U-boat vs submersible

U-boat

noun
  • Any German submarine of the First or Second World War, or any Austro-Hungarian submarine of the First World War. 

  • Nickname given to the General Electric Universal Series diesel-electric locomotives. 

  • A narrow, utility cart with tall handles on both ends that is used to carry loads of retail products to be sold, so named because its shape is roughly that of a capital U. 

submersible

noun
  • A small nonmilitary, non-nuclear submarine for exploration. 

  • A term used primarily by some navies for nuclear submarines, termed "true submersibles", because they cannot retroactively declare that their non-nuclear submarines should be called by a different name. 

  • A very small "baby" submarine designed for specific localized missions, usually while tethered to a submarine or ship for life support and communications. Slang synonyms: midget-submarine, anchor. 

  • A retroactive term used for non-nuclear submarines; nuclear submarines are termed "true submarines". 

adj
  • Able to be submerged. 

How often have the words U-boat and submersible occurred in a corpus of books? (source: Google Ngram Viewer )