= The Cog (German: Kogge) =
This shipstype probably arose from the frisian mudflat ship known as Coga or Coggo. In the 8th century it appeared also in the Baltic sea and was developed to a keel ship until the turn of the millennium. The cog has retained one quirk in the structural form from its time in the mudflat navigation: the typical kink at the transition from the keel to the stem. Illustrations of this can already be found on the Haithabu coin in the 9th century. Discoveries from later periods also prove this. However it cannot be precluded that the naval architecture in the Baltic sea adapted the structure, as the necessity for a flat floor was lacking due to the absence of mudflat in the Baltic.
The cog is clinker built ship with only one mast.  To the front it was secured with a big stay. To the sides and the back bars provided the needed stability. The mast was rigged with a big square sail. The yard was fasted to the mast with a kind of rope noose the rack.
Normally the cog carried some dinghies. Due to improvements in construction the cog became more seaworthy and agile. Among the alterations are closed decks, subdivided payload bays and improved lodging for the crew.
On the cog pictured on the Stralsund Seal from 1329 a cover under the aft castel redoupt is already visible. Thereby the aft deck under the castel became a closed room. The castell on the foreship is still on a beam framework. The castells were a perceptible improvement for the devensive capability and seaworthyness. The upper deck was sheltered from spilled over waves and bowmen and and crossbowmen could better aim at their targets from this elevated position. At the end of the 14th century the cog could carry between 40 and 120 bales.
Source: Günter Krause, Handelsschifffahrt der Hanse, 2010,  S. 133 ff.