Cog (ship)

A cog is 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 onward. Cogs had clinker-built sides but a flush-planked bottom which was built shell-first. The timber used was, generally, 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, with a few considerably larger at 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.

Cogs were characterised 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.

A cog, compared with the carvel-built vessels more traditional in the Mediterranean, was expensive and required specialist shipwrights. However, their simpler sail setup meant that cogs only required half the crew of similar-sized vessels equipped with lateen sails, as were common in the Mediterranean.^{[failed verification]} A structural benefit of clinker construction is that it produces a vessel that can safely twist and flex around its long axis (running from bow to stern), which is an advantage in North Atlantic rollers, provided the vessel has a small overall displacement. A limitation of cogs is that they lack points to mount additional masts: at least some fore-and-aft sails are desirable for maneuverability but clinker-built cogs were effectively limited to a single sail. This made them unhandy, limiting their ability to tack in the harbour and making them very reliant on wind direction at the start of voyages. The flat bottom permitted cogs to be readily beached and unloaded at low tide when quays were not available; a useful trait when purpose-built jetties were not common. Cogs were expected to have a working life of approximately 40 years.

The earliest origins of the cog are believed to be logboats from around northern Germany. These developed into larger craft built in the same basic shape, but with planking instead of hollowed-out logs. Another development was into Kahnen, flat-bottomed boats, with pointed ends fore and aft that were constructed by splitting a hollowed-out log and widening the bottom with planks that were nailed to knee-shaped ribs attached to the sides. The pointed ends (called Block locally) of the log would be cut off and attached separately to the widened hull which resulted in so-called Blockkahnen, variants of which are still in use. The earliest evidence of a cog-like craft is a clay model found in Leese on the middle Weser from the grave of an adult male who died around 200 BC. Fragments of similar clay models have been found in nearby regions.

Trade from Germania in Roman times was mostly carried on Mediterranean-style sailing vessels and controlled by Roman merchants. After Roman power collapsed in the middle of the first millennium AD, transports on the large river estuaries and the sheltered waters of the Wadden Sea was taken over by Frisians who used vessels based on indigenous, flat-bottomed designs that were precursors of later medieval cogs. These had much lower sides than later cogs and would have been very similar to contemporary Scandinavian craft, such as knarrs. The oldest depiction of a vessel identified by contemporary sources as a cog is the Lübeck city seal from 1223. The early cogs were fitted with a side-mounted rudder oar that also functioned as a leeboard and was rigged with a single broad square sail that functioned similar to a lug sail.