ocean liner vs packet boat

ocean liner

noun
  • a narrow-beam passenger ship designed for high speed trans-oceanic travel 

packet boat

noun
  • A boat specifically designed to provide transportation to passengers on a regular schedule up and down a river or canal. 

  • A boat that carried mail, cargo, and, in later years, some passengers along a regularly scheduled route up and down a river or canal. 

  • An ocean-going ship chartered by the government to carry the mail and official communications. 

How often have the words ocean liner and packet boat occurred in a corpus of books? (source: Google Ngram Viewer )