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The torpedo control system as installed on destroyers and light cruisers provides means for solving the problem of torpedo fire at a distant station and transmitting the resulting angles to the torpedo tubes, so that the tubes may be properly trained, or the gyros set, to make a hit at the target with the torpedo.

The installation differs somewhat on different ships but consists essentially of the following component elements:

The director described is the Mark XXV designed and manufactured by the Naval Gun Factory. The Mark XXVI is now in service but does not differ greatly from the Mark XXV. Any of the self-synchronous transmission systems could be used in the torpedo control system. This description deals with the Arma transmission system and tube train indicators as installed on some of the old destroyers and light cruisers.

Definitions. (See Plates 3 and 4)

1. Point of Aim.

The particular point on the target at which the (line of sight) is directed when firing.

2. Line of Sight.

The axis of the director telescope prolonged when it is directed at the point of aim.

3. Point of Intercept.

The intersection of the track of the torpedo and the track of the target ship.


4. Sight angle.

The angle between the line of sight and the virtual torpedo bar (torpedo track when gyro angle is used) of the director with the director correctly set. It is the angle by which the torpedo tube must be trained from the line of sight to secure a hit when using straight fire.

5. Target Angle.

The angle between the course of the target ship and the line of sight. It is measured clockwise from the target bow from 0° to 360°. It is relative bearing of own ship from target.

6. Tube angle.

The angle between the longitudinal axis of a barrel of the torpedo tube and the fore and aft center line of the firing ship. It is measured from the firing ship's bow from 0° to 360° clockwise.

7. Gyro Angle.

Gyro angle is used when it is desired to fire a curved shot, It is the difference between the desired course of the torpedo and the longitudinal axis of the tube at the time of firing.

8. Spread Angle.

In firing a salvo of torpedoes, they may be given divergent courses, forming a spread. The angle between succeeding shots is the unit of spread.

9. Tube Offset.

The groups of tubes may be individually offset from the torpedo bar by angles varying from 0° to 26° right or left. This offset is used in salvo firing.

10. Torpedo Director.

The torpedo director is the instrument by which the triangle of torpedo fire is quickly solved.

11. Torpedo Bar.

Torpedo bar is an imaginary line thru the pivot of the torpedo director which represents, by its length and direction, the speed and track of the torpedo in the triangle of torpedo fire.


Plate 3


Plate 4


12. Torpedo Salvo.

A torpedo salvo is a number of torpedoes fired approximately simultaneous from a firing unit at the same target.

13. Torpedo Range.

The distance, in yards, along the track of the torpedo from the point of fire to the point of intercept.

14. Maximum Effective Range.

Maximum range, in yards, that the torpedo can run. The torpedo range must be less than the maximum effective range.

The Mark XXV Torpedo Director differs considerably from the previous conventional designs in which the solution of the torpedo triangle was accomplished mechanically by setting the enemy course, enemy speed, target angle arid torpedo speed upon sliding bars and dials. The Mark XXV Torpedo Director solves the triangle graphically by the use of three logarithmic scales and the sight angle curve as shown on Plate 5. The quantity desired is sight angle, and is obtained by the logarithmic solution of an oblique triangle of which two sides (torpedo speed and target speed) and one angle (target angle) are known or estimated.

The calculator of the director solves the triangle of torpedo fire in the following manner:

There are three log scales on toe director, (1) target speed scale from 1 to 50 knots, which is fixed, (2) torpedo speed scale from 25 to 50 knots and (3) a target angle, scale. Scales (2) and (o) are on the same piece of metal and can be moved relative to scale (1). By operating the speed setting hand wheel the log scales (1) and (2) are moved relative to each other and by matching torpedo speed with estimated target speed, part of the equation, log Es - log Ts, is solved as on an ordinary slide rule. The sight angle setting handwheel is now turned, which moves the sight angle curve relative to the log scales. The sight angle curve is a log sine curve and is matched with the estimated target angle on the target angle log sine scale. This gives the solution of the triangle and the answer is given on the sight angle scale at the top. In practice it is


not necessary to read the sight angle, since in matching the sight angle curve with the target angle scale, the telescope is offset from the virtual torpedo bar by the proper sight angle. This gives a quick and accurate solution of the torpedo fire triangle.

