Number constructed vs number lost in a year doesn't really tell the story.
According to this source, in 1940 there were rarely more than 10 boats at sea at any one time.
http://books.google.com/books?id=tdY...um=7&ct=result
I'll try to get to the library and look in the book that gives the monthly totals of boats at sea. I don't recall the title right now.