Havhingsten (The Sea Stallion) in the North Sea on a journey to Ireland. She is an authentic replica built with viking age tools by the Roskilde Ship Museum in Denmark
is there any way the viking longship design could be improved?
Depends on what you're looking for. Longships were very good at what they needed to be - light to be able to be rowed and ported, but they were choppy and uncomfortable to be in for long periods of time. Some modifications to the sail's jib and cut might grant you the ability to better sail windward.
Was watching a documentary where these researchers built and sailed a viking longship. As they talked about her (the ship) qualities and the things they were discovering about how she sailed they kept asking things like "How we're they able to do this or that?" or "We don't understand how they could, blah blah blah..." And I just thought that the answers were very simple. The men and women sailing on those ships had the experience and knowledge of generation upon generation of previous sailors and adventures on how to handle those ships! And at least some, probably most, of the people on board were experienced sailors themselves! They could make those ships do anything they were capable of doing. The scientists were especially puzzled as to how the Vikings would keep large fleets together, considering they would be made up of many different classes of longship. And each type has different builds, riggings, trims, etc. But, as before, the people knew their individual ship well, and knew what must be done to hold their place in the fleet. Idk, it just seemed that these "experts" were pretty ignorant about their supposed subject of study.