Originally Posted by derstosstrupp
So there are some common misunderstandings about what the German TDC could do. And unfortunately the SH series doesn’t get the German one quite right. It does however not force you into a static gyro, even SH series gets that right.
The German computer indeed did not have a position keeper, but what it could do, when told to, was update the AOB in the computer for every degree of bearing change, which is what the SH series gets right. This was called “Lage laufend”. It would also update the gyro angle continuously based on this as well. What the SH series does not get right is, on the most common model of German TDC during the war, the model S3, it had connectivity to the gyrocompass, and could take your own course changes into account as well. What that meant was, once the switch for this function was switched on, the U-boat could change course at will, and as long as the optics were kept on target, the target AOB would be correct. After all, it’s all just angles. The range dial really only was there to introduce the correct parallax correction on the Gyro angle. This of course only really matters when the gyro angle is greater than 20° or so, especially at close ranges. Range only matters because of the lateral separation of tubes and optics - if you shot torpedoes literally out of the lens of the optics, there would be no range dial on the computer.
Major disadvantage of the American computer compared to the German one, was the lack of direct connectivity between optics and computer. The American system required a bearing update to be introduced by hand, whereas the German computer received bearings continuously from the optics. The implication here, and the real advantage of the German one, was that fire could be shifted rapidly between targets, and the bearing would not need to be updated by hand for the subsequent target. AOB of course updated automatically with the bearing shift as well (“Lage laufend”).
Position keeper was an incredible tool to refine the solution, but when it came time to actually shoot, in practice, the Germans simply just kept their gyro angle low, making range irrelevant. And Lockwood stressed this practice anyway to his American skippers, so really at the end of the day the biggest value of the American computer was the ability to refine the data all the way up to the shot (by comparing observed bearing/range with generated). At the point of the shot, the Germans had the clear advantage, especially when engaging multiple subsequent targets in a convoy.
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