Vikings were the original marine engineers, and a new British Museum exhibition shows we can learn plenty about navigation and shipbuilding from them.
Attacks by Viking longships on the isles of Britain were common between 800AD and 1000AD. Now one of the biggest Viking ships ever built is being displayed in the friendlier environment of the new Sainsbury Exhibitions Centre at London's British Museum.
The ship, Roskilde 6, was discovered by chance in 1996 when an extension was being added to the Roskilde Ship Museum in Denmark. At just over 36m long, it is four metres longer than Henry VIII's flagship Mary Rose, built 500 years later, and six metres longer than the recreated Viking ship Sea Stallion, which sailed from Scandinavia to Dublin in 2007.
Roskilde 6 dates from around AD1025, when the rule of the Vikings was at its peak. England, Denmark and Norway were ruled by Cnut the Great. It would have needed a crew of 100 to man the 39 pairs of oars and its speed would have been similar to that of the Sea Stallion, which reached a top speed of 20 knots.
Only about 20 per cent of the original timber of the ship survived and this was waterlogged, a problem facing all ships that have been underwater for extended periods. When suddenly exposed to the air, the timbers start to disintegrate.
How do you dry out a ship?
Conservator of the Danish National Museum, Kristiane Straetkvern, was given the job of saving it. First, the ship was carefully lifted up piece by piece and numbered, while scale drawings were made of the individual timbers to give the dimension and shape of the overall ship, which was crucial for the reconstruction. "Even though the timber looked quite intact when it was first exposed, the material was very degraded and required support during recovery," Straetkvern says.
Cleaning the timbers while still wet does less harm to the wood and provides an opportunity to fit in fragments and put loose parts in place. Cleaning also contributes to improved conservation and a better final result as the preserving agents can penetrate the wood and are more evenly distributed. A'clean surface with no deposits of clay, silt'or mud ensures that the outer layers dry uniformly and that the need for later treatment of the wood is reduced.
After the initial cleaning and documentation, the wood was transported to the National Museum of Denmark's conservation laboratories in Brede, north of Copenhagen. Wood that has spent 1,000'years in a low-oxygen or oxygen-free environment under the surface of the water is badly degraded. The cellulose and hemicellulose that wood usually contains entirely or partially disappears, leaving wood cells full of water. If the wood is allowed to dry in an uncontrolled manner, it will collapse and split. To prevent this, water was removed from the Roskilde 6 by a vacuum freeze-drying process. The wood was placed in a watery solution of polyethylene glycol (PEG) 2000, a type of water-soluble wax that soaks into the timber and gradually replaces the water in the wood. Over several years, the concentration in the tanks was raised from 10 per cent to 40 per cent PEG 2000, at which point it was ready for drying. The wood was then frozen in a large vacuum tank at a temperature of -27°C. A vacuum was created in the tank to freeze the water within the wood. When the vacuum was established, the ice turned to vapour and was removed from the tank. This prevented the wood from collapsing, and a shape close to the original was retained. The first of a total of nine freeze-drying processes began in 2009, the last being in July 2012.
"The surviving parts of the wreck consisted of around 200 pieces and most of them were broken in many places," says Straetkvern. "As the components had to be reassembled after impregnation and freeze-drying, the parts needed to be held in the right shape throughout the entire freeze-drying process. The ship was reconstructed on the drawing board, using 1:10 down-scaled drawings from the initial documentation of the timbers. This was an exciting phase as it revealed the original length, breadth, height and shape of the ship. The detailed knowledge gained about the individual parts was transferred to the wet, impregnated wood, which was held in the correct shape by wedges, sandbags and steel braces during the drying."
The final part of the conservation process was to glue the broken pieces together, collect the fragments and remove surplus impregnation agent from the surfaces of the planks, frames and sections of the keel. All the reconstructed lines of the ship - the gunwales, planks, stem and stern, thwarts and floor - have been created in steel, and the positioning of the original frames is reproduced in the stand that holds the original planks. This ensures that the conserved parts fit into the purpose-built stand giving the best possible idea of what the ship would have looked like. The stand itself can be dismantled and consists of parts that can be snapped together."
To ensure safe transport of the ship to London from Denmark, the parts were flat-packed in 35 specially designed crates.
How the Vikings built ships
The rise of the Vikings can be attributed largely to their creation and mastery of the advanced technology of the day. They built long, narrow ships that could travel fast and reach far into rivers and shallow inlets, fully exploiting their natural resource of oak.
They made sure that their ships were robust from the start by using axes, not saws, to shape them. They chose straight oak trunks, split boughs into planks with 'hammers and wedges and then used axes for shaping. The boughs formed the keel, and the joints between the trunk and the branches were used to make the ribs. Stem and sternposts were fitted, while the strakes and planks on the sides were placed so that the lower edges overlapped the top edges of the planks below. This is called building in 'clinker' style.
The mast was then added, holes were carved into the sides for the oars, and the steer-board rudder was fixed at the ship's stern. Ships were made watertight by putting tarred and twisted woollen fibres between the planks.
Viking shipbuilding superiority lasted until other nations developed ships with superstructures or castles, from which they could drop rocks or missiles onto the Viking ships below.
The Vikings did not have maps but used observation and memory. They were rarely at sea for more than a few days at a time, so they used their senses, noting the position of the stars and the sun, the direction of the waves and how the sea changes colour when it mingles with soil from approaching land. Anyone who has sailed down the coast of South America can tell when the blue of the Atlantic meets the yellowy green of the outfall of the Amazon. The taste and smell of the sea is also an indication of land, as are birds. Observations like these were passed on from voyager to voyager.
Recent discoveries have shown that the Vikings made use of a navigational aid called a sun-shadow. A circle of wood or stone was placed where the sun would have shone on it all day. A small post in the middle of the circle would cast a shadow and the length of this would be marked on the board throughout the day. When connected, the marks would form a curve from which the position of north and south could be deduced. The sun is at its highest point at midday, which is where the shortest shadow will appear. A north/south line could be marked by drawing across the board at the point where the curve was nearest the centre.
Once at sea the sailors would align the board according to the sun. If the shadow fell over the line, the ship was too far north; it if fell inside the line, the ship was too far south. Each bearing dial would only have been accurate for a few days each year because of the angle of the sun. Navigators may have had a variety of boards for different journeys and for particular times of the year.
Round-the-world yachtsman Sir Robin Knox Johnson carried out an experiment with the method. "Navigation was entirely by the sun compass. When the sun was hidden by clouds, the boat was steered by keeping it aligned with the waves. The sun compass can only give direction, but the approximate error at the destination was three miles. Further experiments have shown that with the right clear sky conditions, a one-mile error in 50 miles is quite achievable."
There has been a long-held belief that Vikings were able to determine the azimuth of the sun in cloudy weather by using a crystal such as Iceland spar, which has the property of depolarising or fracturing light along different axes. A dot was made on top of the crystal so that when viewed from below two dots appear. The crystal was then rotated until the two dots were of equal intensity. The angle on the top gave the direction of the sun. Credence for this belief was given when the British ship Alderney, wrecked in 1592, was found to have a similar crystal on board.
'The Vikings: Life and Legend' runs until 22 June. Details and tickets at www.britishmuseum.org.