Reflections The Magazine of the C S FORESTER SOCIETY ISSN 2042-1389 Number 25 – June 2013 http://csforester.eu Book-of-the-Year The Ship : the Transmitting Room, Italy not amused, PENELOPE in the Arctic, the early Editions of The Ship , AGM 2013, The Prisoner of Trafalgar, Hornblower’s Seamanship. The Transmitting Station in The Ship Elementary Disquisition on Naval Gunfire Control John Forester 1 The Problem does not tell the man at the gunsight how rapidly to One warship is shooting at another warship. Both ships change the elevation or the direction of his gun. Elaborate are moving; they may be a mile apart, or twenty miles calculations are required to convert the available apart, moving in any direction at any practical speed. It is information into gun aiming instructions. useless to shoot at where the target ship is now, because Mechanization started in 1902, when an RN officer, John by the time that the shells fall into the target zone, the Saumarez Dumaresq, invented an instrument for target is no longer there. The shooting ship must be able mounting on the bridge, with sights for training it on the to calculate where its target ship will be at the time when target. That gave target bearing relative to own ship’s its own shells arrive at the target. At the time that Artemis direction. Own ship’s speed was set by changing the was in action, the machinery to solve this problem had length of a bar fixed directly fore and aft, the direction of been invented, designed, and improved over forty years, movement of one’s own ship. and was the most complicated set of machinery ever On the distant margin of the instrument, out where the devised. (Nowadays, all the difficult parts are done by image of the target was seen, was another bar electronics.) representing the target ship. This bar could be set at the angle that the target ship showed; how far it was pointed 2 The Data Available coming toward or going away from one’s own ship. This The own ship knows its direction and its speed. It can bar could also have its length set, for the speed of the easily measure the angle (bearing) from its own direction target ship. The linkage between the two bars moved to that of the target. It can measure the range to the pointers over a graph that converted this information into target. That would enable it to hit the place where the more useful information: speed at which the distance target is now, but that’s useless. It also has to know the between the ships was changing and the speed at which target’s direction of movement and its speed. But that info the target ship was moving across the line of sight. Dumaresq Mark VIII from a 1946 handbook ( http://www.hnsa.org/doc/br1534/ ) June 2013 Reflections 25 2 The Dumaresq illustration shows the parts in color. The The line between the two dials indicates the line of sight gray bar above the blue bar is fixed parallel to the between them (it is not drawn on the table), the nearer dial centerline of the ship, with direction of movement indicates own ship angle to the line of sight, the further indicated by the small black arrow in the background. The dial indicates the angle of the target ship. Some data are big black arrow indicates the line of sight to the target, with provided electrically, but most are input by operators the sights shown at its ends. The yellow slider and pivot is turning handles, and all the calculations are done by moved along the gray bar to indicate the ship’s speed, as machinery. shown on the scale. The blue bar and gearbox keeps That was done by the Admiralty Fire Control Table constant the target ship’s inclination even when one’s own machinery down in Artemis’s Transmitting Station, ship turns. The red pointer represents the target ship, its presided over by the Commissioned Gunner, Mr. Kaile. angle represents the inclination and its length the target’s The ton or more of machinery was built into a table, estimated speed. The operator reads the point on the around which sat its operators. The Director Control graph directly below the tip of the red pointer. The lines Tower, in which sat the Gunnery Lieutenant and his parallel to the line of sight give the target’s speed across assistants, sent down information on bearing, range, the line of sight. The lines perpendicular to the line of sight enemy course, enemy speed. The ship’s own systems give the rate of change of range. provided own course and own speed. The bearing was The orange roller visible through the slot carries graph accurate; it was easy to measure. Range was less lines. When the roller (a different roller for each type of accurate, and also came from another range finder in gun) is set to the current range, the curves give the angle turret B. Enemy course was estimated by the view of the which the gunsights must lead the target (deflection angle) target provided to the inclinometer operator in the DCT. for the speed across at that range. The rate of change of Enemy speed; that was a large assumption at the start. range told the gunner how rapidly his guns must be After shooting started, spotting information showed elevated or depressed to stay on target. The speed across whether the salvos were falling short, over, or straddling needed an additional calculation. The gunner also needed the target. range to the target, and, once that had been measured, dividing the speed across by the range gave the rate of change of bearing, the rate at which the gun turrets must be rotated to keep the guns on target. 3 The Machinery & Its Operation However, installing a range finder and a Dumaresq (the US Navy called its similar instrument a resolver) on the bridge and giving the gunner a slide rule to divide range into speed across was not sufficient. It was insufficiently accurate, too slow, and it did not do the other calculations required. Besides, it provided only a snapshot of information, with no history of how the information was changing. And it could not provide prediction, for salvo after salvo, as the battle went on, even though the target might become invisible. In World War I, the Royal Navy’s Fire controll table of HMS Belfast (picture from Wikipedia) Dreyer Table, in the transmitting station deep inside the ship, provided this information by plotting ranges and rates The machinery had many parts. There were gears that of change of range and bearing on moving paper graphs, added or subtracted two shaft positions and sent the so that the officer in charge, by analyzing the graphs, output to a third. These were called differentials, because could decide which information best represented the they worked just like the differential gears in a car’s driving target at that moment, and send the information to the axle. There were linkages that divided one shaft position gunners. by another, as in dividing speed-across by range. There At the close of World War I a completely mechanical were precision-cut cams that converted range into gun system was developed by the American engineer elevation, or range into time-of-flight of the shells, for the Hannibal Ford. All future fire control systems, until particular shells in use. The most delicate were the mechanical computation was overtaken by electronic, integrators, requiring extremely delicate maintenance and descended from the original Ford system. The adjustment. Just as the odometer of your car shows how Royal Navy called its version the Admiralty Fire Control far you have traveled, no matter at what speeds you have Table, built in various versions for different types of ship. been driving, an integrator sums up all the different The photograph of HMS Belfast’s AFCT shows the key speeds and times that have been put into it, but with the concept. Near the center of the AFCT are shown two ship- far greater precision required for gunnery. shaped rotatable dials. June 2013 Reflections 25 3 Think of a rotating disk, like a phonograph disk, but up to Mr. Kaile, conducting his assemblage of smooth, and running at a very precise speed. Set on this instrumentalists, to decide which changes in input are disk is a ball, held in a cage. The cage runs on rails so most likely to close the differences between calculated that it crosses the disk from one side to the other, directly values and measured values. He may decide to change over the center of the disk. If the cage is moved to the far the target’s inclination angle, in which case the side of the disk, the ball will rotate one way. If the cage is inclinometer table operator turns his cranks to follow the moved to the center of the disk, the ball won’t rotate at all. order. Mr. Kaile orders his adjustments to try to close the If the cage is moved to the near side of the disk, the ball difference between calculated values and measured will rotate in the other direction. Thus the speed and values. When he gets very close, he can telephone the direction of the ball’s rotation is exquisitely controlled by Gunnery Lieutenant, “Table tuned for deflection.” From the ball’s position on the rotating disk. Then set on top of then on, there is cooperation between the two.