Evershed Bearing Indicator
Evershed Bearing Indicators were a family of transmitters, receivers and indicators designed to communicate throughout the ship a target's relative bearing. Generally, the granularity of the bearings as read off the indicators was only sufficient (accurate to within 1 degree[1]) to establish a shared understanding of which ship was the target. In this way, the devices were really a check against problems such as a rangefinder ranging against the wrong ship or the spotter observing and reporting on another ship's fire.
Contents
History and Deployment
Technical Analysis
Much of the detailed working of the devices and their installation in particular ships is recorded in the Handbook for Fire Control Instruments, 1914[2]. Explanation of the equipment follows, but details on particular installations taken from this rich source are relegated to the articles pertaining to the ship or class of ship in question.
Power
Power for an Evershed installation came from a generator powered by an electric motor (called an isolator) which was in turn powered by the ship's mains. A reserve isolator was provided in case of failure. These generators were preferably located near the forward transmitting station[3].
Transmitters
Location
Each transmitter was generally connected to a periscope of telescope in a control position, and it would transmit the training angle of the host optical device. There were 2 types of transmitters, Type 1 and Type 2, with Type 2 also having the ability to receive.
Transmitters of either type were often found in the fore top in pairs to port and starboard, connected to sighting telescopes on pedestals. A local C.O.S. permitted a choice of which transmitter would be in service and connected to the output cables.
Either type could also be connected to periscopes with illuminated cross wires of 5x magnification in the conning and director towers. In some cases, these were single periscopes on the centre line, but sometimes they were paired for port and starboard control with a C.O.S. to select the one in use.
On some ships with a RF mounted on the gun control tower, a Type 2 transmitter would be fitted to transmit the RF's training angle[4].
Details
Each transmitter featured a control key (integrated or simply near-at-hand) which could be set to indicate Control or No Control[5], and this would be signaled to connected receivers. In this way, the indicated bearings could be disassociated from the implied imperative that the receiving station(s) should do anything with the indication, such as train to the same bearing. A tell-tale dial and pointer in the housing showed the number of receiving positions that were connected to this transmitter through the selector switch in the TS[6].
When controlling turrets, deflection can be dialed in by a hand-wheel, and a moveable zero allows for entry of drift corrections. When no such corrections were in place, the relative bearing of the local optics would be bearing transmitted.[7]
Type 1 transmitters were fitted to a telescope or periscope with illuminable cross wires. The scope could freely elevate, and could either train freely or by hand-wheel within the arc of control the transmitter was permitted.
Type 2 had an open-faced indicator for receiving. A peculiarity in design created the possibility that a large difference in signaled versus local bearing might cause the indicator to prompt a change in local bearing in the wrong direction; instruction plates were provided near the indicators to warn of the issue. Only transmitters deployed in port-and-starboard pairs rather than on the centre line that trained toward the quarter were subject to this issue[8].
Receivers
Receivers also came in 2 types, with Type 2 being fitted able to transmit as well as receive. Both types displayed the relative bearing by a divided dial and index, not unlike a compass.
Either type of receiver might be fitted in the handing room and geared to the turret trunks so they would provide a local reading of the training angle[9]. They were also wired to indicators in the gun house[10]. Those ships employing a Type 2 receiver for this purpose had a control key mounted in the centre sighting position near the trainer so he could choose Control or No Control.
Indicators
The indicators were the key to the Evershed equipment, and their name's subtle difference from "receiver" is that they gave a simpler answer than a relative bearing expressed in degrees. Instead, a galvanometer moved a pointer to right or left of a central zero mark, indicating in which direction the operator was to adjust his training to chase the transmitted bearing. As he neared the correct training, the pointer would swing to the center mark, indicating the local position was within 1 degree if the transmitted bearing.
Type 1 indicators were for use by trainers in turrets, fitted under the central sighting hood on the right hand sight of the left gun; the operator could look into the indicator with his left eye while his right was on his sighting scope[11]. The indicator, however, was illuminated only when the transmitter's control key was in Control, and was otherwise invisible. An adjustment knob allowed the operator to adjust the intensity of the illumination to suit himself[12].
Type 2 indicators were open-faced for greater general visibility. They would be situated near the officer's position within a turret so he could verify the trainer's work, and in those transmitting positions that were also required to receive[13]. A flag at the top of the device read either On or Off to indicate whether the transmitter had engaged his control key[14].
Selector Switch
A ship with a number of transmitting and receiving stations required a switching matrix to dictate which receivers were connected to which transmitters (or not connected).
A selector switch (actually, a switchboard) located in the forward TS enabled any receiving position (those containing any type of receiver or a Type 2 transmitter) to be connected to any transmitting position (those containing any type of transmitter or a Type 2 receiver). Each receiving station was represented by a rotary handle that could be clicked to a numbered position representing which transmitting station (or none at all) it should be connected to. This system provided the means for one transmitting position to send to 0 or more receiving positions, and each receiving position could connect to 0 or 1 transmitting position.
Those positions capable of both transmitting and receiving (i.e., those with a Type 2 receiver or a Type 2 transmitter) would generally have the knob representing their receiver switched to an off position. One exception cited was that a transmitting position might monitor the transmissions of another position in case it lost sight of the target, in which case it was permitted to match the indications of the station it was monitoring and thereby pass them on to its switched-in receivers. However, this daisy-chaining could not be repeated to another stage.
See Also
Footnotes
- ↑ Handbook for Fire Control Instruments, 1914, p. 32.
- ↑ Handbook for Fire Control Instruments, 1914, Chap VI (pp. 30-42), and Plates 34-48.
- ↑ Handbook for Fire Control Instruments, 1914, p. 31.
- ↑ Handbook for Fire Control Instruments, 1914, p. 30, Plate 36.
- ↑ Handbook for Fire Control Instruments, 1914, Plate 34.
- ↑ Handbook for Fire Control Instruments, 1914, p. 32.
- ↑ These corrections were necessary, as the receivers at the turrets were not connected to the gun sights, but to the turret; consequently, they would require these lateral deflections to see the target when the turret was correctly oriented for firing on the required deflection. Deflection and drift were separately entered, I think, as not all gun sights had drift correction features and thus the drift would be directed as an increment in the deflections being sent to the guns.
- ↑ Handbook for Fire Control Instruments, 1914, p. 31.
- ↑ Handbook for Fire Control Instruments, 1914, p. 30, Plate 37.
- ↑ Handbook for Fire Control Instruments, 1914, p. 32.
I am not sure what is meant by this! - ↑ Handbook for Fire Control Instruments, 1914, pp. 30-1 and Plate 38 (fig 1).
- ↑ Handbook for Fire Control Instruments, 1914, pp. 32 and Plate 38 (fig 1).
- ↑ Handbook for Fire Control Instruments, 1914, pp. 30-1 and Plate 38 (fig 2).
- ↑ Handbook for Fire Control Instruments, 1914, pp. 32.
Bibliography
Admiralty, Gunnery Branch (1914). Handbook for Fire Control Instruments, 1914. G. 01627/14. C.B. 1030. Copy 1235 at The National Archives. ADM 186/191.
