A cog was a type of ship that was used during the Middle Ages, mostly for trade and transport but also in war. It first appeared in the 10th century, and was widely used from around the 12th century on. Cogs were clinker-built, generally of oak. Cogs were fitted with a single mast and a single square sail. They were used primarily for trade in north-west medieval Europe, especially by the Hanseatic League. Typical seagoing cogs were from 15 to 25 meters (49 to 82 ft) long, 5 to 8 meters (16 to 26 ft) wide, and were of 30–200 tons burthen. Cogs were rarely as large as 300 tons although a few were considerably larger, over 1,000 tons.
Although the name cog is recorded as early as the 9th century, the seagoing vessel of that name seems to have evolved on the Frisian coast during the 12th century. Cogs progressively replaced Viking-type vessels such as knarrs in northern waters during the 13th century. Cogs could carry more cargo than knarrs of a similar size. Their flat bottoms allowed them to settle on a level in harbour, making them easier to load and unload. Their high sides made them more difficult to board in a sea fight, which made them safer from pirates.
Description and construction
Cogs were a type of round ship, characterized by a flush-laid flat bottom at midships which gradually shifted to overlapped strakes near the posts. They were propelled by a single, large, rectangular sail. Typical seagoing cogs ranged from about 15 to 25 meters (49 to 82 ft) in length with a beam of 5 to 8 meters (16 to 26 ft) and were 40–200 tons burthen. Cogs were rarely as large as 300 tons, although a very small number were considerably larger, over 1,000 tons. A rule of thumb for crew size was that one sailor, exclusive of any dedicated fighting men, was required for every 10 tons burthen of the cog, although this may generate a suggested crew size on the low side of Medieval practice. Crews of up to 45 for civilian cogs are recorded, and 60 for a 240 ton cog being used for military transportation.
Cogs were typically constructed largely of oak, and had full lapstrake, or clinker, planking covering their sides, generally starting from the bilge strakes, with double-clenched iron nails for plank fastenings. At the stem, chases are formed; that is, in each case, the land of the lower strake is tapered to a feather edge at the end of the strake where it meets the stem or stern-post. This allows the end of the strake to be fastened to the apron with the outside of the planking mutually flush at that point and flush with the stem. This means that the boat's passage through the water will not tend to lift the ends of the planking away from the stem. Before the next plank is fitted, the face of the land on the lower strake is bevelled to suit the angle at which the next strake will lie in relation with it. This varies all along the land. The new strake is held in position on the preceding one before the fastening is done.
The keel, or keel-plank, was only slightly thicker than the adjacent garboards and had no rabbet. Both stem and stern-posts were straight and rather long, and connected to the keel-plank through intermediate pieces called hooks. The lower plank hoods terminated in rabbets in the hooks and posts, but upper hoods were nailed to the exterior faces of the posts. Caulking was generally tarred moss that was inserted into curved grooves, covered with wooden laths, and secured by metal staples called sintels. The cog-built structure would be completed with a stern-mounted, hanging, central rudder on a heavy stern-post, which was a uniquely northern development. The single, thick, mast was set forward of amidships, stepped into the keel-plank and equipped with a single large, rectangular, square-rigged sail. The masts of larger vessels would be of composite construction. Complicated systems of rigging were developed to support the mast and to operate the sail. Cordage was usually hemp or flax and the sail hemp-based canvas. From the 13th century cogs would be decked and larger vessels would be fitted with a stern castle, to afford more cargo space by keeping the crew and tiller up, out of the way; and to give the helmsman a better view.
