Regular Uboats in WW-II had 18.5mm thick pressure hulls made of ST52 steel. That allowed diving depth of 100m with max depth of 150m and crush depth calculated at 250m. Since the war boats were already risking diving to that crush depth to escape depth charges, they moved to deeper depth.

For the Type VIIC/41 they moved to 22mm thick pressure hulls of ST 52 steel and this allowed the figures to rise to 120m dive, 180 max dive and crush depth of 300m.

For the VIIC/42 design the pressure hull moved to CM 351 steel which was a special armored steel and combined with still thicker pressure hull [28mm?] they claimed crush depth of up to 500m.Since they only made 2100 tons of this metal per year they could only fabricate ~20 Pressure hulls of this size per year!

How deep would the thermocline layer be in the North Atlantic in winter and summer? To evade ASDIC at this time all you really need is to be below the thermocline and turn on the speed.

Looking at Rosslers UBOAT book, its clear that the underwater speed of these boats was severaly hampered by the conning tower design and all the weapons mounted. Had they took Walters fish boat designs and applied them, they could have increased the under water speed to 10 knots, just by removing the armaments and adopting an enclosed bridge. Move from the 375 electric genetrators to the 580 genetrators [developed for the Type XX-III] and they could have increased speed to almost 12 knots underwater, which was the top speed of the Type XXIII electro boat.

Then all they had to do was to swap out internal hull space for more battery cells to enlarge the endurance. Since it was clear by mid war that reserve torpedos were not really being used since their was little chance of reattacks, then replace the reserve torpedo space with Battery cells. There is enough space to double the battery capacity this way. Next remove 1/2 of the fuel bunkers and cut the deisel range in half , which should give enough space to triple the battery capacity of the Type VII UBOAT. That should give you about 150-160nm @ 4knts, not far below the 175nm @ 4knts for the Type XXIII boat [roughly 39 hours compared to 44 hours @ 4knts] .

Historically increasing the underwater speed by 50%, doubled the Uboats survival against surface escort attacks. Combind with earlier schnorkel deployment, this should reduced the air threat to a minimum level.