The Type XIV U-tankers carried four torpedoes for resupply to other boats. They were stored below the wooden planks on the upper deck. It was time consuming and difficult to transfer them to other U-boats - obviously only possible in fine weather - and the "world record speed" for transfer of a single torpedo was about 20 minutes. On one occasion, a heavy torpedo fell out of its lines and crashed hard onto the saddle-tanks of the U-tanker, causing severe local damage to the diving cells (but no explosion).
All the U-tankers had constant worries about maintaining proper trim and buoyancy. They were pumping out light oil and replacing it with heavier water. So they got heavier as the mission went on. The designers had assumed that transfer of other supplies (food, drinking water, torpedoes) would compensate. But, in practice, very few U-boats wanted the torpedoes. So the U-tankers ended up taking out disposable iron ballast that they would jettison as the mission proceeded.
The U-tankers also could carry out minor repair work, and carried a ship's doctor. They really were floating, very small, bases and were very popular with other U-boat crews. The on-board bakery apparently created a fantastic bread - wonderful for those who had become accustomed to eating mouldy bread.
Stiebler.
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