Okay, it is not quite right to call this a true viking boat as it is even older than the Vikings.
The oak boat was the first boat found, and the only one still preserved. It has been dendro dated to 310-320 AD. The oak boat is considered the oldest Nordic shipfind and the oldest known clinker built boat. It is 23 m long, c 4 m wide, of clinker type, and built for 15 pairs of oars. The Nydam Boat is the largest and best preserved of the boats found in Nydam Bog and is now displayed at Gottorp Castle in Schleswig. It once weighed over three tonnes* and was rowed by thirty men.
* which is LIGHT!!
This simply means we have here the oldest, still complete, Viking-style-built boat known!It lacks some of the features more modern Viking ships imposed. Those used to be multi-purpose (merchant, traffic, war, even more sea-worthy, i.e. ocean going), and of course: with mast! This meant: with a minimum of effort you can change these boats to transport goods, lifestock, passengers or your war band! Or your war band plus horses (of course you did watch "The 13th Warrior"!).... and go up a river, pull them over land, go down another river... and thus find a (mostly) shipable way from the Baltic to the Black Sea... oh, read it yourself (book tip at bottom of page).
Nydam Mose - where they found the boat
Schloss Gottorf - the museum where it is now
Haithabu museum - a Viking settlement. German only, but click at least through navigation bar at the top to see some pics!
Haithabu and Hedeby - Wikipedia links
The Long Ships - definitely the best fiction (but no-nonsense and historically correct) read about the the Vikings. There simply is no other novel you can read and still learn more facts about them!