When did the three elements of the song came together (disgruntled apprentice, blackbirder, pirate). Were they there from the start (the ca. 1906 referenced version cited by Malcolm Douglas)?
With red face I admit that I should have checked Lomax 1934, otherwise I would not have attributed the composite to the "folk song era." It is the kind of song McColl loved to sing, and the two are stuck together in my mind. Now it seems that the song may be of 1860-1900 vintage, when pirate songs were very popular (long after the western pirates' heyday).

The Lomax version (ABFS)is an admitted composite in which the Flying Cloud is a "Spanish ship" of 500 tons, not the American ship. Where did this come from? The tune in Lomax is "from Shay's More Drunken Friends and Pious Companions (New York)." What else has this tune been used for, and what other tunes have been used for "Flying Cloud"?