Unlike modern vessels, in which the aim is maximum stiffness in the construction and materials chosen, Viking Age ships were very flexible. A stiff hull gives maximum speed, because it makes it possible to get exactly the shape you want. With a flexible hull, the shape will change under the impact of the rig and the sea, which is a disadvantage as far as speed is concerned.
It is a matter of discussion as to whether the Vikings deliberately built their ships to be flexible or whether it is the result of their technology and the way the ships were built. Some people think that a flexible hull gives good seaworthiness. I think that if you try to build a light ship which is also long, you will get a flexible ship. It is important that the flexibility is very evenly spread through the clinch pins, wooden rivets, the evenly cut board planks, the compass timbers and the whole construction. The stiffest part of the construction will be the first to break.
In more recent traditional, clinker built open square-rig boats, the builders and the people who sailed them acquired considerable experience with flexibility, and they could “shake” the boat at one end and judge whether the boat was a good, not so good or perhaps even a poor boat. I am sure the Vikings had the same discussions about flexibility. I think that what they would have looked for was a uniform flexibility throughout the construction.
To learn more about this subject, we keep an eye on the Sea Stallion all the time. What gets worn out or breaks and how flexible is the ship really? Flexibility is very hard to measure, and we have concentrated on just demonstrating that the ship does move, both as a whole, and individual parts in relation to each other. The measurements will have to come later, made by people who understand that sort of thing. But with regard to flexibility and movement, I can state that:
- Thwarts move in from and out to the ship side, and diagonally in relation to each other
- Braces move in and out
- Joints in the lengthwise stringers twist
- Beams move diagonally in relation to each other
- The ship’s sides are pushed in and out
- The rigging can be completely tightened up one moment, and hang loose the next
- The mast fish moves from side to side every time the sea Stallion hits a new swell
- Clinch pins break due to the many movements – in boards, braces, stringers and planks
- Spikes get pulled out of braces and joints
- Wooden rivets spit wedges out.
Most surprising and almost scary is the way the whole fore ship and stern move 20–50 cm up and down and to the sides. When the fore ship is on the way down a swell, the stern twists the opposite way up the same swell. It really shakes you the first time you see it, and after the first few sail trials, we also thought we should strengthen the ship – to limit the flexibility. We looked again at the original ship and other ships of the same time and here we found an option for strengthening the ship which is described elsewhere on this site.
Both out here on the North Sea and when we “tossed around” on the Celtic Sea, the thought struck us: How many times can this ship make such a trip and how long can it last? The archaeologists think Skuldelev 2 was about 25 years old. If we sailed the Sea Stallion a couple of times a year to Dublin or the British Isles for 25 years, I think it would be much more flexible to sail with than it is now because everything would have moved so many times. The holes for the clinch pins would be worn so big the ship would leak, and a lot of boards, stringers and braces would have been replaced or repaired. We would be on our 8th or 10th rudder, because the others would have been worn out by the birch tackle. The Sea Stallion would have worn out up 3–4 sails and a lot of rope.
After 25 years and a lot of long trips, I would probably politely decline the chance to sail on the Sea Stallion in the North Sea or the Celtic Sea. But that is far from the same as to say the Vikings would not have done so.