Cogs first appeared along the Frisian and North German coasts in the 10th century-evolving from earlier clinker-built vessels-and was widely used from around the 12th century onward. Their name, recorded sporadically since the 9th century, came to denote any large merchant ship in medieval northern Europe. By replacing the deeper-keeled Norse knarrs, cogs offered increased cargo space and simpler rigging that suited the burgeoning trade of the Hanseatic League. Typically 15–25 m long and 5–8 m wide, a standard cog displaced 30–200 tons. Constructed almost entirely of oak, they featured lapstrake planking from bilge to sheer, caulked with tarred moss under wooden laths. Their straight stems and stern-posts were joined to a slightly thicker keel plank via “hook” timbers, with a single stern-hung rudder providing steering—the hallmark of northern shipbuilding. A single mast, stepped forward of midships, carried a large square sail of hemp canvas. Hemp or flax cordage rigged the mast, and later cogs added a simple deck and small stern-castle to clear the hold and give the helmsman a better vantage. Though clinker construction let the hull flex in heavy seas, cogs could carry only one sail, making tacking difficult and leaving them dependent on favorable winds. Their flat bottoms let cogs settle upright at low tide, simplifying loading and unloading at primitive quays—or directly on tidal beaches. High freeboard and sturdy hulls also made them harder to board, offering some defense against piracy. A typical crew numbered one man per 10 tons of burthen, so a 200-ton cog might sail with 20–45 hands, half the manpower of a lateen-rigged Mediterranean trader of similar tonnage. By the 13th century, Hanseatic merchants had adopted cogs as their workhorses, linking Baltic ports to Bruges, London and beyond. Their dominance waned only in the 15th century, as carvel-built, multi-masted ships with fore-and-aft sails offered better maneuverability and greater capacity. Yet for three centuries, the cog’s simple power, spacious hold and rugged seaworthiness underpinned northern Europe’s medieval trade network.

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Cog ship