'Vikings at sea' by John Fraser. Using longships, the Vikings set out from Scandinavian countries like Norway and Denmark every summer and raided other countries. Britain was one of the places that they raided. They also used their ships for trading. Viking ships had one square sail made of wool, and a row of oars on each side. There was a steering oar at the back on the right-hand side. The ship was built by the shell method, and the planks overlapped, which is called clinker building. Gaps between the planks were stuffed with animal hair to keep the water out.