A large single square rigged sail on a mast set mid-ship is one of the defining visual elements of a classic “Viking” ship.  Although the single square rigged sail was also used in various parts of Europe both before and after the Viking period, and continues to see some limited use even today,   its use on the large Scandinavian warships represented a high point, with  Viking ships likely being by far the most widely recognized of the vessels that  used it.


Although the first crude sails were probably hides, or woven reed mats, tied to and supported on two poles expediently attached to the gunwales of primitive boats, or even affixed on rafts, recognizable square rigging of a sail on a single central mast appears very early. A clay disk from 5000BC in Mesopotamia depicts a square rigged boat. It can be reasonably  inferred that if there is a surviving depiction from a particular early date like that, the technology was likely already in use for some time before then.  Subsequently, single square rigged sails were commonly used by the Egyptians, the Phoenicians, the Romans, and later the Celts, the Germanic peoples, and the Slavs, as well as the Scandinavians.


There is debate as to when the Scandinavians first used sails.  Although  a lack of archeological evidence of masts on early Scandinavian ships causes some to contend that sails weren’t used before  about the 7th century, there is evidence of earlier trade by sea with places where ships were rigged with single square sails centuries earlier -   e.g. Ireland, where a beautiful gold model  from the first century BC definitely depicts a ship with the mast and yard of a square rig. It thus seems highly unlikely that, having seen sails in use on foreign vessels, the Scandinavians would not have copied them. Given that some Irish sea-going vessels that had square-rigged sails were constructed of relatively light wooden frames covered with animal hides, a mast for such a sail rig could obviously be set in a vessel without the heavy timber mast block on keel arrangement seen on ships of the Viking period. Such vessels likely used leeboard type keels or may even have improvised such with oars lashed to the gunwales. Thus, lack of mast blocks and keels in archaeological finds of early vessels in the Baltic area is not conclusive evidence that sails were not in use. 


The keel running along the full length of a Viking ship was well suited to, and undoubtedly developed due to the use of, the square-rig sail. When on a “reach” with the wind abeam, such keels resisted sideways slippage the entire length of the ship, making it easy to maintain course while converting the lateral force on the mast at the center of the ship to forward motion  The ready availability of long, large, straight timber in the Baltic Sea area and the British Isles made it easy and economical to lay down such keels


The typical Viking ship rigging consists of a fore-stay running from mast top to the stem post, usually a back-stay running from mast top to the stern post, and one or more shrouds on each side running from mast top to the tops of the port and starboard gunwales somewhere toward the stern.  In some period depictions of Viking ships there is no back-stay shown, while there are multiple shrouds shown on each side of the ship. Some of the reconstruction Viking ship use that configuration with good results. However, wind pressure on the sail pushes the mast forward and puts tension on the shrouds, which then put upward and forward stress on the rivets of the top strakes, with the possibility of  the  stakes splitting along the rivet line. Thus having a backstay running to the heavy stem post take most of the stress provides an extra margin of safety. The mast is also rigged with two halyards to raise and lower the sail, and the sail is controlled/trimmed by a brace line at each end of the yard, and a sheet line at each bottom corner of the loose-footed sail..


Although the term “square-rig” is used, there is no requirement that the sail in such rigging  be a perfect square. A rectangular or trapezoid sail qualifies as long as it is suspended from the mast by the middle of  its head and the yard to which it is attached.  There is some debate as to whether the sails of Viking ships were rectangular with the long dimension horizontal as appears in almost all period depictions of Viking ships, or whether the long dimension was in the vertical orientation as was used in Scandinavian square-rigged  vessels in the modern period (often with a trapezoid sail with the head narrower than the foot) , and as is used on some of the European reconstructions of Viking ships.(1)  Each configuration has some advantages and drawbacks;  e.g. vertical orientation requires a taller mast which shifts the center of gravity of the ship upward, whereas horizontal orientation requires a wider yard which makes sail handling more difficult.  However, given that virtually all period carvings and images of  Viking ships across several centuries from many different areas show the long dimension horizontally, the contention by some writers on the subject that  such images were all distorted due to their carvers all taking the same artistic license, and thus that the sails of Viking ships actually had the long dimension vertically , is an opinion with no real foundation. Rather, given that the period images show the horizontal dimension as the larger one, it can be reasonably inferred that the difference between vertical and horizontal dimensions was obvious enough to make that consistent impression, and thus sails of the period may have had even greater width to height ratios than the 5 to 4 ratio of the sail normally used on the Longship Company's Viking ship reconstruction Sae Hrafn.  In any case, unless and until archaeological evidence to the contrary is found, the graphic depictions must be relied on as to which configuration is most authentic.





(1)   Some of the European positions in this debate are given in:  Time to revise our view of Viking ships.   By: Asbjørn Mølgaard Sørensen

 Science Nordic April 21, 2013