Dreyer Fire Control Table
The Dreyer Fire Control Table was the Royal Navy's highest-level Fire Control instrument during World War I.
Bearing plot, deflection drums and deflection totaliser on left (Wind Dumaresq obscured behind this equipment), Mark VI* dumaresq with integral range and bearing clocks beneath gyro-compass repeater and Forbes log in centre, and range plot, gun range counter and spotting corrector on right.
Spotting Corrector, Range plot and range plot grid on left, emergency manual drive hand-crank and stop-watch governor housing in centre (dumaresq, etc in rear), axle and chain drive to actuate bearing plot is prominent, and the wind dumaresq is visible here with the bearing plot, bearing grid, and deflection drums beyond. Though the Mark III table was introduced in 1912, this image depicts it after it had ripened through the addition of several accessories, most notably by the addition of deflection drums, deflection totaliser, wind dumaresq, and an improved range plot typewriter. These improvements typified the continual refinement process the Dreyer tables' open-faced nature seemed to welcome.
The tables existed in various Marks, though they were not developed in the order of their Roman numeral Mark numbers (Mark III was first). The Dreyer tables were based on semi-automated plotting of range cuts and bearings versus time on separate sheets of paper and employing a dumaresq and other appliances to relate these data, guess their derivatives, and compute a continuous range and deflection for use at the guns and/or the director.
For many decades, Dreyer tables have been a scapegoat for poor shooting by the Royal Navy. A technical evaluation suggests that this criticism is misdirected.
Overview
Dreyer Fire Control Tables were early mechanical computers ("calculating workbenches" might be a better term) meant to process data to permit a ship to engage a distant target with heavy artillery. They were fairly involved pieces of equipment, and grew more intricate between the invention of the first prototype table in 1911, their first deployment on dreadnoughts in 1912 (?) and the creation of the Mark V table in 1918. The dreadnoughts of the Grand Fleet relied on Dreyer equipment at Jutland to convert sporadic and imprecise estimates of range and bearing into workable firing parameters. Sadly, the level of success was spotty, as visibility conditions made such systematic data collection too occasional for performing the sort of graphical analysis on which the Dreyer relied.
The Dreyer FCTs were literally sturdy iron tables fitted with a number of fire control devices tied together by rotating shafts, bicycle chains and other linkages, worked by 7 or more men simultaneously as a corporate endeavour. They were housed deep within the ship in the Transmitting Station (T.S.) located beneath the armoured deck. By the time the Mark V table was fitted in H.M.S. Hood, as many as 30 people might occupy the T.S., working the table and its many ancillary devices or serving as liaisons to the fighting positions of the ship. It is fair to suggest that the Dreyer and its environment and attendants resembled a premonition of the Mission Control centres of the Apollo program 50 years later: a human/machine system on a broad scale to factor down a torrent of real time sensory data to create a manageable environment for exerting command at a rate tolerable to humans.
Dreyer Table as CPU in a Ship/Computer
It is not possible to study the Dreyer tables without developing a familiarity with the ship-wide art of fire control, the process of calculation and articulation by which the shells can be made to rapidly and continually fall in a pattern around a distant maneuvering enemy. The Dreyer's role was akin to that of a CPU within a modern computer system, and its "socket" was the T.S.
Examining a dreadnought as a computer system is the best way to develop this understanding. Just as a CPU processes input received via keyboard, mouse, and network adapter, the Dreyer table was given data on what the enemy appeared to be doing. And, just as a CPU might generate output on a screen or printer, the Dreyer's output peripherals were powerful naval guns. Without paying too much attention to the input and output devices, let's examine the T.S.'s role in the system by drawing a circle around it and observing which inputs and outputs crossed this membrane.
