Click image to for a larger version fit for printing (485k).The following information is excerpted from U.S. Submarines Through 1945: An Illustrated Design History, by Norman Friedman.
Click image to for a larger version fit for printing (485k).
The two key dials of the position-keeper section of a torpedo data computer (TDC) are shown. They model the approach and attack situation. The upper dial shows the target, the lower the submarine; the line between represents the line of sight from submarine to target (postwar systems designed for attacks on submarines called this the line of sound). Outer rings show true bearing; inner ones are relative bearing. The submarine image is aligned to show the angle between the line of sight and the submarine heading; the target shows the angle on the bow, i.e., the angle between the line of sight and the target’s course. The submarine attack officer steered according to true (compass) bearing, but the progress of the approach was reflected in the changing relative bearings of target and submarine, as American submariners learned from the British during World War I.
The small symbol below the target ship symbol represents a torpedo; it shows the track angle, i.e., the angle at which a torpedo fired to hit will approach the target. The similar symbol on the bottom dial shows the corresponding torpedo gyro angle, the angle through which the TDC turns the torpedo to approach the target. The approaching submarine commander sought to minimize gyro angle while obtaining the broadest possible track angle (closest to 90 deg) to make up for possible errors in target course and speed.
Data - target course, target speed, relative target bearing, and range - were entered by hand using cranks. Own-ship data were entered automatically. The odometer dials shown here were added during World War II to existing TDCs as a field change. The TDC contained.separate angle-solver sections. Power switches could energize either the position keeper or the angle solver or both. Typically the angle solver was turned on only after a valid enemy course had been computed by the position keeper. Angle-solver dials showed the ordered gyro angle (at top) and the desired spread; the expected torpedo run was shown. Buttons could be pressed to indicate to the angle solver that the solution in the position keeper was correct.
A real TDC would have white numbers and symbols on a black background, but this version of the display is more legible.
No other navy developed a comparable instrument. The Germans and Japanese used angle solvers without position keepers (at least in the Japanese case, the device also had a timer that allowed it to dead reckon target position for indirect fire through smoke or mist). Probably because the Japanese had no TDC, they abandoned stern torpedo tubes in their later cruiser and fleet submarines on the grounds that they would require excessive gyro angles (Top). (U.S. Navy via Capt. James Patton, USN, Ret.)
These U.S. Navy diagrams show how the TDC was to have been used. At left is the pair of own- and target-course dials, showing the target and the submarine. The officer making the approach had to work out the course needed to obtain a given track angle. If the two dials were mentally superimposed (as at center), they showed the angle between the courses of the submarine and the target. For a stern shot, the necessary course angle appeared opposite the angle on the bow corresponding to the desired track angle. For a bow shot, the necessary course angle was read off 180 deg from the angle on the bow. True (as well as relative) bearings were shown on these dials because they corresponded to directions actually steered. (U.S. Navy via Terry Lindell)
Arrows on the TDC indicated torpedo course and gyro angle, as shown at right (solid arrow for bow tubes, broken arrow for stern tubes). The arrows on the target dial indicated present track angle, the angle at which the torpedo would hit if it were fired at any given moment (i.e., how good a shot was likely to be). Present track angle for a straight shot could be read off the dials by transposing the number on the outer ring opposite the submarine’s disengaged axis to the target outer ring, then reading the corresponding inner number. In this example, submarine and target are on converging courses. (U.S. Navy via Terry Lindell)
Last modified: