Bloody Jack by L.A.Meyer

  This is the first book of a large series.  A young girl is orphaned and left to fend for herself on the streets of 1797 London.  She dreams of getting away to anywhere but London, but joins a group of other orphans living on the streets.  Soon she is alone again and signs up to serve as a ship's boy, and her adventures and "deception" begin as only boys can serve on ships.  This is a great tale and very informative of the life of a sailor during that time.