Thanks, Steve, I was intending to revisit the passage to see what had struck me as unclear. I don't have the text with me, so I am entering here the passage as cited in Hugill's SfSS:

Our seamen having left the ship, the harbour work was performed by a gang of Negroes. These men will work the whole day at the capstan under a scorching sun with almost no intermission. They beguiled the time by one of them singing one line of an English song, or a prose sentence at the end of which all the rest join in a short chorus. The sentences which prevail with the gang we had aboard were as follows...

It can be fairly assumed that their songs were sung at the capstan, however it does not say explicitly. (For example, they may have used a capstan but along with other tools, too.) To be honest, I would assume the men working working the cargo by means of a capstan and whilst singing the songs. My only hang-up is the dissonance between the use of capstan here and what seems like a more sensible use of simple hauling on a rope as cited in similar cases (e.g. the excellent photo in Parrish's book). The cargo must have been very heavy, as for example in the sketch of loading timber through the bow-port, by capstan, that appears in Hugill's book. But I can't really say; can only accept with slight disappointment :) that, yes, it was a capstan they were using!