Norway-based historical textile researcher Amy Lightfoot has made several woollen sails based on old sailcloth found in the eaves of a church . An important step, she wrote, was recreating the resinous goo found in old sails. She smeared her sails with a combination of fir tar, fish oil, and sheep tallow—all easily available 600 years ago. Her sails repel water and have substantial wind resistance.
Based on her first sail, Lightfoot estimated that a 100-square-meter sail (about one-quarter the size of a basketball court and big enough for a 30-meter longship) used two tonnes of fleece, the annual production of about 700 sheep. Researchers at the Viking Ship Museum in Roskilde, Denmark, calculated that by the mid-11th century, the Viking fleet—fishing boats, coastal traders, cargo ships, and longships carried roughly one million square meters of sail, requiring the equivalent of all the wool produced in one year by about two million sheep.
“It’s actually more time-consuming to produce the textiles than to produce the boat,” Lightfoot said in a 2009 documentary about woolen sails. Building a boat might take two skilled boatbuilders a couple of weeks, she estimated, but creating its sail would take two skilled women a year.
Woollen sailcloth production at Roskilde Ship Museum