Unless I'm misreading this, the error is in adding two things together. One method is to guess the speed, use an angle-off. Example target moving left to right at 10 knots, lead angle about 12 degrees. Aim scope straight ahead, send range and bearing to TDC. SPEED SET TO ZERO.
That sets the fish to a zero gyro angle, they head straight out with no turns. Turn scope 12 degrees left (348 degrees relative) and OPEN OUTER DOORS (also important, otherwise there's a delay and the fish will miss behind). When target is in crosshairs fire, target and torpedo meet dead ahead of sub.
Now.
If you estimate speed at 10, set AOB to 90 and set speed at 10 in the TDC, along with using angle-off, the TDC will set the gyro angle to 12 degrees right, you're firing with the scope 12 degrees left, add the angle-off aiming of 10 knots to the TDC speed of 10 knots, the effect would be a target speed of 20 knots - so you miss ahead of a 10 knot target every time.
Use one method or the other, adding them together results in the wrong speed and therefore wrong firing angle.
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