The Typ XXVI Walter U-boat was intended to become the most important weapon in the German naval arsenal, with performance figures that would not be found in other navies until the nuclear age. Whether it would actually have lived up to these theoretical qualities is, of course, open to conjecture. There were four Typ XXVI boats under construction when World War II ended. None were actually completed, so the true performance figures remain a matter for speculation among engineers. (And, of course, naval adventure writers.)
The actual boats would have been numbered U-4501 through U-4504, had they been completed. (There was a real U-2317, but she was a Typ VIIC/42, ordered in 1943 and canceled before completion.) The degree to which German industry had declined might be illustrated by the fact that these four boats were all that were begun out of an order for 100 units.
The projected performance figures included a submerged speed in the area of 25 knots, and a surfaced speed of at least 18 knots. A diving depth of about 1,000 feet was built into the design, though whether it could have been safely reached is open to debate. Other German designs intended to operate at such depths proved incapable of actually reaching it without serious leaking problems.
Armament would have consisted of ten 533 mm (21-inch) torpedo tubes. Four would have been located in the usual place in the bow, with six more located approximately amidships, firing aft. Access for both sets of tubes would have been from the forward torpedo room.
The Typ XXVI would have been the first German combat submarine to have it's attack center located in the control room. Previous designs put the captain in the conning tower. In the Typ XXVI, the conning tower contained only an escape trunk. The usual two periscopes would have been provided, a night periscope with a large objective lens, and an attack periscope with a very narrow head to minimize the chances of it being spotted during the day.
While the Walter turbine would have allowed a very high submerged speed for chasing down a target, or escaping an attacker, the need to carry huge supplies of the highly-corrosive hydrogen peroxide fuel limited the use of turbines. For normal propulsion, one of these boats would have used its diesel engine or electric motor. A Schnorchel was fitted, and the head would have been covered in the usual anti-radar coating and contained the most advanced radar detector available.
Plans also called for inclusion of the most sophisticated sound gear available. German designs were sophisticated, indeed. Late war German designs included phased array hydrophones, which could be electronically "steered," and became the basis for post-war American systems development. Other refinements, which were being tested at the time and might have been fielded given only a little more time, included the first wire-guided torpedoes.