Dreyer Calculator

The Dreyer Calculator as Proposed
Visible are the means of setting the Air Density, Range Rate, Plot Range, and Wind along the line of fire. The range spotting correction is read off of scale H. Only the grayish piece N is peculiar to the gun system in use, containing a "Wind Curve" (label obscured in this image by compositing) at the top, which affects the impact of Air Density and Wind on the output and a "Time of Flight Curve" at the bottom which affects the Range Rate's contribution.

Dreyer Calculator showing action

Dreyer Calculator, in use

The Dreyer Calculator was a British Fire Control Instrument that determined the aggregate effect on gun range attributable to a number of factors. It was invented by John Tuthill Dreyer, brother to Captain Frederic Charles Dreyer and worked in concert with his brother's fire control table.

Genesis and Nature

The Dreyer calculator was a small calculating board that eventually enjoyed a place on the bulkhead of Royal Navy Transmitting Stations. It calculated several separate factors that would cause the clock range and gun range to differ and summed them together to produce an aggregate spotting correction for range.

It appeared in 1908, along with definite mention of its adoption and manufacture.

Inputs and Mechanical Design

A wooden case with a carry handle was envisioned, and the full size drawing seems to hint at dimensions of 33cm by 55cm with a depth of 65cm or so.

The calculator was customized to a given Gun System by the choice of which backplate (bearing the range scale) was fitted. Milled curves along these backplates expressed ballistic relationships taken from the Range Table, such as range to time-of-flight.

The inputs were:

• Range Rate
• Air Density (difference from the standard atmosphere of the range table)
• Wind along the line of fire
• Plot Range

By altering the plot range with a large knob, the backplate's "time-of-flight vs range curve" would interact with the range rate and a "wind vs range curve" would influence the degree to which air density and wind along affected the output.

Application and Use

In ships with Dreyer Fire Control Tables, this correction would be entered as a straddle correction. into the Spotting Corrector to be summed with clock range and accrued spotting corrections for range.

As the battle progressed, it was of course common for all the inputs (except air density) to change. It would be a fairly manual task for someone to keep the Dreyer calculator updated and to feed the changing result back to the Dreyer table's spotting corrector. I can imagine that ships lacking a tool similar to the spotting corrector would forego use of a Dreyer calculator and simply rely on spotting to account for these dynamics.

The Grand Fleet Dreyer Table Committee found that in practice the Fleet's Dreyer calculators' output was being handled in a variety of ways by the various personnel. Focusing on ships equipped with Mark IV tables and later:

• nine ships had a worm shaft by which the calculator operator could shift the zero of the Spotting Corrector on the table, for follow-up by its operator
• five ships applied the correction only initially, relying on spotting corrections thereafter
• three ships applied the correction due to own ship's motion only, in deference to a lack of confidence in tracking enemy movement
• two ships did not use the calculator's indication at all

The calculator continued to have a role in British fire control alongside Mark III* and IV Dreyer tables into the 1930s, and probably into World War II.

See Also

• Wind Dumaresq
• Ballistics
• Range Table
• Transmitting Station
• Dreyer Fire Control Table

Bibliography

• Template:BibUKFireControl1908
• Template:BibUKDreyerTableHandbook1918
• Template:BibUKReportsDreyerTableCommittee1919