grappling iron vs mooring

grappling iron

noun
  • A metal hook, attached to the end of a line, used to hold two ships together. 

mooring

noun
  • A place to moor a vessel. 

  • The act of securing a vessel with a cable or anchor etc. 

  • Something to which one adheres, or the means that helps one to maintain a stable position and keep one's identity - moral, intellectual, political, etc. 

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