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Type XIV Milch Cows (supply boats) - U-boat Types - German U-boats of WWII - Kriegsmarine - uboat.net

The size of these boats and their intended purpose resulted in the nickname 'Milch Cow'. They were built to resupply the fighting U-boats in the Atlantic and thus had no offensive power, only anti-aircraft weapons for defence.

German U-Boat Type XIV (Milchkuh Milk-Cows) - history, specification and photos

Because of their role as supply U-Boats, the Type XIV was nicknamed "milk-cows" (milchkuh). They acted as force multipliers wherein a network of supply U-Boats would replenish operational boats with the much needed torpedoes, food, fuel, and other provisions. They also carried a doctor onboard and a bakery which could provide freshly baked bread. In effect, the Type XIVs enabled operational boats to remain much longer in their patrol zones, significantly increasing their presence.