By James Gray & Nathan Williams
On Sunday, 65 men and women will embark on one of the most ambitious, dangerous and important experimental archaeology projects ever undertaken.
The Viking warship was a high point of boat-building technique
They will attempt to sail a reconstructed Viking warship from Roskilde, Denmark, to Dublin, across some of the roughest seas in the world.
The ship, the Sea Stallion from Glendalough, is the most authentic Viking warship built in nine centuries. It's based on the largest of five ships that were excavated from the bottom of Roskilde fjord in 1962, opposite the small village of Skuldelev.
One of these ships was the holy grail of Viking archaeology: the remains of a 30m longship. This was the largest Viking ship yet found and would have carried a crew of more than 60 men.
It represented the very pinnacle of Viking technology and ship-building technique, designed to be very fast and manoeuvrable and capable of transporting a large contingent of warriors.
The ship would have provoked terror when sighted by the Vikings' intended targets.
All five of the Skuldelev wrecks are now housed in a purpose built museum in Roskilde. The museum also embarked on the ambitious project of building accurate reconstructions.
The rebuilding of the longship was a lengthy process. In 1998, the Viking Ship Museum constructed a 1:10 scale model of the ship. The model demonstrated that the construction of a full-scale ship was feasible and work began in 2000.
The construction was undertaken relying solely on traditional Viking tools and building methods - the Vikings did not use saws and so every single piece of the ship had to be hand cut and then shaped and hewn using axes, an extremely skilled and lengthy process.
More than 7,000 iron rivets, 2,000m of rope and 300 ancient Danish oaks were used in the reconstruction. After four years of painstaking work, the Sea Stallion from Glendalough was launched on 4 September 2004.
Now, the team behind the project wants to know whether the reconstruction is capable of making the kind of journeys the Vikings once undertook. To find out, the ship is being taken over 1,000 miles (1,600km) across the North Sea to Orkney and on to Dublin.
This voyage is not simply a frivolous adventure; it has a serious point - to understand how the ships managed to carry the Vikings so far and wide.
Privacy will be virtually non-existent on the journey
The entire crew understands the importance of the project and what the museum hopes to achieve. Triona, one of the Irish crew members, is well aware of the reasons for undertaking the voyage.
"The museum staff on the ship will be involved in recording everything that happens. And all this will be analysed when the ship gets back.
"This is crucial to understanding the Viking Age as these ships were really the backbone of it - we can't really understand how they got where they did, and why they did it, without knowing how they did it."
The 65 members of crew on board face many challenges. Each has a specific and vital role in the smooth running of the ship. From captain to cook - everyone has their own responsibilities and worries.
Carsten, the skipper of the Sea Stallion, is in charge of getting the ship to Dublin in one piece: "It's a big responsibility; I am always thinking about it, day and night, and now the time to leave is coming I wake up in the night thinking how will it go?"
While Carsten faces the unenviable challenge of getting his crew safely through some of the most unpredictable and hostile waters on Earth, the two cooks on board must sustain the crew with limited rations, often in terrible weather conditions and with very limited space.
These challenges would have been the same for the original members of the Viking crew.
Every member of the crew has less than one sq metre in which to live, sleep and eat.
"It's hard to get proper rest, sleeping conditions are hard - you don't get the rest that you need," says Erik, one of the older members of the crew.
"Often you don't get as much food as you need and this is cumulative, so as days go on, it gets worse and worse."
Pushing the limits
Privacy is also impossible in such close quarters. The ship has no shelter from the weather, no cleaning facilities and no lavatories.
They will be living virtually on top of each other for six weeks and this will test their friendships to the limit.
All of the crew are volunteers and despite the difficulties involved, most are relishing the prospect.
The crew faces real challenges and risks on the voyage
"It's the adventure of crossing the North Sea, in an open boat like this, and also the social project; with 65 people crabbed down on 65 sq m, to see how you will perform. It pushes boundaries, in all sorts of ways," explains Hans, a young Danish crew member, summing up the feelings of many on the expedition.
The voyage will also be a challenging test for the boat itself. No one knows if it will be able to withstand the rigours of the unforgiving North Sea. Even some of the experienced sailors on board think the ship may run into trouble. Dylan, a New Zealander, has some concerns.
"I was sailing with it to Norway last year and I noticed that when we were surfing with the waves, when the bow of the boat got pushed into another wave that there was only about two centimetres of free board.
"And this was in waves of about three-and-a-half or four metres high. So I am excited to see how it goes in waves of six metres high," he says.
If the team has made a mistake in its reconstruction, the ship could be destroyed by the huge forces of the waves and wind.
Louise, a Viking historian and member of the crew, is well aware of the risks that she and the others face.
"It's mainly the weather conditions and the fact that we are sailing in quite risky areas, that is the biggest worry," she explains.
"The prospect of being shipwrecked is all too real and every member of the crew has received training in cold water survival."
It is the first time in nearly a thousand years that a fully laden Viking warship will sail across the North Sea.
The Sea Stallion represents the highpoint of Viking maritime achievement and it will give historians a once in a lifetime glimpse at how these mysterious raiders from the North spread so far and wide.
The ship's crew will be writing a weekly diary. More regular updates and a satellite map of the ship's latest position can be found at BBC History's Viking Voyage website.
The ship's voyage is also being filmed for a BBC Two Timewatch programme in the autumn.
THE SEA STALLION FROM GLENDALOUGH
1. The crew of 65 men and women will sleep on the open deck, as the Vikings did, and take turn keeping watch
2. Satellite navigation equipment will make sure the ship stays on course. Vikings had to rely on the position of the sun and stars, the colour and movement of the sea and wind direction
3. Oak planks were cut radially for maximum strength, overlapped and nailed together. Axes and other tools used to make the planks were replicas of those used by the Vikings
4. The sail, mast, rigging and rudder on the original were missing so these have been copied from other finds
5. Shields, vital in battle, were tied over the oarports when the ship was in port
Sources: Viking Ship Museum, Denmark; National Maritime Museum, UK. Photos: Werner Karrasch and Erwan Crouan