By Nathan Williams
Six months ago I set out on the most unusual and the wettest journey of my life.
I was on board a replica Viking longship, the world's largest and most authentic, which was attempting to sail from Denmark to Dublin.
The aim of the voyage was to test the ship and gain an insight into how the Vikings would have fared when making the same journey.
One of the first lessons was just how dependent we were on the weather.
With two days of strong winds in the right direction, we had managed to sail over 250 miles (400km) in one stretch.
But once we reached Norway, the situation changed. For 10 days we were stuck on land with the wind blowing in the wrong direction or not at all.
A Viking ship is very poor at tacking (sailing into the wind by zigzagging).
Using the oars is possible for short distances but getting all the way to Scotland would take hundreds of hours of continuous rowing.
So the only option was to wait for the wind to change.
At first it was a welcome relief after 48 hours in an open boat in the rain. But boredom soon turned out to be the biggest hazard.
Faced with the same adverse winds, the Vikings would have faced an additional problem and I wonder if this could be at least part of the explanation for the Viking reputation for rape and pillage.
They would soon have exhausted their own food supplies and so, naturally, they would turn to the local population to supply them - whether the local population wanted to or not.
Their voyages would also have been far more hazardous without the benefit of modern weather forecasts.
When we did eventually set out into the North Sea, we were warned of a gale and reluctantly accepted a tow rather than face winds that would at best have blown us back to port.
Wind and waves
In fact the greatest test the Viking ship faced came towards the end of the voyage when too much wind, rather than too little, was the problem.
The wind had appeared to be moderate as the Viking ship set off from Scotland heading for the Isle of Man.
But as we reached open sea, the waves started to get larger and the wind rapidly increased.
Soon there were waves coming over the side of the ship and we were all ordered into our survival suits.
Wind and waves provided a severe test of Viking ship design
We knew the situation was serious when we were suddenly ordered to take down the sail and bring the ship to a halt.
The stability of the Viking longship depends on the flow of water over the hull. And the rudder is only effective when moving forward.
Without control the ship was turning more side on to the waves and so more waves were crashing over the side.
The noise from the wind and waves made communication very difficult and no one seemed to know why the sail was being taken down.
Eventually I found the captain who was busy organising repairs. The strap holding the rudder in place had snapped. Until they could repair the rudder, the ship was helpless.
Meanwhile the life rafts were being prepared and it felt to me like there was a real chance the ship might not survive.
Survive it did, however, and with a new rudder strap in place, we were able to sail once more and make it safely to the Isle of Man.
The ship was made as far as possible using the original Viking construction methods but clearly something had gone wrong.
Carsten will sail the Viking ship again in summer 2008
"We have this weak point - the rudder - but everything else seems to work very well," said Carsten Hvid, the skipper.
"I think it has something to do with the materials we're using for the rope and the leather strap. We have to find a new way to make these things."
The ship builders have until this summer to work out a better way of attaching the rudder.
In July, the ship will set off from Dublin on the return journey to Denmark. This time they will travel via the south coast of England and may even visit London, taking a week long detour to travel up the Thames.
"Just imagine the ship going through Tower Bridge - that would be a wonderful image," dreams Carsten.
He feels he now has a much better understanding of the characteristics of the ship.
"We've learned how to handle the ship in different conditions," Carsten says. "The ship is so narrow and so long you have to sail it in its own way compared to other ships.
"For instance, in hard weather we have to reef down earlier than we're used to, because of the very long keel."
There is still a huge amount of data to sift through generated during the voyage. But already they have discovered that the longship is slower than expected.
Overall though, Carsten is very impressed with how the ship performed and is confident it will make it back to Denmark safely. "Basically it works!" he says.
Timewatch: Viking Voyage is on BBC Two on Saturday 5 January at 1955GMT.
THE SEA STALLION FROM GLENDALOUGH
1. The crew of 65 men and women sleep on the open deck, as the Vikings did, and take turn keeping watch
2. Satellite navigation equipment makes 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