Staff Writer (Updated: 4/14/2016):
The Type XXI hull was of all-welded construction and pieced together from eight preconstructed major sections. The vessel featured an extremely streamlined shape from bow to stern with the elongated fin conventionally jutting out amidships. The Type XXI separated itself from its predecessors by implementation of several vital amenities including improved food refrigeration storage, crew showers, integrated air-conditioning and air recycling systems. Armament consisted of twin automatic turrets mounted on the sail as these could be comprised of either 4 x 20mm cannons or 4 x 40mm cannons help forward and aft for anti-aircraft defense. Primary anti-ship armament was 6 x forward-facing 533mm torpedo tubes with 23 reloads. There were no stern-facing torpedo tubes common to other submarines of the war.
The heart of the Type XXI was the propulsion system which was made up of 2 x MAN M6V40/46KBB supercharged 6-cylinder engines, 2 x SSW GU365/30 double-acting electric motors and 2 x SSW GV232/28 "silent running" electric motors. Active and passive sonar systems were the power behind the potency of the Type XXI and supplemented periscope use when aiming, targeting and firing the torpedoes. This addition alone brought about a new age of submarine warfare consistent with philosophies still in use today.
The Type XXI U-boat series began with the U-2501 and ended with the U-3530. These types of German U-boat vessels were also internally known as "Elektroboote" (or "electronic boat") due to their heavy use of electrically-powered machinery. Two variants were proposed in the Type XXIB and Type XXIC models for the class. The former would have increased the forward torpedo tubes to 12 total while the latter would have seen this increased to a unbelievable total of 18 tubes. The increase in forward armament would have necessitated the elongation of the hull to make room for the added weaponry. The two variants, however, were never produced.
So pivotal was the Type XXI to German Navy operations in the Atlantic that it was given precedent over all other submarine designs in construction towards the end of the war. The target production goal was to have some 1,500 examples in service - a number which no doubt would have devastated Allied navy and marine efforts in the region - and a target production goal of of three submarines per week was envisioned. Though construction of the 118 Type XXI's began as early as 1943 and lasted into the final months of the war, the vessels never had time to achieve full operational capabilities and thusly never reached its full potential in wartime. Like other German late-war developments, this breakthrough design fell into Allied hands once the submarine pens and manufacturing facilities were taken over. Some Type XXIs saw service after the war.
Construction of Type XXI submarines was handled by the German concerns of Blohm & Voss, AG Weser and F. Schichau shipyards.