[Hitman]

Hitman - Where are you getting your details of the torpedo director from?

It's a long story, I was involved long ago in a mod about british subs that finally didn't see the light. I have still some stuff, including diagrams, etc.

Here's a part of a russian book that CCIP translated:

Despite certain successes achieved by British submariners during the First world war, in general the attitude towards this type of vessels remained negative at the Admiralty. Not the least role in this was played by the invention of ASDIC in 1918, which admirals decided was a panacea for subs in future wars. Coupled with the predominant policy of economy in naval spending during the early 1920s and 1930s, this led to the British submarine service entering the new world war with the most primitive TDC, compared to those in US and German fleets.

The electromechanical "Torpedodirector" was founded on the principle of calculating the submarine's course for the moment of attack. This was realized through an intricat system of logarithmic scales. The only element entered electrically into the device was the sub's own course from the main gyrocompass. The TDC was not connected to the torpedo tubes. Changes in the running depth of torpedoes were made manually by torpedo operators via special devices, installed on the tubes themeselves, but changing torpedo speed without removing them from the tubes was impossible. Gyro angles could be corrected without removing torpedoes from tubes, but the device only calculated solutions for straight or "angle" shots (90 degrees).

The device itself worked in the following fashion:
During the first periscope observation, the torpedo officer would enter the bearing and estimated heading of the target on the commander's orders.
Based on this the device calculated the target's angle on bow and the relative position of the sub to the ship (this was shown visually on the two rotating gauges in the centre of the device). Then the height of the target's masts and the angle of their rise calculated by marks on the periscope. Distance was determined in this way. Based on all earlier-entered data, the device calculated traverse distance - i.e. the distance at which the target will pass the sub if it maintains the current course. Seeing this, the commander could decide where and at what speed he should move for conducting the torpedo attack.

In the last stage, the device calculated the attack course. For this the speed of the torpedoes and the length of the target were entered (the speed of the target was estimated by comparing bearings, later in the war - by counting screw turns on the hydrophone). If neccesary, by turning a handle the device could be set up for an "angle shot". After enering all neccesary numbers, the image of the submarine on the lower gauge "doubled" - the upper, lighter image matched the current position, and the lower - the one neccesary to achieve the attack course. All that remained was to read the angle from the scale on the side of the device and pass it to the helmsman. After the submarine was on the correct course (which could be judged by the two images coinciding) the commander read the lead angle from another scale on the side, turned periscope to that angle and waited for the target to enter the crosshairs of the periscope.

[footnote - due to the amount of gauges and scales the TDC was nicknamed "fruit machine". This is how submariners called a gambling device which today is commonly called the "one-armed bandit" (i.e. slot machine).]

Relatively simple and light (weight 52kg; size 487x645x368mm) and visual device, in practice it had many deficiencies. Only one parameter was entered into it automatically, with the rest having to be set by hand. The absence of a position keeper meant that after each periscope observation all numbers had to be reset. In combat conditions, there was often insufficient time for this, as a result of which many commanders had to calculate solutions in their head without consulting the "torpedodirector". A large number of data was calculated not by the device, but subjectively, which increased the risk of error. And finally, the need to take a strictly determined course (attack course) took up time, which could often not be afforded in combat.

All of the above deficiencies led to the fact that the degree of success with single-torpedo attacks was less-than-satisfactory. There was a hurried effort to develop methods for solving this situation. The answer turned out to be very obvious - switching to salvo fire, but because the torpedodirector could not set gyro angles, then the best method - firing "fan shots" - was unavailable. The solution was found in the stop-gap method of firing in intervals. This is how this method was characterized by A.V. Tomashevich in the "Collection of materials on combat experience of the USSR Navy" No.6 (1943):

"Launching torpedoes one after another, the British continued fire during the time required for the target ship to pass a distance equal to one, two or three times its own length. From this there were three firing options. The first was called concentrated fire, the second - scattered A, and the third - scattered B. With concentrated fire, if the target motion parameters were calculated correctly, all of the torpedoes launched would hit the target. If there were errors in the calculation, there will be hits only if the margin of these errors did not exceed half the length of the target ship. The aiming point was the bow of the ship. Therefore the middle torpedo (out of those fired) would hit the center of the ship. This firing method was prescribed for attacks from short distances and when the target's motion was calculated correctly enough in the commander's opinion.

In the case if the target's motion parameters were calculated correctly while using the scattered firing method A, the ship would be hit by 50% of the launched torpedoes. If there were errors, there would be hits only if the margin of these errors did not exceed the length of the target ship. The aimpoint was switched from the bow of the target to halfway between the bow and midships. This method was used when the target's motion parameters could not be calculated accurately enough.

With a correct firing solution using the scattered firing method B, the ship would be hit by 33% of the launched torpedoes. Errors in the solution could not exceed 1.5 lengths of the target ship. The aimpoint was switched to midships. This method of firing would be used in attacks from long distances, when the target calculations could contain serious errors.

In all scenarios, only the minimum number of torpedoes to be fired was determined: with concentrated fire - no less than two, with scattered A - no less than three, with scattered B - no less than four. This norm, in the British understanding, guaranteed a hit with at least one of the torpedoes. Otherwise the commander himself decided how many torpedoes to fire in each individual scenario.

Firing calculations were made using three tables. The first table determined which method should be used in the given situation, the second - to calculate the length of the overall torpedo firing interval, depending on the number of torpedoes fired and the length of the target, and the third table allowed to calculate the intervals between individual torpedo launches based on the overall firing interval and target speed"

Thus, in addition to the device which required eight manual settings, there was also a set of three tables. Only in modified versions of the "torpedodirector" which started to appear from early 1943 there were automatic calculations for recommended time interval depending on earlier-entered target length, distance, speed and the number of torpedoes to be launched. That said, since all the torpedoes followed the same course, if the target evaded one torpedo it evaded the entire salvo. It seems that in 1944 the British gained the capability to enter gyro angles between 0 and 90 and switched to using fan shots, however no descriptions of the accompanying TDC devices could be found. It is most likely that the dispersion angles were entered manually by the torpedo operators themselves, although the latter seems to have been done by British submariners rather rarely.

The British TDC system turned out to be the most primitive compared to those used by Germany and the US. Many of the parameters had to be obtained visually and entered manually. The weakness of the firing apparatus could only be compensated by very professional and disciplined work of the crew - which judging by the overall success of British submarine attacks was exactly the case.

Diagram on that book (Including translation of terms by CCIP):