cockboat vs mother ship

cockboat

noun
  • A small rowing boat, especially one pulled behind a larger ship, or used to ferry goods between a ship and the shore. 

mother ship

noun
  • A vessel or aircraft that serves or carries a smaller vessel or vessels that operate independently from it. 

  • Fond term of address for one's mother. 

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