gangplank vs promenade deck

gangplank

noun
  • A board used as a temporary footbridge between a ship and a dockside. 

promenade deck

noun
  • An upper deck on a passenger ship on which passengers may walk, which can be covered but open to the air. 

How often have the words gangplank and promenade deck occurred in a corpus of books? (source: Google Ngram Viewer )