Eric Illott, a shantyman from Bristol, told me that this song was used when scraping growth from the ship's hull during long voyages - barnacles, seaweeds and the like, which would slow the ship down. They used a device called the "Devil's Scrubbing Brush", two planks joined by a hinge and with long nails driven through. This was lowered into the sea over the "sharp end", and hauled by ropes backwards under the hull, scraping off the fouling. The verses, which had to be kept going as long as the job lasted, mentioned all the things the shantyman would rather be doing instead of scratching his bottom in mid-ocean.

"Nelson's Blood" is brandy, (not rum). After Trafalgar, Nelson's body was put into a cask of brandy to preserve it for the voyage back to London. When the Victory docked, it was discovered that the crew had tapped into the cask and drunk the lot. That, according to Eric, was always the first verse. The last verse, he said, should always be "A night in jail wouldn't do us any harm", since sailors reckoned that conditions were better in jail than at sea. (It was also safer when tunneling out). In between, sing anything you like.

Well, that's the story as I heard it.

Andrew