I don't think the question about the appearance of the gaff and boom mizzen rig has a simple answer.
I waded fairly deeply into this one - deep enough to get thoroughly confused - when I was working on my little model of the Hancock. I satisfied myself pretty firmly that that particular vessel did have a "driver boom." Sir George Collier's letter about her (which I slightly misquoted in my last post; sorry about that) said she had "a fore-and-aft driver boom with another across." Earlier in the same paragraph, he said the same ship had "a whole mizzen yard." My eventual conclusion was that she normally sailed with a lateen mizzen (the sail, presumably, only extending as far forward as the mast), and carried a separate sail, called the driver, for use in fine weather. (William Falconer's Universal Dictionary of the Marine, 1776 edition, defined "driver" as "an oblong sail, occasionally hoisted to the mizzen peak, when the wind is very fair. The lower corners of it are extended by a boom, or pole, which is thrust out across the ship, and projects over the lee quarter.") I think the idea of that arrangement was to let the aft, lower corner of the sail be stretched out further than that of the lateen-rigged mizzen.
Sources that give "about 1800" as the date for the appearance of the gaff-and-boom-rigged spanker may not be exactly incorrect. I think it was about that time that the "whole mizzen yard" went completely out of fashion. (That meant, in practical terms, that the section of the mizzen yard forward of the mast got sawed off. What was left was, in practical terms, the same as the driver gaff.) The term "driver" seems to have morphed from its original meaning as a fine-weather replacement for the mizzen, to a permanent replacement for it. Just when the word "spanker" came to replace "driver" I'm not sure; the change seems to have been pretty universal by the middle of the nineteenth century. I have a general impression that American terminology adopted the word "spanker" slightly before the British did, but I'm not at all sure about that.
It should perhaps be noted that what we're talking about here is the full-rigged ship. Smaller vessels, such as sloops, cutters, and brigs, were shipping gaffs and booms well before the American Revolution.
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