Cog ( ship ) A cog ( or cog - built vessels ) is a type of ship that first appeared in the 10th century , and was widely used from around the 12th century on . Cogs were generally built of oak , which was an abundant timber in the Baltic region of Prussia . This vessel was fitted with a single mast and a square - rigged single sail . These vessels were mostly associated with seagoing trade in medieval Europe , especially the Hanseatic League , particularly in the Baltic Sea region . They ranged from about 15 meters to 25 meters in length ( 49 ft to 82 ft ) with a beam of 5 to 8 meters ( 16 ft to 26 ft ) , and the largest cog ships could carry up to about 200 tons . Design . Cogs were characterized by a flush - laid flat bottom at midships but gradually shifted to overlapped strake s near the posts . They had full lapstrake planking covering the sides , generally starting from the bilge strakes , and double - clenched iron nails for plank fastenings . The keel , or keelplank , 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 keelplank 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 " . Finally , the cog - built structure could not be completed without a stern - mounted hanging central rudder , which was a unique northern development . Cogs used to have open hulls and could be rowed short distances . In the 13th century they received decks . The most famous is the Bremen cog . History . Cogs are first mentioned in 948 AD , in Muiden near Amsterdam . These early cogs were influenced by the Norse Knarr , which was the main trade vessel in northern Europe at the time , and probably used a steering oar , as there is nothing to suggest a stern rudder in northern Europe until about 1240 . Current archaeological evidence points to the Frisia n coast , Western Jutland , as the possible birthplace of this type of vessel . The transformation of the cog into a true seagoing trader came not only during the time of the intense trade between West and East , but also as a direct answer to the closure of the western entrance to the Limfjord . For centuries , Limfjord in northern Jutland offered fairly protected passage between the North Sea and the Baltic . Due to unusual geographical conditions and strong currents , the passage was constantly filling with sand and was completely blocked by the 12th century . This change produced new challenges . Bigger ships that could not be pulled across the sand bars had to sail around the Jutland peninsula and circumnavigate the dangerous Cape Skagen to get to the Baltic . This resulted in major modifications to old ship structures , which can be observed by analyzing evolution of the earliest cog finds of " Kollerup " , " Skagen " , and " Kolding " . The need for spacious and relatively inexpensive ships led to the development of the first workhorse of the Hanseatic League , the cog . The new and improved cog was no longer a simple Frisian coaster but a sturdy seagoing trader , which could cross even the most dangerous passages . Fore and stern castles would be added for defense against pirates , or to enable use of these vessels as warships , such as used at the Battle of Sluys . Eventually , around the 14th century , the cog reached its structural limits , resulting in the desperate need for a quick replacement . The replacement , the hulk , already existed but awaited reconditioning . Although there is no evidence that hulks descended from the cogs , it is clear that a lot of technological ideas were adapted from one to the other and vice versa . The transition from cogs to hulks was not linear . According to some interpretations , both vessels coexisted for many centuries but followed diverse lines of evolution .