Easing the ravages of time

In the 17th century, vessels were not built from plans or drawings but from “rule of thumb”, based on a shipwright’s instincts and his sea experience. The builder of the warship Vasa poorly estimated its proportions. The underwater section of the vessel was far too small for the visible part above the waterline, making the ship completely unstable. On the day of its maiden voyage in 1628, the Vasa was loaded to full capacity. 64 bronze cannons were proudly displayed with all gun port windows open. As the Vasa left the harbour and its sails filled with wind, the boat rolled dangerously to one side and was swiftly filled with water coming in from all the open gun port windows.

Although the ship sat in less than 40 metres (131 feet) of water, it remained to a large degree unscathed in Stockholm’s sheltered harbour until 1961. Now 333 years after sinking, the Vasa is an almost untouched piece of 17th century Swedish history that has returned to the surface. Before being moved to the museum where it now rests, the Vasa was stored in a harbour shipyard with little protection to elements. It was coated with PEG, a chemical compound that replaced the water in the wood, and helped to prevent shrinkage and cracking of the 300 year old timber. The first visible signs of change were seen in the 1990’s when white spots appeared on the surface of the wood caused by sulphur and iron used to build the ship.

These visible signs ultimately led researchers to focus on changes taking place below the surface and the first monitoring of the ship began in 2000. At that time, it was decided the best device to monitor the vessel was the Leica TDA5005. The data collected proved that even with PEG treatments, the structure of the ship was seriously altered by gravity. The 300 year old wooden structure of the ship had already lost over 40 % (in certain parts, even up to 80 %) of its mechanical strength. It was absolutely necessary to collect precise data in order to decide how to stop deformation and best conserve this historic vessel.

对结构的正确支持
今天，VASA坐落在专门为船上建造的博物馆中的标准存储块上。这些常见的存储块可以追溯到1960年代，并且没有为船提供适当的支撑。研究表明，为了正确地支持这艘脆弱的历史船，需要迫切需要建立一个新的存储系统。但是，为了使木匠构建正确的支撑结构，需要收集和分析大量准确的数据，以确切确定木材和船舶的结构如何在时间过程中进行化学和机械变化。选择了使用Leica Smart-Worx Viva软件的Leica Nova TS50来收集数据以制作一个可以设计，测试和使用的原型。

Monitoring
监控“瓦萨”号每年发生两次。每一个surveying epoch takes roughly ten working days to accomplish, depending on the how many visitors are at the museum. Measurements are made by museum staff and the Department of Geodesy and Satellite Positioning at Kungliga Tekniska Högskolan.

In order to collect data from the Vasa with absolute precision, no less than twenty-nine prism points are attached to the museum’s static walls. These serve as reference points to get the exact position of the Leica Nova TS50. Once these are calculated, the operator measures thirty-three prisms that have been attached to the inside of the ship’s pliable hull and about 330 customised reflector tapes on the outside of the Vasa to collect information regarding deformation of the ship’s structure. To ensure that data collection has been carried out correctly, a second measuring procedure is carried out from different setup position. This process is done over several years, again and again, in order to determine just how fast the deformation is occurring.

每个时期之后，将处理数据并将其与船上进行的早期监视时期进行比较。结果证明，随着时间的流逝，该船已被重力倾斜和拉下。

由于最近15年的监测，将开发一个新的VASA支持系统。超过600万瑞典krona（690,000美元 / 650,000欧元）已用于研究用途，并将研究木材质量和船舶结构在随着时间的流逝中发生变化。研究将持续到2016年，此后将建立设计新的工作结构的基础，并且可以开始进行船舶存储块的实际结构。

除了帮助设计新的支持存储块的VASA外，使用Leica Nova TS50收集的测量数据肯定会有助于确定瑞典船上时间破坏的扩展和速度，并将使研究人员能够更好地预测未来的变化在木材中，这对于VASA的未来非常重要。

由Rikard Evertsson和Mattias Bornholm撰写