...crosspost...
Yep.. some boats were lost without a clear reason. In some books i have you can read about many ways to sink a sub in the later years.
From bast shoes in the conning tower hatch, over to loose screws on the exhaust caps and flood tank caps and so on...
Afaik it was U123 (Hardegen) who returned to Lorient after one day. Boat was far away from being watertight!
