I have heard this but never saw a good reference ( somebody maybe has one). Seems kinda dubious, unless they're hand-forged nails, so earlier than 1830 or so- cut iron nails after that become pretty cheap. But I have also heard it for wooden boats, at the Mystic Seaport museum. Here , it does make sense: wooden boats tended to develop problems with rot, with hogging ( the middle part of the boat sagging down), with sprung joints after years of stress and strain, or with bad leaks from years of being wet through, then hauled out to dry on a beach. It would be hard to rebuild them, and the metal fasteners would be bronze or copper and worth reclaiming- even as scrap metal.