| Inputs to a Mark III Dreyer Table, c1918 | |||
| Information Received | Source | Input Mechanism | Nature of Data |
|---|---|---|---|
| Estimates of Target Heading/Speed | Visual estimates | Manually | sporadic, authoritative |
| Range Rates | from Rate Officer, aloft | Verbally | sporadic, authoritative |
| Estimates of Target Range | Optical rangefinders | Manually | sporadic, imprecise |
| Observations of Target Relative Bearing | Optical device | Automatically | sporadic, granular |
| Spotting corrections | Verbal reports | Manually | fairly often, authoritative |
| Own ship's heading | Gyro-compass | Automatically | continuous, fairly precise |
| Own ship's speed | Forbes Log (a speedometer) | Manually | continuous, fairly precise |
| Apparent wind speed | Anemometer | Manually | continuous, fairly precise |
| Apparent wind direction | Wind vane | Manually | continuous, fairly precise |
| Adjustments to range (for Wind Along, etc) | Dreyer Calculator | Verbally | occasional, consistent |
A Dreyer table delivered the only 2 outputs a gunner with a modern gun sight needed to hit a target he could see.
| Outputs from a Mark III Dreyer Table | ||
| Output Generated | Source of Data | Nature of Data |
|---|---|---|
| Gun range | Range clock + accrued spotting corrections | continuous, 25 yard granularity |
| Gun Deflection | Computed totals + accrued spotting corrections | continuous, "1 knot" granularity |
Primary Components
Dreyer tables' differed in their particulars by model and by refinements applied to each over time (which were seldom heralded and are difficult to track reliably), but they generally had 4 primary components and some number of ancillary subsystems.
Range Plot
The range plot was a semi-automated scatter plot of optical range estimates versus time. A wide expanse of paper (46-in wide paper in a Mark IV* table[1]) scrolled away from the observer on a flat surface at a constant speed, and as range cuts from rangefinder topside arrived, he plotted them on the paper at the appropriate position as measured by a graduated range scale arranged horizontally across the paper at its near edge. The means of plotting these points evolved over time. In some early tables, the plotting from a single rangefinder was entirely automated. This was soon withdrawn in favor of the use of Commander John Brownrigg's eponymous keyboard, which saw use through Jutland before being replaced by the range typewriter which could allow up to 9 rangefinders to have their data plotted, each with a distinctive mark.
A range rate grid positioned over the plot could be moved over the recent range data samples and a skein of wires within it deflected to what the operator perceived was a linear trend suggested by their alteration over time. In this way, the range plot was a means for allowing the operator to visually see through the errors in the rangefinder data while also taking the derivative over time. This linear correlation provided a workable instantaneous range rate to establish on the table's range clock.
Also marking the face of the range plot's sheet was a pencil in a carrier on a worm screw that could be moved to any position near the range scale under the influence of the table's range clock. This range pencil could trace a continual hypothesis of the clock range (the current range to the target). The operator was able to manually position the pencil so it would be within the recent ranges being reported should the hypothesis and observational data diverge significantly. On some Dreyer tables (or added to some after their initial installation), a second pencil could be offset laterally from the first (often from a device called the Spotting Corrector) by the accrued spotting corrections and the corrections of a separate Dreyer Calculator so it would plot the gun range.
Bearing Plot
The bearing plot was similar to the range plot, but generally smaller in size (16-in wide paper in a Mark IV* table[2]) and it plotted bearings corrected for yaw versus time. In most of the pivotal battles, the bearing plots in use were widely criticised and seems to have been a true shortcoming in the Dreyer tables[Citation needed].
The shortcomings of the bearing plot used through Jutland seemed to be these:
- the relative bearings taken were transmitted in coarse 1/4 degree steps[3].
- the gyroscopes and the relays conveying their indications were prone to wander[4].
- the paper plotted 120 degrees across its width, producing for moderate bearing rates subtle angles difficult to measure confidently[5].
