How was the viking ship made wartertight?

Answer 1

Vikings were very skilled craftsmen. Their boats were built with overlapping planks, then waterproofed with a mixture of resin or tar and reindeer hair, cow hair or moss.

ANS 2 Near my home, in Vancouver B.C. we actually have a half size fully operational replica of a Viking longboat, the Munin. It was built by a group of Scandinavian seamen including two professional wooden boat builders. It took a year and half to build the 40 ft ship and apparently everything was done the original way with original materials. The seams are caulked with rope hammered in between them. I know this for a fact as I was once in her, far out in English Bay on a very hot summer day and the tops of the seams began to dry out. The crewmen threw buckets of water into the bilge to seal them tighter.