The cleaved plank

The saw was not used when making planks in the Viking Age. Instead, planks were cleaved as illustrated to the right. When cleaving planks for boatbuilding, it is important to find the right tree in the forest with the necessary dimensions and quality. The tree must be straight-growing and knot-free, and it must be big.

The outer sapwood  - which is lighter in colour - rots too quickly and cannot be used, and the centre of the trunk (pith) grows irregularly, so that must also be cut away. Therefore, the tree essentially needs to be oversized. If many broad planks are required, then the diameter must be increased.

In an oak tree, the pith rays (see illustration to the right) extend from the pith to the tree’s outer edge, just like spokes on a bicycle wheel. When oak is cleaved, it splits along both these pith rays and the tree’s own fibres. If the tree is twisted, the cleaving will also follow this twist. Usually, you can tell if the fibres are twisted or straight-growing by looking at the bark.

When boatbuilding, a straight-growing tree is always optimal, as it is easier to produce planks with an even thickness if the cleaved sections aren’t twisted. However, a twisted plank still has the same strength, so long as the fibres are not cut over in an attempt to straighten the plank.

After cleaving, the pieces are roughly shaped with an axe. The planks are then smoothed using a broad-bladed axe, the long edge of which serves to even out the surface of the planks. On some boats, tool marks from axes can be seen while others bear traces of the use of planes. A plank is called a board when it is placed on a boat.

Cleaving oak for Gislinge III

Production of the planks began at the start of June, with the cleaving of the oak. The initial cleave into halves went smoothly and took just 50 minutes from start to finish. The work then continued splitting the halves further into quarters, eights and finally sixteenths. Occasionally, with a very large oak, it can possible to cleave down to thirty-two parts, but that wasn’t the case this time.

By clicking on the images, you can follow the process from start to finish. 

A line is cut into the root end of the oak using an axe and small wedges are inserted.
The log is ready for cleaving.
Asger begins to hammer in the wedges.
Larger wedges are then introduced.
The split at the root end initially looked to be quite curved but quickly straightened itself out.
Martin and Birger use an axe and a froe to cut through the fibres that cross over the line of the split.
Wedges are now added along the sides as the split progresses.
Asger and Birger continue to expand the split at the root end, while Martin and Ture continue working along the side.
Martin uses a froe to cut through any wayward fibres.
The split works its way along the length of the log. The yellow line marks the spot at which Birger estimated the split would occur. He was close...
Asger and Martin begin to rock the log from side to side at the root end, while Birger cuts through the last stubborn fibres with the froe.
Exactly 50 minutes after the first blow of the hammer, the log cleaves in two.
The boatbuilders immediately set to splitting the first half into quarters.
The half-log splits into quarters.
Ture uses an axe to mark up the line for the next cleave into eighths.
He inserts small wedges along the line.
Larger wedges are then added as the split progresses.
Birger works larger wedges in along the side of the log.
A large knot at the end of the log put up a bit of the fight but eventually gave up and the piece was split into eighths.
Martin marks up the line for the next split down to sixteenths.
Birger hammers in the first small wedges.
Larger wedges are added...
And the work continues as the split works its way along the oak.
The log is cleaved to sixteenths. These thin sections will then be dressed to make planks for the Gislinge Boat.
The boatbuilders assess the day's work. Now the long weeks of plank production can begin.

Some of the cleaved pieces were roughed out by cutting scores and removing the joggles in between, while others were thin enough to just begin hewing straight away. As Gislinge III was built as a reconstruction of an archaeological find, there were specific parameters that had to be respected when building the boat. The dimensions – and in some cases, the shape – of the planks was dictated by the surviving planks on the original boat, so the boatbuilders had very specific measurements to match.