copilot vs coxswain

copilot

verb
  • To serve as relief or assistant pilot. 

noun
  • A backup or assistant pilot of an aircraft. 

coxswain

verb
  • To act as coxswain for. 

noun
  • The member of a crew who steers the shell and coordinates the power and rhythm of the rowers. 

  • In a ship's boat, the helmsman given charge of the boat's crew. 

  • A rank or position. 

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