Radke was killed in a fire while the boat was undergoing trials on 14 December 1941.
German Type VIIC submarines were preceded by the shorter Type VIIB submarines. U-657 had a displacement of 769t when at the surface and 871t while submerged. She had a total length of 67.1m (220.1feet), a pressure hull length of 50.5m (165.7feet), a beam of 6.2m (20.3feet), a height of 9.6m (31.5feet), and a draught of 4.74m (15.55feet). The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of NaNPS for use while surfaced, two Siemens-Schuckert GU 343/38–8 double-acting electric motors producing a total of 750PS for use while submerged. She had two shafts and two 1.23abbr=onNaNabbr=on propellers. The boat was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, the boat could operate for at ; when surfaced, she could travel at . U-657 was fitted with five 53.30NaN0 torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one 8.82NaN2 SK C/35 naval gun, 220 rounds, and a 2sp=usNaNsp=us C/30 anti-aircraft gun. The boat had a complement of between forty-four and sixty.
The boat's career began with training at 8th U-boat Flotilla on 8 October 1941, followed by active service on 1 March 1942 as part of the 3rd Flotilla in La Pallice, France. Four months later, she transferred to 11th Flotilla, in Bergen, Norway, for the remainder of her service.
In seven patrols she sank one merchant ship, for a total of .
U-657 took part in six wolfpacks, namely:
U-657 was sunk on 17 May 1943 in the North Atlantic, E of Cape Farewell, Greenland, in position 58.9°N -75°W, by depth charges from Royal Navy frigate . All hands were lost.