Yeah, I hate to say it, but my later research into Alden's shanties (on one of those other threads) makes it very unlikely that he'd known "Shenandoah" or *any* others for "thirty" years.

The most we can say is that Alden's source or sources told him in the '70s or '80s, or gave him the impression, that they'd known them all in the '50s.

Shanties were not a subject that nineteenth-century writers thought they needed to be very precise about. They were just songs sung by laboring sailors - for centuries as far as they knew.