[kiterunner]

I've just discovered that Google Books has several mid-19th century editions of the Mercantile Navy List, which lists everyone who was qualified as a Master or Mate in the merchant navy. I'm still finding my way around them, but each edition has an alphabetical list of people and also a list showing what grade they had reached, which examining board, date qualified, etc. Definitely worth looking at if any of your ancestors were ship captains or mates.