grappling hook vs grappling iron

grappling hook

noun
  • A type of hook attached to a line and designed to be thrown at a target (e.g. rigging of an enemy ship) for the purpose of catching hold of it. 

grappling iron

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

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