Like that of the Type II, the design of the Type VII seagoing boat had export origins in a Finnish-built series of 19301 (the 'Veteranen' class) and, beyond that, in the UB III of 1918, To permit the greatest number of hulls to be built within the ceiling tonnage agreed, size was severely limited in the 10 Type VIIA boats (626/745 tons). With performance and offensive capacity optimized, conditions aboard were somewhat spartan even with internal space saved by mounting the after tube in the casing (where it could be reloaded only with difficulty and then on the surface) and by the stowage of spare torpedoes and part of the bunker capacity externally (where they were vulnerable to depth charging). The Type VIIB and Type VIIC were, therefore, stretched to increase internal volume to rectify some of the shortcomings and to allow more powerful diesels to be fitted, a significant factor in surface operations. This modified boat was highly successful, nearly 700 units being built in various sub-variants until the war's end. Later improvements included greater operational depths, reinforced towers, enhanced AA armament and snorts, all features reflecting developing Allied antisubmarine procedures. Significantly, most lacked a deck gun as surface operations became impossible.
While mines configured to the standard 533-mm (21-in) torpedo tube could be laid by all German submarines, these weapons could not guarantee a sinking as opposed to disablement. ' To lay the largest moored mines, therefore, six Type VIIs were stretched by the addition of an extra 10-m (32.8-ft) section amidships containing five vertical free-flooding tubes, each containing three complete mine assemblies, These tubes protruded upward to 01 level into an extended tower. The class was known as the Type VIID, A further four boats, the Type VIIF sub-class, were similarly lengthened, with the additional space given over to spare torpedoes for transfer to extend the operational duration of other boats. Up to 25 torpedoes could be carried but transfer operations with both boats temporarily immobilized on the surface became increasingly upopular and were abandoned. The Type VUE, a study in improved propulsion, never progressed beyond the drawing board.
Specification Type VIIC Type: sea-going submarine Displacement: 769 tons surfaced and 871 tons submerged Dimensions: length 66.50 m (218 ft 2 in); beam 6.20 m (20 ft 4 in), draught 4.75 m (15 ft 7 in) Propulsion: surfaced diesels delivering 2,800 bhp (2089 kW) and submerged electric motors delivering 750 hp (559 kW) to two shafts Speed: surfaced 17.5 ktsand submerged 7.5 kts Range: surfaced 15750 km (9,785 miles) at 10 kts and submerged 150 km (93 miles) at 4 kts Armament: one 88-mm (3.465-in) gun, one 37-mm AA gun, two (later eight) 20-mm AA guns, and five 533-mm (21 in) torpedo tubes (four forward and one aft) with 14 torpedoes Complement: 44