The British captured a German submarine, the U-570 in August 1941. Following repairs, she was brought into Royal Navy service as HMS Graph on the 19th September 1941. A long period of trials and analysis followed, as the RN, and later the USN, tried to learn everything they could about the U-boat and its performance. Once every scrap of information had been wrung out of her, Graph made three war patrols. The first started on the 8th October 1942, and took her to the Bay of Biscay, where she unsuccessfully engaged U-333, another U-boat returning from patrol. On her next, she tried and failed to find an Italian blockade runner. Her final patrol took her to Norwegian waters, where she attempted to attack German ships returning from the Battle of the Barents Sea. Following this, a shortage of spare parts and an increasing number of defects, led to her being removed from active service.
The RN also used a captured Italian submarine, the Galileo Galilei. Captured in the Red Sea, she was brought into RN service as the X-2, and served as a training ship, and as a battery charging station for submarines at Port Said.