Type XIV submarine

The Type XIV U-boat was a modification of the Type IXD, designed to resupply other U-boats, being the only submarine tenders built which were not surface ships. They were nicknamed "Milchkuh/Milchkühe (pl.)" (milk cows).
Design.
German Type XIV submarines were shortened and deepened versions of the Type IXDs. 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 3000 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 bakeries 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. Cargo was transported by means of a 20-foot 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 dryer 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 other 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. The Type XIV had no torpedo tubes or deck guns, only defensive armament of anti-aircraft guns.
The milk cows operated 300–400 miles off the North American mainland which was the so-called mid-Atlantic gap, far enough from Allied anti-submarine patrols and aircraft while still close enough to provide logistical support to U-boats. In 1942, the milk cows enabled Type VIIC boats to remain on station for a couple more weeks off of the American coast during the "Second Happy Time" raids of the Battle of the Atlantic. Type IXC boats otherwise only carried 12 weeks of supplies.
The milk cows were priority targets for Allied forces, as sinking one milk cow would effectively curtail the operations of a dozen attack U-boats and force them to return home for supplies. Ultra intercepts provided information concerning sailing and routing, and this, coupled with improved Allied radar and air coverage in the North Atlantic, eliminated most of them during 1943 including 4 lost in the month of July alone. By the end of the war all ten had been sunk. Milk cow duty was especially hazardous; 289 sailors were killed out of an estimated complement of 530–576 men.
List of Type XIV submarines.
Ten boats of this type were commissioned:
Fourteen planned Type XIVs were cancelled. Three of them ("U-491", "U-492", "U-493") were about 75% complete when work was stopped in 1944. The other eleven boats had not been laid down when they were cancelled on 27 May 1944. On that same day Karl Dönitz stopped construction on the Type XX U-boats, large transport boats that would not have been ready until mid-1945.