mother ship vs supply vessel

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. 

supply vessel

noun
  • A ship used to transport supplies to offshore installations such as oil and gas platforms. 

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