The bolts holding Vasa together are corroding and thereby weakening the ship's structure. To improve structural stability, we are now replacing around 5000 bolts. The work is reckoned to be finished by late 2017.
Vasa was originally held together with iron rivets which corroded away during the hundreds of years of burial. In order to raise and give structural stability to the ship, more than 500 mild-steel bolts were inserted into the original rivet holes in the 1960s.
Over time, the bolts have corroded and the ship has deformed. The corrosion weakens the bolts and they have deformed along with the ship. Corroding iron also causes chemical reactions in the wood which can result in loss of wood strength.
New corrosion-resistant bolts
In 2011, The Vasa Museum and the Swedish steel company Sandvik initiated a long-term research and development project to replace the bolts. Specially-designed bolts of high alloy corrosion-resistant steel were developed in a joint collaboration. The steel, SAF 2707, is normally used in the demanding oil-and-gas industries and can withstand exposure to the harshest environments.
The material has many advantages. Its high mechanical strength means that the bolts are lighter in weight, which in turn reduces the total weight of the ship, and its high corrosion-resistance reduces the risk of chemical reactions in the wood. It is also an environmentally friendly alternative, since steel is recyclable.
Each bolt is a new challenge
Replacing the bolts is complicated. The 1960s bolts may be stuck fast or sit in inaccessible places. Each bolt is a new challenge.
The old bolt is pulled out by a jack, sometimes with the help of a sledgehammer. Corrosion and other loose material are cleaned out using a steel brush and vacuum cleaner and the hole is enlarged slightly to take the new bolts, which have a slightly larger diameter than the originals. To assist insertion of the new bolt, the hole is then sprayed with soapy water solution, an environmentally-friendly lubricant.
Gentler to the ship
The new bolt design is much more complex than the old. While the 1960s bolts were composed of a bolt, washer and nut, the new bolts have eight parts: a tube, threaded rod, compression spring, nut, bolt head, a special washer to hold the spring and a cylindrical cone.
First, the tube is pulled into the bolt hole by a jack. The threaded rod is then inserted into the tube and fastened to the washer and nut on the inside of the ship. The spring which sits inside the nut makes it possible to determine just how tightly the bolts are held, which is gentler on the ship.
To minimize the stress in the timbers, only every other bolt is replaced in a timber at one time, and the remaining bolts are replaced at a later stage. A maximum of ten bolts can be replaced every day and the goal is to replace 1000 bolts per year. We reckon the bolts will not need to be replaced for another 150 years.