How to build a viking ship - Basic elements of norse wooden boat construction

One mast and a square sail

The viking ship had only one mast which could be taken down. The sail was square attached to a cross beam in top. In addtion the mast was supported by ropes from the top down to the stern and stem and to several places along the gunwhale. The crossbeam could pivot around the mast so that the ship could be sailed with wind from behind and from both sides. The main problem was to sail against the wind. It was possible to some degree, but could not sail as sharp as with a modern rig.
The sail was made of woven wool or linen. The ropes were usually tar-proofed hemp. In more rare cases ropes were made from horse tails or even from walruses.

Phoenician inspired rig?

I believe that the scandinavians were strongly influenced by mediterranean rig designs when they started to put mast and sail on their ships. The transition from pure rowing ships to sailing ships probably took place in the 7th and 8th century.
The phoenicians used rig designs with one mast and a sqare sail some 2000 years earlier. Although the phoenicians eventually disappeared, their rig design survived in greek and roman ship designs. Somehow the ideas of such rigs must have travelled to northern Europe. Eg. the romans used sailing crafts when they expanded the Roman empire to include parts of England. Roman coins have also been found as far north as Herjolfsey in Iceland, which may indicated that the romans undertook long voages deep into Scandinavia long before the Viking age. But the scandinavian ships remained rowing ships until the keel was strong enough to carry the strain from a sail in the 7th century. Perhaps the keel was developed in direct response to just meet these demands? The further development of the keel, mast base and rig then must have followed hand in hand.

Carved picture stones from the 8th century from Gotland in Sweden shows several varieties of ships with rig. The height-width ratio of the sail should probably not be regarded as accurate in these images as they are very rough in design and limitated to the shape of the stone. But they all confirm the sqare shape of the sail. Some of these images also picture a lower yard in the bottom of the sail.
The ships in the Bayeux tapestry do not have these lower yards. Instead they show problems with the sail collapsing in its lower part. These ships were built in Normandie by William the Conquerer, and may not be good examples of a viking rig design or the embroidery may be inaccurate in displaying correct rigs.
This indicates that several rig designs existed side by side. Most (or all?) modern viking ship replicas have left out a lower yard, but sometimes include a lower beam for keeping the sail wide at the foremost lower corner while tacking or sailing downwind in modest winds.

The rig of the Gokstad ship

A major problem for maritime researchers and historians is that a viking ship with a complete rig has never been found. Some remains of ropes and parts of a sail was found in the Gokstad ship. The mast was cut or broken and some wooden beams belonging to the rig was found. But still the rig was incomplete, so one is left to some guessing to get the entire picture. The size of the sail and heigth of the mast is widely discussed ever since the Gokstad ship was excavated.
The 1892 replica Viking had a sqare sail, but this was probably too tall. Later replicas have been equipped with a lower and broader sail. An old norwegian rule of thumb says that one should be able to lower the mast within the stern of the ship. In the case of the Gokstad ship this limits the height to about 12 m.

Later wooden boats from western and northern Norway are direct descendants of the viking ships, and are likely to have rigs and sails very much like the vikings used.
The sail and rig designs were usually slightly different in each geographich area and of course in size according to the size of the boat. But the square forms (quadratic, trapezioid or rectangular) were dominant into the latter part of the 19th century. Boats from western Norway usually had lower sails than in northern Norway, and some with trapeziod sails.

From about 1880 triangular sails took over as the main design. This also changed the way the norse wooden boats could be sailed, and in many ways the sailing tradition of the vikings was broken.

The rig of a viking knarr.
(Click to enlarge)

Phoenician merchant ship with similar rig, ca 1000 BC.
Image University of Texas, permission pending.

The Bayeux Tapestry

Nordfjord færing 19th century.

These are the key elements in norse shipbuilding as I see them. Before you eventually go ahead building a boat, seek as much information as you can. Study finished boats in detail and listen to people who have done this before you (thus.. not me!). Take care selecting only good materials. Oak is what the vikings preferred, but pine is also fine. In northern parts of Norway spruce was used, as the climate there is to cold for growing pine in suitable dimensions.

Interior of the norse færing	Splitting logs into strakes. The point is to avoid the marrow.
Attaching the first strake to the keel	Keel support arrangement