Here's what happened, in short: near Malta, I was jumped by a destroyer in the storm. I dived and realized I was almost in the middle of a small convoy. When I escaped, i follow them. The weather cleared and I identified the ships as Aquitania and another smaller troop ship, doing some crazy course changes, almost running in circles. I positioned myself in front for an attacked, when they changed course again. Aquitania ran directly overhead, and we touched.

I quickly turned and fired full salvo at her stern from 500m. Her draft is 14m, so i set the depth to almost 15. At least one torpedo must have exploded, but nothing happened. Then I watched Aquitania getting sank by the Luftwaffe. They didn't bother to attack two destroyers who were pounding me nearby.
So I created a single mission with Aquitania, to see what did I do wrong. After many fired torpedoes, I came to the conclusion that
her draft is around 12.3m, not 14m as stated in the recognition manual.
The test and the actual attack were done in dead calm seas.
Can someone confirm this?