Me and me friends sang this at primary school back in the 50's in the school yard, as did me mam. Was told it was about the Manchester Ship Canal, and being from Manchester, we were dead proud of it at school. We'de make a long line and hold hands and go under the arm of the kid who were chosen, and he or she would put their hand on the wall like it were a bridge on the canal. 'The big ships sail on the Alley Alley O' Can just hear us all now, we had nought, but they were happy uncomplicated days.