Hm... hm.. hm.. did 6 more tests on Liberty Cargo (draft 8.2 meters) at TDW Torp Tutorial.
1. Torpedoes on 8.3, 7.8, 7.5 and 7.3 meters. First 3 passed under ship, 4th just hit hull and exploded.
And then started to happen strange things, torpedoes ran right where they was set:
2. Torpedoes on 7.8, 7.6, 7.5 and 7.4 - all hit hull.
3. Torpedoes on 8.2, 7.8, 7.5, 7.3 - all hit hull.
4. Torpedoes on 8.4, 7.8, 7.4, 8.4 - first and last passed under ship, 2,3 - hit in hull.
5. Torpedoes on 8.4, 8.3, 7.8, 7.5 - 1,2 passed, 3,4 hit.
Exit to Windows, launch game, 6th try:
6. 8.4, 7.9, 7.8, 7.5 - 1 passed, 3-4 hit hull.
Torpedoes hit ship exactly at 90 degrees (was shooting at bearing ~10 to get 0 gyroangle). Range 1400 meters. Speed of Liberty Cargo was 6 knots in 1st try and 5 knots in others.
In 2-5 cases i was on VIIB and first torpedo was TII (electric, second tube in game). 1 and 6 - VIIC boat. In 1-5 cases speed was set to slow, in 6th - medium.
So, magnetic pistol don't work on Liberty Cargo for sure. At least in this mission.
And still don't know why torpedoes ran on set depth after 1st try?