U-boat vs nuclear submarine

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. 

nuclear submarine

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