Since the telescope is offset from the torpedo bar by the correct sight angle, training the telescope on the target brings the torpedo bar to bear on the point of intercept. A tube train order, combining target bearing and sight angle is automatically transmitted to the tubes where it is received upon an indicator as the required tube train order.

The torpedo director is equipped with two sets of target-speed, target-angle, torpedo-speed and sight angle scales, one on each side of the director. The torpedo-speed and enemy speed scales are the same on both sides. The target angle scale on the right is marked from 1° to 179° and on the left from 181° to 359°. The sight angle drum on the right is marked from 0° to 70° and on the left "from 290° to 360°. This allows the director to be installed on either side of a ship without any changes in design.

If it is desired to fire a spread, the telescope may be offset right or left by operating the spread-angle knob. This moves the telescope relative to the rest of the director by the desired amount, but does not introduce any change into the train order transmission system unless the director is trained. Each notch or click on the spread-angle worm shaft represents 1/2 degree of spread. Having selected a unit of spread, the director operator offsets the director by the proper number of degrees by turning the spread-angle knob the required number of notches or clicks.

Divergent tube train orders may be transmitted to the groups of tubes by operating the tube offset angle setting knobs until the desired offset angle is shown on the tube offset angle drums. This rotates the stator of the transmitter so that the tube train order going to the indicator is modified by the proper amount. The torpedo tube will therefore be trained at an angle equal to the sight angle plus or minus the offset angle when zero readers are matched.

A latitude correction for the gyros is provided in the tube train order transmission system to compensate for errors of the gyros which would be present when firing in latitudes other than that in which the gyro was calibrated. The correction is introduced thru the staters of the transmitters, by setting the proper correction on the latitude correction dial. This moves the tube offset index of the transmitter and when the stator is moved to line up with the index mark, latitude correction is applied to tube train order.


The transmission system is of the self-synchronous, alternating current type. Two tube-train transmitters are mounted on the torpedo director, one being electrically connected to the tube train indicator on the forward tube, the other to the aft tube on the same side of the ship (destroyers) . The rotors of the transmitters are directly geared to the fixed pivot of the director and when the director is trained the rotors turn thru definite angles. The motor shaft in the indicator at the tubes moves in synchronism with the transmitter. Since the stator of the transmitter is moved to take care of sight angle, spread angle, offset, and latitude correction, the tube train order as sent to the indicator, is the angle which the tube must train to line up with the vertical torpedo bar of the director.

The tube train indicator (see plate 20) receives and indicates the quantities tube angle and gyro angle as transmitted by the director and indicates tube train on a mechanically operated dial. It is connected electrically to one of the transmitters on the torpedo director, and is mechanically geared to the torpedo tube stand. The indicator is mounted upon a bracket secured to the torpedo tube.

The construction of the instrument is such that when the director is set and trained, an indication is received as to the direction and amount that the tube must be trained to coincide with the virtual torpedo bar of the director, or if curved fire is being used, it indicates the gyro angle that is to be set on the torpedo gyro. The tube is properly trained when the zero reader pointer is caused to match its index.

The arrangement of the dials of a port indicator is shown in Plate 20. At the top is the tube train dial, graduated uniformly thru 324° of arc. The starboard indicators are marked from 0° to 180° and the port from 180° to 360°. The indicators are therefore not interchangeable as are the directors. This dial is mounted on the stator of the motor, and since the stator is geared to the tube stand, when the tube is trained the motor is rotated. The tube train index is fixed to the indicator frame and the tube train dial is read against this index, indicating the relative train of the torpedo tube. The director train dial is graduated the same as the tube train dial and is also carried on the stator. The zero reader pointer and gyro angle dial are carried on the rotor of the motor. The rotor is electrically connected to the transmitter of the director and rotates in accordance with the tube train order sent out from the director. As the zero reader pointer turns, the window over the director train dial moves, uncovering the angle of director train. When the tube is trained to match the zero reader pointer, both rotor and stator rotate, so that when the pointer is matched with the index, the Director train dial and tube train dials will read the same.


The gyro angle dial in later instruments is graduated differently from that shown in Plate 20. Instead of having 270° for the zero position the dials have 0° for the zero position. When using straight fire the zero of the gyro angle dial will be matched with the fixed index. For curved fire the mean angle set on the torpedo gyros is matched with the index line.


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Transcribed and formatted by Thomas Wildenberg