The most significant "gem" follows. I'm taking it directly from its original appearance in James Kennard, Jr.'s article, "Who Are Our National Poets?" (Knickerbocker, Oct. 1845, p. 338):

"One day during the early part of the Indian war in Florida [i.e., 1835] we stepped into a friend's boat at Jacksonville, and with a dozen stout negro rowers, pushed off, bound up the St. Johns with a load of muskets, to be distributed among the distressed inhabitants, who were every where flying from the frontier before the victorious Seminoles. As we shot ahead, over the lake-like expanse of the noble river, the negroes struck up a song to which they kept time with their oars; and our speed increased as they went on, and become [sic] warmed with their singing. The words were rude enough, the music better, and both were well-adapted to the scene. A line was sung by a leader, then all joined in a short chorus; then came another solo line, and another short chorus, followed by a longer chorus, during the singing of which the boat foamed through the water with redoubled velocity. There seemed to be a certain number of lines ready-manufactured, but after this stock was exhausted, lines relating to surrounding objects were extemporized. Some of these were full of rude wit, and a lucky hit always drew a thundering chorus from the rowers, and an encouraging laugh from the occupants of the stern seats. Sometimes several minutes elapsed in silence; then one of the negroes burst out with a line or two which he had been excogitating. Little regard was paid to rhyme, and hardly any to the number of syllables in a line: they condensed four or live into one foot, or stretched out one to occupy the space that should have been filled with four or five; yet they never spoiled the tune. This elasticity of form is peculiar to the negro song."

In other words, a halliard shanty without the halliards. Such rowing songs must have been just as important to shanty development as were the cotton-screwing chants.

I'll revise my theory of shanty creation. Regardless of how later shanties were "composed," the first "halliard shanty" may have been nothing more than a rowing song sung in a new context. And maybe "Cheerly Man!" was that song!