The submarines were powered by two Germaniawerft supercharged four-stroke, six-cylinder diesel engines producing a total of 2,800–3,200 metric horsepower (2,060–2,350 kW; 2,760–3,160 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/38-8 double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged.

In addition, the boats were equipped with a small bakery in order to provide the luxury of fresh bread for crews being resupplied.

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.

[1] The milk cows operated 300–400 miles (480–640 km) off the North American mainland in the so-called mid-Atlantic gap, far enough from Allied anti-submarine patrols and land-based aircraft while still close enough to provide logistical support to U-boats.

[5] 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.

This intelligence, coupled with improved Allied radar, air coverage, and hunter-killer groups in the North Atlantic, eliminated most of the milk cows during 1943 including four lost in the month of July alone.

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.