Unterseeboot 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 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) and fitted with an experimental schnorkel. It is possible Stephenson picked the number not for historical accuracy, but for its being prime.