- a lack of integration to automatically pass gun deflections onward
A comprehensive and compact solution to at least some these issues appeared in the form of the Standard Bearing Plot documented in the 1918 Dreyer Table handbook, which
- coincided with a change to a finer quantization (4 arc-minute[Citation needed]) of gyrocompass data
- coincided with adoption of Sperry gyroscopes which wandered less[Citation needed]
- added a Deflection Totaliser which acted like the Spotting Corrector did for range and established an proper mechanism for the output of an aggregate gun deflection
However, the "Standard Bearing Plots" never lived up to the ubiquity implied by their name, as Gyro Director Training Gear was being conceived by war's end[Citation needed].
Dumaresq
A dumaresq situated on the table could relate range rate, speed-across, target bearing, and the speed and heading of the two vessels in a manner that permitted the operators to divine the data they lacked from those they had in hand, or to judge the plausibility of the data they were receiving. The dumaresq might convert an initial assessment as to enemy bearing, speed and heading into a range rate for initial use. Taking the indicated speed-across and a range estimate could allow the operators to reference special charts or deflection drums and arrive at an initial gun deflection.
Most vitally, the relationships a dumaresq modeled could permit a good firing solution against a non-maneuvering enemy to be maintained even while one's own ship maneuvered and range and bearing reports slacked off. The more feature-rich Dreyer tables automated the way in which the dumaresq could fulfill these purposes.
Range Clock
A clock-work or electrically driven range clock was an essential tool for reducing the workload for fire control and to permit a reasonable representation of the firing scenario to persist during temporary interruptions in visibility. All Dreyer tables possessed a range clock of some kind, and the better tables increased the ease with which the clock was kept in agreement with the dumaresq's configuration or even allowed an electric dumaresq to set the clock's range rate automatically.
The range clock's output was directed toward a Spotting Corrector which in turn drove the plotting pencils.
Bearing Clock
Most Dreyer tables also had a bearing clock which would help keep the target bearing updated on the dumaresq. In a simple installation such as a Mark III table, this clock applied a constant bearing rate to the present bearing, and the operator would periodically update this rate by hand. In a fancier installation, when the bearing rate grid was correctly aligned with the bearings visible on the plot below, the bearing clock was automatically set by the same action and the corresponding gun deflection entered into the first column of the deflection totaliser. As ever, the later and larger Dreyer tables smoother out the edges progressively while the smaller and earlier designs generally needed more attention to meet the task.
Spotting Corrector Gear
The range clock drove this differential gear, and through it, the red clock range pencil drew on the range plot. A dial and differential within this box, however, permitted incremental spotting corrections for range to be applied to this figure, along with a separate little addend from a Dreyer Calculator found on the transmitting station's bulkhead. In later Dreyer tables, a second gun range pencil was added to the range plot. A digital gun range counter similar to an old odometer sat on the table and provided a continuous display of the gun range. On a very late model Dreyer table, the clock range might also be brought to the standard bearing plot to rotate the deflection drums as needed and to set the change of bearing gear for gun range.
Distribution of Dreyer Fire Control Tables
As of June, 1918 Dreyer Fire Control Tables were allocated as shown:[6]
See Also
Footnotes
- ↑ Captain F.C. Dreyer's Fire Control Apparatus, Mark IV., sheet 11.
- ↑ Captain F.C. Dreyer's Fire Control Apparatus, Mark IV., sheet 11.
- ↑ Brooks, John. Dreadnought Gunnery and the Battle of Jutland, p. 171.
- ↑ Brooks, John. Dreadnought Gunnery and the Battle of Jutland, p. 171.
- ↑ Captain F.C. Dreyer's Fire Control Apparatus, Mark IV., sheets 44-5 & Fig. XII.
- ↑ Handbook for Capt. FC Dreyer's Fire Control Tables. p. 3.
Bibliography
- Template:BibUKDreyerTableHandbook1918
- Template:BibUKDreyersFCSystemForLocalControl1913
- Template:BibUKDreyerTableMarkIVRoyalOak1916
- Template:BibUKPamphletDreyerTableMarkIII*1930
- Admiralty, Gunnery Branch (1930). Pamphlet on the Mark IV* Dreyer Table. O.U. 6196 (C). Copy at Admiralty Library, Portsmouth, United Kingdom.
