How old is the tree?
For every year that a tree lives a new growth layer is added to the tree -- a year ring. The year rings can be either wide or narrow depending on climate and soil conditions in the area where the tree grows. The various rings form a pattern that varies from area to area. Every area has its own growth conditions and trees from the same area display similar patterns.
The drawing shows the principle behind year ring dating / dendrochronology.
The inner part of the year ring pattern in a younger, still living, tree matches the outer part of the pattern of an older logged tree. Thus the year ring patterns form an overlapping series. By comparing to even older trees, that have been used as construction materials in, for example, churches, bridges or wells, is is possible to trace the series even further back in time.
Toughness and pliability
The toughness of wood depends on how much it can be bent before it breaks. Tough wood can be bent a lot before it breaks. Trees with tough wood are hazel, birch, elm, willow, ash, hornbeam and maple. Less tough wood is alder, spruce (fir), pine, larch, oak, and beech. The toughness of the wood grows in relation to the water content and green wood is therefore tougher than dry wood. The longship is built from green wood.
The pliability of the lumber depends on how much force is necessary to bend it. The pliability of the lumber increases with the water content and this can be controlled by the boatbuilders through a variety of methods. The lumber can be immersed in water, it can be steamed, or it can be heated over an open flame while being sprayed with water. When the water content of the lumber is suitable it is clamped to the desired shape which it will retain after drying/cooling.
Lumber strength is defined by compression, tensile stress and deflection force coefficients. This is measured in kg/cm2 and normally relates to density. The higher the density of the lumber (i.e. the heavier the lumber), the stronger it is.
Compressive strength
The compressive strength of the lumber is how much force can be applied before the lumber breaks.
Tensile strength
The tensile strength of the lumber is how much one can pull at it before it breaks.
Deflective strength
The deflective strength of the lumber depends on how much it can be bent before it breaks.
Strength is affected by:
Water content - The greater the water content, the less strength.
Knots - The strength and workability of lumber is reduced by knots.
Workability - The lumber is strongest when its fibres have not been cut.
External factors - Fungi, bacteria, and insects destroy the lumber.
Density and strength of different types of wood
Every woodworking tradition puts up its own particular demands as to the quality of the lumber. The carpenter wants wood which has a pleasing appearance, is dry, easy to work, and sands smooth. A wood carver wants soft woods that are easy to work. The cooper needs straight lumber, easy to split, tough, pliable and odorless for making good vats and barrels. A boatbuilder wants durable, strong, pliable and knot-free lumber. If the lumber is healthy then the right type of wood always be used for its proper purpose.
The difference between good and bad lumber of the same type can be very great. The strength properties of willow vary a lot from species to species. In the table below the strength of willow is inferior to both oak and pine. In the experience of the boat builder, however, the goat willow is both tougher and more resistant to impacts than both pine and pine.
Density and strength in different species of wood
SpeciesDensity kg/m3Tensile strength kg/cm2Compression strength kg/cm2Deflection strength kg/m2Modulus of elasticity 1000 kg/cm2Hardness kg/m2
Black Alder53088047085077440
Norway Spruce430900500780110270

The durability of the lumber depends on various factors, e.g. species, growth conditions, and how it is treated after logging. Fungi, rot, and insects attacks lumber and, over time, destroys its strength. Some species of wood, like oak and pine, contain fungicidal chemicals like tannin and resin. An oak post, driven into the ground, will last from 50 to 100 years while a post made from beech will only last 10 years. Beech, on the other hand, will last longer when stored under water.
The more moisture the lumber is exposed to, the greater the risk of fungus attacking the wood. The lumber can be protected with, for example, tar, oil, or paint. The boatbuilders treat the wooden ships with wood-tar, which can make them last for up to 100 years. In the exhibition at the museum you can see how the tar is made.
The difference between good and bad lumber of the same type can be very great. The strength properties of willow vary a lot from species to species. In the table below the strength of willow is inferior to both oak and pine. In the experience of the boat builder, however, the goat willow is both tougher and more resistant to impacts than both pine and pine.
