mainmast vs royal mast

mainmast

noun
  • The tallest mast of a sailing ship that has more than one mast; particularly a full-rigged ship. 

royal mast

noun
  • The fourth and highest part of the mast from the deck, commonly made in one piece with the topgallant mast. 

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