merchant marine vs seamanship

merchant marine

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

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

seamanship

noun
  • Skill in, and knowledge of, the work of navigating, maintaining, and operating a vessel at sea. 

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