Viking longships, starting in the 700s AD, had a mast with a sail. To make a boat with a sail stable, a keel is needed to counteract the tipping forces the wind places on the sail. Prior to the sail being added, longships did not require or have keels.
Viking Longships ranged any where from 10 tons to 40 tons or more unloaded depending on the size of the ship and whether it was a Warship or a Cargo Ship as the Warships tended to be lighter than the heavier built Cargo Vessels.
There are many decks for longships that are way huger than the Serpent, so Vikings could live and shelter in there. For longships smaller than the Serpent, Vikings hold up their shields to shelter themselves.
A typical Viking ship looked somewhat similar to a modern day canoe. The longships were used for exploration, trade and warfare. They were long, narrow boats with shallow hull's that were designed for speed.