Durability of the wood species used in the construction of the Skuldelev ships
SpeciesUnprotected in the open air (years)Under cover in the open air (years)Indoor and dry (years)
Ash15 - 6030 - 100300 - 800
Birch3 - 403 - 40500
Beech10 - 605 - 100300 - 800
Oak50 - 120100 - 200300 - 800
Alder3 - 403 - 40400
Elm60 - 10080 - 1801500
Larch40 - 9090 - 1501800
Willow5 - 305 - 40600
Norway Spruce40 - 7050 - 75120 - 900
Pine40 - 8590 - 120120 - 1000
The right wood for the right place
It is important to chose the lumber for the individual parts of the boat based on the natural shape of the tree -- for example, crooked growth lumber for the frames and beams. In this manner the natural fiber arrangement of the lumber is preserved giving the viking ships their inner construction strength and stability.
Lumber selection
RødgranNorway sprucePicea abies
SitkagranSitka SprucePicea sitchensis
Douglas fir do not exist in Europe at all. They are a North American/Asian species. Obviously, they are not period.
Douglas firPseudotsuga menziesii
BøgBeechFagus sylvatica
EgOak (probably white)Quercus (Unsure of what kind)
AskAshFraxinus excelsior
(Skov-)ElmWych ElmUlmus glabra
Ahorn (Ær)(Sycamore) MapleAcer pseudoplatanus
Almindelig AvnbøgHornbeamCarpinus betulus
Rød-ElBlack AlderAlnus glutinosa
Småbladet LindSmall-leaved LindenTilia cordata
Skov-FyrScots PinePinus sylvestris
Obviously not period...
Impossible to readand I am tired trying tofigure out what it says...
Selje-PilGoat WillowSalix caprea
Species of wood
The selection of a particular species of tree is an exercise in prioritization of the many properties of the lumber such as strength, weight, durability, growth method and appearance. The boatbuilder wants durable, strong, pliable, and knot-free lumber.
Lumber for boats
Many viking ships are constructed from oak which is stronger than, for example, birch. Birch, on the other hand, is stronger than oak.Fro this reason birch is used to manufatcure easily accesible parts of the ship which can be quickly maintained with tar or replaced. Here durability is prioritized higher than strength.
Lumber for wooden nails
In many viking ships the wooden nails are made from goat willow, which is tougher than oak. However, it rots much faster. Toughness, in this case, is more important than durability.
Lumber for the keelson
The mast of the ship is placed in, and pushes upon, the keelson. Beech has higher compression strength than oak but even so most keelsons in viking ships are made from oak because it is far more durable than beech. In this case, durability is prioritized higher than compression strength.
Lumber for the mast
The masts are usually made from some sort of pine which is elastic. However, pine is not as tough as birch but it is light and grows straight. The low weight and the straight growth are in this case prioritized higher than durability.
The wood in the viking ships
The drawings show which species of wood were used in the construction of the Skuldelev ships.
The original ships can be seen in the museum.
The recontructed ships can be seen in the museum harbor.
Skuldelev 1
Skuldelev 1 was a powerful, ocean-going cargo ship. Length: 16 m. Width: 4.8 m. Date: approx. 1030. Place of origin: Western Norway.
Skuldelev 2
Skuldelev 2 was an ocean-going warship. Length: 29.4 m. Width: 3.8 m. Date: approx. 1042. Place of origin: Dublin, Ireland.
Skuldelev 3
Skuldelev 3 was a small, elegant cargo and exploration ship. Length: 14 m. Width: 3.4 m. Date: approx. 1040. Place of origin: Denmark.
Skuldelev 5
This is probably wrong: Skuldelev 5 was a small, maneuverable warship. Length: 13.3 m. Width: 2.5 m. Date: approx. 1040. Place of origin: Denmark.
Skuldelev 6
Illegible, I forgot my book at work so I can't get the facts.