A new development: Hanseatic trade and the cog.
With the Viking's powers dwindling, trade became the economic flywheel on the North- and Baltic Seas and along northwestern-Europe's Atlantic coasts. Of course there had always been some merchant-marine traffic, but only small-scaled and short-distanced; now the English, French, Dutch and German traders could deploy their capacities without Viking threat. The only threat remaining, even till this day, was piracy.
Within a short period this development led to innovations in shipbuilding: a host of shiptypes, mostly based on the Viking knorr, were launched: single-masted square-sailed ships, round-built and without a deck or only fore- and after-decked. Some of these were furnished with a cutwater built in front of the round stem, to give them a better course-stability. Most of them could be rowed too, but as the crews were smaller, that was only done in times of dire need.
In the 13th. century the Hanseatic League was founded by German, Friesian and Dutch cities and their traders; it included Lübeck and Hamburg in the north and had it's influence as far south as Dunkirk. And somewhere around that time it's means of transport was developed along the Friesian coast: the Hansa cog.
It was built as a much sturdier, faster and more seaworthy successor of the knorr-clones. Apart from that it was a more efficiently rigged, fully-decked and pure sailing-ship.
It was a keeled double-ender with a flat, lancet-shaped bottom, (carvel-built, some planks higher up replaced by clinker), long, straight, slanting stem and stern; the rudder was, for the first time in history, hinged from the sternpost; and the waterlines fore and aft were swept concave, in order to get an efficient hydrodynamic underwatershape.
Given the large cargo-capacity and draught it sailed (comparatively) very fast (some 7-9 knots) on beam-, broad and running reaches; the most efficient close-hauled course was some 70 degrees. (These are the results of Marin testruns with a replica, the Dutch KamperKogge) See for pictures http://www.kamper-kogge.nl/kamperkogge/nl/foto_album/070407_sail_ kampen_2007/index_2.html and
Further information: http://en.wikipedia.org/wiki/Cog_(ship)
And for a nice impression (also meant as a tip for the model-boat builders): http://www.modelshipgallery.com/gallery/misc/sail/cog-55-ep/ep-index.html
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