without refueling from a tender or U-boat tanker. The VIIC came into service toward the end of the "First Happy Time" near the beginning of the war and was still the most numerous type in service when Allied anti-submarine efforts finally defeated the U-boat campaign in late 1943 and 1944. Type VIIC differed from the VIIB only in the addition of an active sonar and a few minor mechanical improvements, making it 2 feet longer and 8 tons heavier. Speed and range were essentially the same. Many of these boats were fitted with snorkels in 1944 and 1945. They had the same torpedo tube arrangement as their predecessors, except for , , , , and , which had only two bow tubes, and for , , , , , and , which had no stern tube. On the surface the boats (except for , and to which used MAN M6V40/46s) were propelled by two supercharged Germaniawerft, 6 cylinder, 4-stroke M6V 40/46 diesels totaling at 470 to 490 rpm. For submerged propulsion, several different electric motors were used. Early models used the VIIB configuration of two AEG GU 460/8-276 electric motors, totaling with a max rpm of 296, while newer boats used two BBC GG UB 720/8, Garbe, Lahmeyer & Co. RP 137/c or Siemens-Schuckert-Werke (SSW) GU 343/38-8 electric motors with the same power output as the AEG motors. Perhaps the most famous VIIC boat was , featured in the movie Das Boot. U-flak "Flak Traps" The concept of the "U-flak" or "Flak Trap" originated on 31 August 1942, when was seriously damaged by aircraft. Rather than scrap the boat, it was decided to refit her as a heavily armed anti-aircraft boat intended to combat the losses being inflicted by Allied aircraft in the Bay of Biscay. Two 20 mm quadruple Flakvierling mounts and an experimental 37 mm automatic gun were installed on the U-flaks' decks. A battery of 86 mm line-carrying anti-aircraft rockets was tested (similar to a device used by the British in the defense of airfields), but this idea proved unworkable. At times, two additional single 20 mm guns were also mounted. The submarines' limited fuel capacities restricted them to operations only within the Bay of Biscay. Only five torpedoes were carried, preloaded in the tubes, to free up space needed for additional gun crew. Four VIIC boats were modified for use as surface escorts for U-boats departing