German submarine U-691

Unterseeboot 691 (U-691) would have been a Type VIIC/41 submarine of the Kriegsmarine, if she had ever been built. Her construction by Howaldtswerke of Hamburg, Germany was ordered on April 2, 1942, suspended on September 30, 1943, and cancelled on July 22, 1944. The new Elektro Boat XXI design had made the Type VIIs obsolete.

Neal Stephenson's novel Cryptonomicon includes a fictictious U-691, a Type IXD/42, launched at Wilhelmshaven on September 19, 1940 (four years before IXD/42s were actually developed). It is possible Stephenson picked the number not for historical accuracy, but for its being prime.