Just found that, on their site:
"All Type VIIs were attack boats
with a single-hull design – which is to say the pressure hull is also the outer hull (wrong, all were two-hull-boats). To withstand sea water pressure up to a crush depth of 250 meters, the pressure hull was further
reinforced with strengthening steel ribs inside the hull (? i think those rings were outside of the inner pressure hull?). Ballast was provided by several ballast tanks situated internally and externally of the pressure hull. The main tank was just below the control room, inside the pressure hull. Outside the pressure hull, external tanks were fitted at the bow and stern. These tanks however did not provide sufficient capacity, so additional saddle tanks were fitted to the sides of the hull.
Diesel fuel tanks were situated inside the pressure hull (also, but not only - fuel saddle tanks) to avoid leakages during a depth charge attack. The earliest variants were also fitted with serrated net cutters on the bow, a specification carried over from World War 1 boats, but these were quickly discontinued"
It really takes some effort to put so many mistakes into one paragraph
