The Viking Ship Museum in Roskilde, Denmark has been conducting research on woolen sails since 1977. Weavers and spinners from Denmark and Norway have been spinning single ply yarn in-the-grease from spelsau wool for the sails. Eight lengths of cloth are woven on warp-weighted looms and sewn together using round seams to construct each square sail. The round seams result in a smooth join on the facing side of the sail.
Historically, depending on the geographic location and resources available, researchers believe that three structures were used to weave the sail cloth. There is evidence of plain weave, 2/1 twill, and 2/2 twill. Click here to read more about woolen sails.