Another sea shanty. Like all such, it would be sung to help the sailors keep time in whatever job they were doing.
The following is an extract from Wikipedia regarding 'The Blackball Line' which is mentioned in this song as well as others:
The Blackball line
Starting in 1818 , ships of the Blackball line began regularly scheduled trips between Britain and America. These "packet ships" (named for their delivery of mail "packets") were infamous for keeping to their disciplined schedules. This often involved harsh treatment of seamen and earned the ships the nickname, "bloodboat".
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