From Songs of American Sailormen by Joanna Colcord:
"Paddy Doyle was a famous boarding-house master in Liverpool. It is told of him that he kep a cow's horn in the back yard, round which he solemnly marched "green hands", so as to be able to tell a doubting skipper that they had "been three times round the horn!"
The shanty was doubtless related in the mind of the singers to the sea-superstition that it brings bad luck to leave port with bills unpaid."
I bought the book in a recent mudcat auction and just read this yesterday.
JohnB