:I fear the conceptual similarity between the Viking's shields and ship armor might have something of a a kind of a XIXth century urban legend, in the way of a flattering, perhaps iconic, image for navies at the time.  The purpose of the disposition was rather the protection of the warriors/rowers from behind the upper half of the shield, than by the bottom half. That disposition dispensed with the need of a parapet, and permitted swiftness of the boat, a bit like in some countries there is usage of particular sport cars on speedy highways for law enforcement. -