Since September, full-time work has been done on the IJssel. Archaeologists, divers, skippers and technicians try to salvage a ship that sank nearly 600 years ago. But how do you get this fragile wooden colossus out of the riverbed? The recovery companies are performing a technical tour de force for the most complex Dutch archaeological operation in years.
The IJssel near Kampen is normally the domain of cargo ships, pleasure yachts and perhaps a lost water sports enthusiast. But since September, an unusual colossus has been anchored in the river:a medium-sized ship, with two pontoons attached to it, a crane on top, which in turn holds a metal structure that sinks into the waves. Divers go under, come back again. People are busy with sonar equipment. Underwater welders are approaching. It is very busy.
What attracts all this activity is the medieval cog buried here in the riverbed, which was discovered by sonar in 2012 along with two smaller shipwrecks.
The merchant ship, which must have sailed the North and Baltic Seas about six hundred years ago, is twenty meters long and eight meters wide. Slightly tilted, it is buried up to meters deep in the sandy bottom, above which the IJssel flows.
How the seagoing vessel ended up in the river here is still a mystery. Concerned archaeologists have their suspicions (perhaps sunk on purpose?), but more research is needed for that…
Normally, the policy for such a large historical find is:let it lie. Then (small-scale) research is possible that is carried out on site and leaves the wreck as intact as possible. Salvaging an ancient vessel can seriously damage it.
But precisely that was not an option with this IJsselkogge – as the ship has come to be called. As part of the large-scale Room for the River project, the IJssel is to be two meters deeper here. And there was only one option to save this wreck:salvage.
No ordinary salvage
The Dutch telephone directory is full of companies that you can call for a salvage. Still, the archaeologists involved, such as Wouter Waldus of the archaeological company ADC, had to be careful who they would hire for this job. “The salvage of a modern ship is usually done with a lot of clumsiness,” he says. “For example, chains are used that pull hard on a wreck. Obviously that was not an option with this hundreds of years old cog. This ship must come to the surface.”
But how do you do it then? “OK, if everything was possible,” says Waldus, “we would remove a whole 'block of sand' with the cog in it from the river bed. On dry land we would prepare the ship in peace. Unfortunately, that is not feasible, it presents too many challenges, both technically and financially.”
Waldus says that a lot of thought has gone into the best recovery method, a job that requires lifting straps or a construction under the colossus and then lifting it. It was immediately clear that it was not possible to have a diver dig in the bottom, because of the danger that the dug hole immediately fills up with sand again and overwhelms the diver.
The solution is to use a so-called spray lance, a tube with holes through which water is sprayed and thus digs through the soil. “This way we can go under the wreckage and make a sort of basket of tires and a metal frame in which the cog is stabilized,” says Waldus. “After erecting the wreck, we lift it straight up from the bottom. Once above water, the ship is given a kind of metal corset, for further stabilization. We also keep it wet to prevent the wood from drying out and being damaged.”
Also important was to remove as much of the excess sediment covering the wreck as possible. “That is a large amount, about two hundred cubic meters,” says Waldus. “Some of this can be done with large pumps, but most of the work has been done with divers who know exactly what they are sucking out and so carefully release the wreckage. Incidentally, all the sand that has been sucked up goes through a sieve that prevents us from missing valuable objects.”
Exceptional good condition
For the time being, a few planks have been brought up from the cog to determine the quality of the wood. That was promising.
“Everything seems to be well preserved, the wood is still relatively strong,” says Waldus. “Furthermore, the cross beams and iron connections between the planks are still present, which keep everything together nicely. So it is still 'connected', stabilized by the soil."
It will be even more exciting when the cog is soon above water and removed from the bottom. The wreck will then have to do without the stabilizing effect of the soil in which it has been lying for centuries.
“There is quite a bit of uncertainty about that,” says Waldus. “To what extent is the wreckage sturdy enough to withstand our approach? It is possible that the wood breaks, or that crossbeams and planks come loose… But if things don't work out in that regard:these kinds of things can often be repaired.”
Waldus says that the project is unique in the Netherlands. Never before has such a large historic ship been lifted, which is also still in a relatively good condition. “We are true pioneers in this field and I think it could be years before such a fantastic find is made again.”
The cog will be lifted on 10 February. Click here for more information.