Type XIV submarine
The Type XIV U-boat was designed to resupply other U-boats, being the only submarine tenders built which were not surface ships. It was nicknamed the "Milchkuh/Milchkühe (pl.)" (milk cows) or U-Tanker. Design The Type XIV was based on the Type IXD long-range U-boat design, but with a shorter and deeper hull. The boats had a displacement of when at the surface and while submerged. The U-boats had a total length of , a pressure hull length of , a beam of , a height of , and a draught of . The submarines were powered by two Germaniawerft supercharged four-stroke, six-cylinder diesel engines producing a total of for use while surfaced, two Siemens-Schuckert 2 GU 345/38-8 double-acting electric motors producing a total of for use while submerged. They had two shafts and two propellers. The boats were capable of operating at depths of up to . The submarines had a maximum surface speed of and a maximum submerged speed of . When submerged, the boats could operate for at ; when surfaced, they could travel at . The boats were not fitted with torpedo tubes nor deck guns. The only armament was for defense, consisting of two SK C/30 anti-aircraft guns with 2500 rounds as well as a C/30 gun with 3,000 rounds. The boats had a complement of fifty-three. Operation Due to its large size, the Type XIV could resupply other boats with of fuel, of motor oil, four torpedoes, and fresh food that was preserved in refrigerator units. In addition, the boats were equipped with a small bakery in order to provide the luxury of fresh bread for crews being resupplied. The Type XIV also had a doctor and medical facility for injured sailors, and even had a two-man brig to imprison sailors awaiting discipline back at home. Type IXC boats otherwise only carried 12 weeks of food supplies, and Type VIIC U-boat carried about 114 tons of diesel fuel. Cargo was transported by means of a inflatable boat and portable cranes. The flat main deck with cargo hatches and davits was designed in theory to facilitate the transfer of bulk supplies, however its low freeboard made this work extremely hazardous in typical North Atlantic swells that made the deck awash, so often supplies had to be hand-lifted through the smaller but drier conning tower hatches to avoid flooding the boat. Resupply and refueling operations often took hours, putting both the milk cow and the submarine it was servicing at risk. If the Germans came under Allied attack during a resupply operation, the milk cow would dive first while the attack submarine might fight it out on the surface for a while, as the Type XIV's bulk and flat deck made it slower to maneuver and submerge, although it could dive deeper than Type VIICs or IX. The Type XIV had no torpedo tubes or deck guns, only defensive armament of anti-aircraft guns. The milk cows operated off the North American mainland in the so-called mid-Atlantic gap, far