The strongest impulse to the coastal seafaring of the Frisians was given by an event far outside Frisia(Ellmers 1985a; 1985c). In the middle of the 6th century Avaric and Slavonic tribes invaded large parts of eastern Europe and interrupted the trade connections from Byzantium to Scandinavia. At this time nobody was able to cross the North Sea directly from the British Isles to Scandinavia and the latter depended for its whole supply of goods from western, central and southern Europe completely on Frisian coastal trade. In all Scandinavia for 200 years or more before the Viking Age, there is not one single find of foreign origin that came there without Frisian intermediate trade (Bakka 1971).
At the beginning of this phase of Frisian monopoly in trade they had no towns which could serve as trading centres. Frisian traders were peasants, skippers and merchants in one person and lived throughout their country in small farms erected on top of artificial hills (terpen) near tidal creeks and streams. In a 7th century layer in one of these, excavated at Hessen in the town of Wilhelmshaven, there was found a slipway on which flat bottomed boats could be built (Ellmers 1972). Another important find was a side rudder of the firrer type which, today, is still in use on the traditional sailing boats of Steinhuder Meer, a lake north of Hannover. This type of rudder was specific to smaller vessels within the ship building tradition of the cog. In the late 12th century, Hanseatic cogs replaced the firrer by a stern rudder (Ellmers 1985b, 15ff).
The firrer of Hessen tells us that the farmer-merchant there intended to build boats of the cog type on his slipway. A flat bottom was essential on Frisian craft which were designed for the special conditions encountered when sailing the shoals of the Wattenmeer along the Frisian shores.
The economic base of the farm at Hessen was sheep of two different breeds with different types of wool. From this raw material cloth of very high quality was woven in many different varieties. Provided with this excellent home-made commodity our farmer-merchant sailed to the beach markets along the Frisian borders to meet neighbouring merchants or customers. One of these rural beach markets has been excavated on the Jutish (Jutland) west coast near Dankirke, south of Ribe (Thorvildsen & Bendixen 1972). Located near the farm of a rich customer, this market place lay close to the shore where flat bottomed boats could beach and dry out at low tide and where merchandise could be sold to visitors to the market. The presence of Frisian merchants is confirmed by stray finds of not less than 13 coins which, among other small objects, had been lost during the process of buying and selling.