Unterseeboot 553

Unterseeboot 553 (U-553) was a Type VIIC submarine of the Kriegsmarine. Her keel was laid down 21 November 1939, by Blohm and Voss of Hamburg. She was commissioned 23 December 1940, with Kapitänleutnant Karl Thurmann in command. Thurmann commanded her for her entire career.

U-553 conducted ten patrols, sinking 13 ships totalling 64,612 tons and damaging two others totalling 15,273 tons.

On 25 April 1941 major engine trouble forced U-553 to return to base.

On 20 January 1943, U-553 sent a radio message: "sehrohr unklar" (periscope not clear), and was never heard from again. She had suffered no casualties to her crew until lost with all hands.

Neal Stephenson's novel Cryptonomicon includes a fictitious U-553 which runs aground in a fictional location near the northern British Isles. It is possible that Stephenson picked the number of the boat not for historical accuracy but rather as the product of two primes, 7 and 79.