merchant marine vs navigation

merchant marine

noun
  • A ship dedicated to commercial, rather than military, purposes. 

  • Synonym of merchant navy. The fleet considered as an entire combined force. 

navigation

noun
  • Traffic or travel by vessel, especially commercial shipping. 

  • A canal. 

  • The theory, practice and technology of charting a course for a ship, aircraft or spaceship or (colloquially) road vehicle. 

How often have the words merchant marine and navigation occurred in a corpus of books? (source: Google Ngram Viewer )