the Vikings could sail in shallow waters, allowing them to invade far inland along rivers. The ships were agile, and light enough to be carried over land from one river system to another. "Under sail, the same boats could tackle open water and cross the unexplored wastes of the North Atlantic." The ships' speed was also prodigious for the time, estimated at a maximum of . The use of the longships ended when technology changed, and ships began to be constructed using saws instead of axes, resulting in inferior vessels. While battles at sea were rare, they would occasionally occur when Viking ships attempted to board European merchant vessels in Scandinavian waters. When larger scale battles ensued, Viking crews would rope together all nearby ships and slowly proceed towards the enemy targets. While advancing, the warriors hurled spears, arrows, and other projectiles at the opponents. When the ships were sufficiently close, melee combat would ensue using axes, swords, and spears until the enemy ship could be easily boarded. The roping technique allowed Viking crews to remain strong in numbers and act as a unit, but this uniformity also created problems. A Viking ship in the line could not retreat or pursue hostiles without breaking the formation and cutting the ropes, which weakened the overall Viking fleet and was a burdensome task to perform in the heat of battle. In general, these tactics enabled Vikings to quickly destroy the meagre opposition posted during raids. Together with an increasing centralisation of government in the Scandinavian countries, the old system of leidang — a fleet mobilisation system, where every skipreide (ship community) had to maintain one ship and a crew — was discontinued as a purely military institution, as the duty to build and man a ship soon was converted into a tax. The Norwegian leidang was called under Haakon Haakonson for his 1263 expedition to Scotland during the Scottish–Norwegian War, and the last recorded calling of it was in 1603. However, already by the 11th and 12th centuries, European fighting ships were built with raised platforms fore and aft, from which archers could shoot down into the relatively low longships. This led to the defeat of longship navies in most subsequent naval engagements—e.g., with the Hanseatic League. Exactly how the Vikings navigated the open seas with such success is unclear. While some evidence points to the use of calcite "sunstones" to find