While most longships held a length to width ratio of 7:1, the Karvi ships were closer to 9:2. These ships were considered to be "general purpose" ships, mainly used for fishing and trade, but occasionally commissioned for military use. A ship with 6 to 16 benches would be classified as a Karvi. According to the 10th-century Gulating Law, a ship with 13 rowing benches is the smallest ship suitable for military use. The Karvi (or karve) is the smallest vessel that is considered a longship. The most common way to classify longships is by the number of rowing positions on board.
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Longships can be classified into a number of different types, depending on size, construction details, and prestige. The Norse had a strong sense of naval architecture, and during the early medieval period they were advanced for their time. They were called "dragonships" by enemies such as the English because they had a dragon-shaped bow. In the same year, 129 ships returned to attack up the Seine. Quentovic, near modern Étaples, was attacked in 842 and 600 Danish ships attacked Hamburg in 845. Rouen was sacked in 841, the year after the death of Louis the Pious, a son of Charlemagne. During the 9th century peak of the Viking expansion, large fleets set out to attack the degrading Frankish empire by attacking up navigable rivers such as the Seine. In the tenth century, longships would sometimes be tied together in offshore battles to form a steady platform for infantry warfare. While longships were used by the Norse in warfare, they were mostly used as troop transports, not warships. They were often communally owned by coastal farmers or commissioned by kings in times of conflict, in order to quickly assemble a large and powerful naval force. Moreover, each Viking longship had particular features adjusted to the natural conditions under which it was sailed. For example, the choice of material was mostly dictated by the regional forests, such as pine from Norway and Sweden, and oak from Denmark. Ships varied from designer to designer and place to place, and often had regional characteristics. Archaeological finds show that the Viking ships were not standardized. The Viking longships were powerful naval weapons in their time and were highly valued possessions. The longships dimensions are 20-30m in length 4-8m in width 3-4m in airdraft and the mast measures in at 15-30m One longship in particular can be seen in Oslo, Norway in The Viking Ship museum. The average speed of Viking ships varied from ship to ship, but lay in the range of 5–10 knots (9.3–18.5 km/h) and the maximum speed of a longship under favourable conditions was around 15 knots (28 km/h). Later versions had a rectangular sail on a single mast, which was used to replace or augment the effort of the rowers, particularly during long journeys.
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Longships were fitted with oars along almost the entire length of the boat itself. Longships were also double-ended, the symmetrical bow and stern allowing the ship to reverse direction quickly without a turn around this trait proved particularly useful at northern latitudes, where icebergs and sea ice posed hazards to navigation.
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The ship's shallow draft allowed navigation in waters only one meter deep and permitted arbitrary beach landings, while its light weight enabled it to be carried over portages or used bottom-up for shelter in camps. The longships were characterized as graceful, long, narrow and light, with a shallow- draft hull designed for speed. They were all made out of wood, with cloth sails (woven wool) and had several details and carvings on the hull. The particular skills and methods employed in making longships are still used worldwide, often with modern adaptations. The character and appearance of these ships have been reflected in Scandinavian boat-building traditions to the present day. The longship appeared in its complete form between the 9th and 13th centuries. The longship's design evolved over many centuries, and continuing up until the 6th century with clinker-built ships like Nydam and Kvalsund. Originally invented and used by the Norsemen (commonly known as the Vikings) for commerce, exploration, and warfare during the Viking Age, many of the longship's characteristics were adopted by other cultures, like Anglo-Saxons, and continued to influence shipbuilding for centuries. Longships were a type of specialised Scandinavian warships that have a long history in Scandinavia, with their existence being archaeologically proven and documented from at least the fourth century BC. They were not always equipped with shields.