Stranger than you CAN imagine

[Hitman]

The parallax between the position of the torpedo tubes and the position of the periscope or TBT has nothing to do with the problems. That difference is automatically compensated for by the TDC. In fact, the distance between periscope and torpedo tubes, both fore and aft, were adjustable! Now why would that be? Well, fleet boat lengths varied from the 253' length of the Gar class to the 311' length of the Gato and Balao classes. The adjustable parallax parameter made the TDC equally deadly in each.

But the TDC can't automatically compensate everything. True it will take into account one side of the triangle, which is known -distance from the optics to the torpedo tube- but it still will lack the variable of distance to target in order to set the proper correction for parallax.