
  4. Manual of Gunnery Volume III, 1920, p. 18.