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Light Cruisers No. 26 Northampton, 13 June 1930
  " 27 Chester, do
  " 28 Louisville, 13 March 1931
  " 29 Chicago, do
  " 30 Houston, 13 June 1930
  " 31 Augusta, 13 March 1931

1. Transmission - Selsyn System for operation on 115 volts, 60 cycles, single phase. Secondary voltage is to be nominally 90 volts.

2. The battery consists of nine 8" - 55 caliber guns mounted in 3 mounts. Full charge M.V. is 3000 f/s and for reduced charge is 2300 f/s. A special target practice velocity of 2700 f/s is also used.

3. For directing the battery there are available two gun di rectors, one forward and one aft, and two Marie VI range keepers. The range keepers are located in the plotting room.

4. The battery may be used entire or may be divided into two; the one including one or two forward triple mounts under control of forward director and No. 1 rangekeeper, the other consisting of the remaining triple mount under the control of the after gun director and the No. 1 rangekeeper. When the battery is used entire either director may direct the fire with the assistance of either rangekeeper.

4. Four methods of fire are available:


6. The point of reference in train is the center of rotation of the gun director in the after main battery control station.

7. The plane of reference for vertical parallax is the horizontal plane through the mean trunnion height of the 8" enclosed mounts.

8. The forward director is provided with a parallax mechanism for both vertical and horizontal parallax.

9. Each gun director is provided with a pointer's and a trainer's sight. A pointer's handwheel turns a pair of selsyn generators to communicate gun elevation to the guns and the rangekeeper. A trainer's handwheel trains the director and turns a pair of Selsyn generators to transmit the target bearing to rangekeeper and elsewhere as necessary.

10. The pointer's sight is provided with.a movable prism or mirror in its optical system whereby the elevation of the line of sight may be adjusted to and maintained in the proper relation to the elevation of the guns. This prism is under remote control from the rangekeeper and the pointer does not ordinarily control directly the elevation of his line of sight, though means are provided whereby it may be set to known sight depression in accordance with the range, by hand, in emergency conditions, ("secondary" and "auxiliary" control).

11. The Mk. VI range keeper contains, in addition to the rangekeeper proper:

12. The range keeper is set by hand for:

It receives automatically:


Plate 1


Ballistic data as to the motion of the projectile is incorporated in the ballistic computer, which is set for one of three initial velocities. The ballistic computer does not compensate for errors due to variation in initial velocity.

13. The range keeper generates range and bearing and solves the fire control problem. It computes D.C. and transmits to the controlling director the director correction which will place the director pointer's line of sight at the correct angle of elevation with respect to the guns for the existing advance range and cross roll error. It modifies the observed relative, target bearing from the controlling director by adding deflection and cross roll correction in train, transmitting the resultant gun train order to the guns.

14. The reason for the method of controlling the elevation system used in the Consolidated System is that the trunnion tilt (cross-roll) compensation originates in the range keeper and, since it varies continuously as the ship rolls and pitches, it was thought preferable that the range keeper should be permitted to weave the line of sight up and down' through the small angle involved rather than to weave the guns up and down.

15. It was not considered feasible to avoid weaving the guns back and forth in train, so the cross roll correction in train was applied direct to the gun train order.

16. The graphic plotter automatically draws curves of present range, advance range, true target bearing, and true gun train.

17. The gyroscopic unit contains a "level" gyro and an "angle" gyro. The level gyro's axis is maintained vertical by applying precessional forces as necessary to level up a system of bubbles mounted in a small auxiliary gimbal system located at the upper left side of the range keeper. In case of casual ty to the level gyro, these bubbles are kept level and used as standard. The level gyro furnishes the range keeper with the cross-level angle. It also furnishes a level reference for firing the guns from the rangekeeper in indirect fire. The level point may be checked accurately against the horizon by "marks" given by the director pointer.

18. The "angle" gyro is a standard of reference in train, its axis pointing always horizontally in the direction of the target. The bearing of the gyro axis is the generated Relative Bearing, and is transmitted through busses on the switchboard to target designators on the directors. It may also be used in plane of observed relative bearing to obtain gun train order (indirect fire). The angle gyro is free of the errors of a north-seeking gyro, and is therefore available for use during


indirect fire at times when the gyro compasses would not be reliable. However, provision is made for using the gyro compass as a standard in case of trouble with the angle gyro. As an additional feature the angle gyro, in conjunction with a bearing integrator, functions as a gyro compass, and may be compared with the ship's gyro compass by means of a dial.

19. The range keeper as a gun director. When the target is obscured, or casualties occur, control may foe shifted to the rangekeeper, and indirect fire used. The-rangekeeper will transmit to the guns the generated relative target bearing (controlled by the "angle" gyro), plus deflection and cross-roll correction. The director pointer operates a crank which sets the gun elevation order into the rangekeeper and transmits it also to the guns. The range keeper computes the director correction just as it does when the top director is in control, but in this case the director correction is visually compared with level angle from the level gyro, by means of a "firing point dial", which enables the pointer to change the gun elevation order until the level angle and the director correction will be equal at some part of the roll. The battery may be fired when these two angles are equal by an automatic firing device, (provided a hand firing key is closed), or independently by the director pointer when the firing point is reached, as shown by the dial. In either case the level gyro is used as the standard.

