Scapa Flow was considered invulnerable at that time.
It was a daring raid and the captain of the submarine showed great courage by bringing it off.
Think his name was Gunther Prien (sp)....called the “Bull of Scappa Flow”.....died in 1943-44 like most UBoat crews.
http://www.uboatarchive.net/U-47RoyalOak.htm
REPORT
ON
SINKING OF “ROYAL OAK”
Reproduced from
British Admiralty translation
(Too long to post here, but good stuff!)