Viking shipbuilding wasn't done quite the way we would do it today. Most of the ships were what's called clinker built. The boards of the hull were split, then shaped with draw knives to fit, and then were riveted together. Around eighty trees were used in the process of building a single ship.
Once the hull was completed you then made the framework to fit inside. Right over the keel between the middle framework pieces you'd install the mast fish which would hold the bottom of the mast in place. Ropes would secure the top of the mast. Any stresses were transferred to the hull as a result.
Of all things a Viking ship builder did it all by eye and by feel. They'd have a basic plan in their head only and nothing was written down. They were quite the craftsman I think.
In this video they are splitting a tree into boards, shaping them, and installing them. They are even using the types of glue and flax caulking that would have been used. This video is about 5 minutes. http://youtu.be/Y12PN8gaQ4Y
This video shows much of the process in less than two minutes. http://youtu.be/78kpzwGmBxk
It's been fascinating contemplating all of the process. I've done some woodworking over the years and some of the projects have been pretty cool. Some of the projects closest to how the Vikings would have accomplished it were railings that were made from deadfall branches. I had to use a draw knife to clean off the bark and had to create notches to join the pieces together.
Today if we were going to build a ship we'd most likely make a framework first and then attach the hull. It's a totally different process.