Besides fighting vessels the navigators of Scandinavia and North Europe used cargo vessels - knorrs. The construction of these ships was caused by a great need in carrying of large quantities of ammunition and horses. In the 13th century one more radical change in the construction of vessels took place - an invention of a stern rudder. But much earlier seafarers of the North had placed a steering oar just behind the stern in its diametric surface. When the steering oar was fastened at the starboard under influence of side winds and a careen it went in and out of the water being useless. At placing it along the center line the steering oar truly carried its obligation steadily converting into a stern rudder.
The hull of a knorr was typical for vessels of Scandinavian shipbuilding. It was distinguished by its outer sweep of boards, highly raised posts with ornaments, a square hatchway and a square sail where they firstly began to use reefs to lessen the sail's area.