20. The rate control mechanism provides a means for automatically correcting the target's course and speed so that the components of the target's motion along and across the fire as resolved by the rangekeeper from the course and speed as set will agree with the observed motion along and across the line of fire. If the generated bearing strays off from the observed, it is only necessary to wait until it has moved off for a period of one minutes, then to push in and turn a crank until the generated bearing again equals the observed, which is indicated by pointers. The rate control mechanism will simultaneously change the target's course and speed settings in such a way as to correct the bearing rate without changing the range rate. Similarly, the range rate may be corrected by the rate control mechanism, which will change the target's course and speed set tings so as to correct the range rate without varying the bearing rate.

Operation of the System.

21. The director trainer keeps his sight on the target by training the director and incidentally transmits the target bearing to the range keeper as basis for the gun train order that finally issues therefrom. In the range keeper the corrections for cross leveling and deflection are applied.


22. The angle of train of the gun mounts, corrected far horizontal parallax, is transmitted mechanically to the gun train indicators and combined in an appropriate manner with the angle of train transmitted from the range keeper, so that when the zero reader dials in the gun train indicators read zero, the gun mount is trained to the desired angle of train; The angle of train of the gun mounts, corrected for horizontal parallax to the point of reference, is transmitted electrically to appropriate dials in the mount train indicators.

23. The range function of horizontal parallax is transmitted mechanically from the sight setter's station to the local gun train indicator.

24. The director pointer observes whether his line of sight is too high or too low and if he judges it to be so, turns his handwheel to make the appropriate change. The turning of the handwheel does not move his sight directly but moves the gunerators that communicate gun elevation order to the guns and the range keeper. The guns respond directly to his signal by elevating or depressing as required. The range keeper responds by transmitting to the pointer's sight and applying thereto, by direct remote control, a new director correction corresponding to the new gun elevation and to the existing range and trunnion tilt. Thus the elevation of the guns is controlled by the director pointer himself arid cannot be disturbed by changes in range or by trunnion tilt compensation as the ship rolls and pitches. Presumably the director pointer will not disturb the elevation of the guns unless the accumulated change of range or list makes it necessary in order that his sight may come on the target properly. In any case he will know when they have been disturbed so as to refrain from firing at that instant.

25. For emergency use means are provided whereby the moving prism or mirror in the pointer's sight, can be coupled through suitable gearing with his handwheel and the Selsyn generators that transmit gun elevation. In this connecting gearing will be provided a range adjustment governed by a range drum whereby a sight setter can set sight depression on the director. It will thus be possible to use the director in the same manner as the "Tickers' Director is used if for any reason the range keeper is unable to exercise control over the pointer's sight. Under the latter condition ranges will be taken from the Battle order indicator or from the local rangekeeper arid set on the directors.

26. Gun elevation order indicators (1 speed) at the directors enable the director operator to match in elevation with the controlling director, to facilitate shifts in control.


27. The angle of elevation of the guns, corrected for inclination of the roller path and loss in muzzle velocity due to erosion will be mechanically transmitted to the gun elevation indicators.

28. The angle of train of the main battery directors or of the anti-aircraft range finder mounts is transmitted electrical ly to target designators at the searchlight operating gear stations. Provision for modifying this angle of train as desired by the illumination control officer is made by means of an arbitrary deflection device (electric differential) in the search light battle order transmitters in the A.A. Control station. These battle order transmitters also transmit battle orders for searchlight control to battle order indicators located near searchlight operating gear stations.

29. Provision is made on the switchboard to control the train of all searchlights from either main battery director or starboard lights from starboard A.A. R.F. mount and port lights from port A.A. R.F. mounts.


1 Gun Director Mark XXIV (foremast)
      (To mount spotting glass Mark VII and Target Designator Mark V)
1 Gun Director Mark XXIV-1.
2 Range keepers, Mark VI
2 Target Designators, Mark V
2 Gun Battle Order Transmitters, Mark XVII
1 Target Designator Mark IT (2 exposed R.F. mount)
10 Battle Order Indicators Mark XVII (indicate range from 0 to 35950 yards)
3 Gun Train Indicators Mark XII-1
3 Gun Elevation Indicators, Mark XI-I
5 Mount Train Indicator, Mark I, Mod. 2
      (Mounts 1 and 2, plot and each MB control)
4 Mount Train Indicators, Mark II (Mount -fo, plot, and each M.3. control)
6 Bearing Indicators, Mark I-I
2 Battle Order Transmitters, Mark XVI (Stbd) (A.A. control stations) and Mark XVI-1 (port)
4 Target Designators, Mark IV-II (Searchlight gun operating stations).
2 Battle Order Indicators, Mark XVI (Stbd) and XVI-1 (port) Searchlight operating station.
7 Exciters, Mark III
1 Switchboard
2 Gun Elevation Transmitters, Mark I (Near each range keeper)
2 Tube Train Order Transmitter (O.S. 437) (On torpedo directors)
2 Torpedo Target Bearing Transmitters (O.S. 437)
2 Tube Train Indicators, Mark III (stbd) and Mark III-1 (port)
3 Motor generators for gyros.
2 Sets, Input and Output units for range keepers, consisting of Selsyn motors, transformers, etc.
3 20 volt Dynamoters.
      To provide current for running the range keepers.


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Transcribed and formatted by Thomas Wildenberg