buttock vs gunwale

buttock

noun
  • The convexity of a ship behind, under the stern. 

  • Each of the two large fleshy halves of the posterior part of the body between the base of the back, the perineum and the top of the legs. 

gunwale

noun
  • The top edge of the hull of a nautical vessel, where it meets the deck. 

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