How to save an aging ship
21 October 2013
Millimetre by millimetre the Vasa ship is collapsing. If the support system is not improved it will finally collapse. To prevent this, a research team at Uppsala University has been commissioned to propose a new support structure for Sweden's dearest museum object.
The Vasa ship has been through a lot. From being a showpiece, disaster area and then concealed below the surface for 333 years the ship has now been on dry land for over 50 years. Visitors from around the world have been impressed by how well-preserved a ship from 1627 can be.
Yet researchers who work with the ship also see something different.
“Stability is poorer than had been expected. The strength of the Vasa oak has been approximately halved. In some places, stability has been reduced by 80 per cent compared with present-day oak,” says Ingela Bjurhager, researcher in applied mechanics and coordinator of the research project ‘Support Vasa’.
The warship Vasa is under attack on several flanks simultaneously. Chemical processes weaken the timbers and we know that conservation using the substance PEG (polyethylene glycol) initiated directly after salvaging operations in the 60's has impaired the mechanical strength of the timbers. Nevertheless researchers are agreed that it was the right decision to use PEG and today the ship contains 50 tonnes of preservative.
Like other structures, the ship has also been subjected to what scientists call “creep”. The timbers are affected and deform over time. Using different methods it has been seen that Vasa is slowly collapsing and becoming wider in the middle. The ship was also designed to float in water, not stand on dry land.
Vasa still stands on the support structure built when the ship was salvaged over 50 years ago. It is a basic structure where the ship rests on keel blocks with bracing along each side of the hull.
“It’s easy to improve on the current support structure but it’s difficult to make one that is optimal. And it must be aesthetic too,” says Ingela Bjurhager.
In two glass containers in her office at the Ångström Laboratory lie stacks of wooden blocks. One contains oak wood from the Vasa and the other contains present-day oak to use as a reference. The doctoral student Alexey Vorobyev performs experiments to see how the timbers of the Vasa react over time when loaded.
A third member of the team, research assistant Nico van Dijk, makes calculations to predict material behaviour far into the future. They share the results amongst themselves. The results from the experiments are added to the calculations, which brings about new questions that need to be tested in the experiments. Everything starts at a micro level to be gradually scaled up to a larger size.
The team also has access to data from previous research and from geodesy measurements performed by the Vasa Museum. Specific points on the ship are measured annually and provide a picture of how the ship moves.
From Nico van Dijk’s calculations, models of the Vasa ship emerge that show how the ship changes over time. From these models, researchers want to be able to conclude which areas of the ship are deforming rapidly and which areas need more support. And when will the ship reach critical deformation, i.e. collapse?
Unquestionably, the ship needs reloading. This means that in the future the ship needs to be loaded in a different way. Points on the Vasa, which today bear much of the ship's weight are poorly suited for this and are unable to do the job.
“Today's support structure is not particularly specialised for Vasa. It could be for any ship,” adds Ingela Bjurhager.
The research now being conducted at Uppsala University will not only benefit the Vasa ship. Important results can be used to solve similar problems with other large timber structures, such as buildings and bridges.
The Vasa team conducts research with a distinct practical objective and with a very popular object in the centre. Research quite different to that carried out only a few corridors away, where most things take place in a world of theory.
Ingela Bjurhager believes it is an advantage that the research is so easy to understand, but circumstances also entail challenges.
“It’s not hard to explain to people what we are doing. However, for those of us working with material modelling it can sometimes be challenging to find the thought-provoking research and research results. Sometimes we see interesting scientific traces that are not really in the remit to follow up in the project. Our aim is to find new things that will generally benefit wood research.”
.. is an interdisciplinary research project conducted by the Vasa Museum and Uppsala University. The project manager is Kristofer Gamstedt, Professor of Applied Mechanics. Research funding bodies are: Formas, Vinnova and the Swedish Research Council. Uppsala University and the Swedish National Maritime Museums are also financially involved in the project.