[UKönig]

Paraphrased and summarized from one of my U-boat texts, here's what they have to say about the supply submarine...

In the early war, Uboats had been resupplied at sea by surface tankers, but as the war dragged on, these were quickly hunted down. From Sept. 1940 to April, 1941 Germany lost 5, one of which checked in at over 10,000 tons. By Dec. 1941 tanker losses had become so severe that they were withdrawn from service until another means could be developed to resupply U-boats while at sea.
The answer seemed to be the supply submarine, affectionately known as 'the milch cow'. These Type XIV (15) boats were 1,688 tons, had a range of 12,300 miles and carried 720 tons of diesel fuel in addition to their own supplies. Ten of these boats were laid down in 1941 and came on-stream in 1942.
Tasked with meeting up with patrolling u-boats at prearranged co-ordinates, the milch cow could restock a U-boat with fuel, torpedoes, and/or other ammunition, fresh food, drinking water, medical equipment or supplies, even evacuate injured sailors and swap with viable crewmembers.
By far, the largest number of boats operational was the Type VII, 517 tons, and with a limited range of about 7,000 miles. But the distance from Lorient to Halifax is about 2,500 miles and to New York, about 3,000 miles, and when assigned to those areas, the type 7s had barely made it to their patrol billets, when they were forced to return.
The Milch Cows were to remove or reduce this handicap, and when sinkings declined in the American coastal waters and patrols were assigned to areas farther away, thanks be to the milch cows that allowed the type 7s to continue battle operations.