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Saving the Cutty Sark


One of London's landmarks is in serious trouble, and only the latest IT technology can save it. Helen Elias reports from the Cutty Sark.

The world famous Victorian tea clipper Cutty Sark is in serious need of major conservation, before its rusting wrought iron frame and rotting timbers cause the ship to be closed to the public.

Conservation awaits the outcome of a £12M funding application to the Heritage Lottery Fund. Meanwhile, computer models devised by the University of Greenwich are being used to save the ship from total collapse.

'Cutty Sark is in peril, ' says Richard Doughty, chief executive of the Cutty Sark Trust. 'Greenwich University's innovative software reassures us that we understand the full implications of our whole programme, and gives us the vital intelligence to inform the decisions we have to take in the conservation process.'

Part of the problem facing conservators is discovering what residual strength remains in the ship's hull. They need to determine the effect of removing any structural member for conservation, or, in the worst cases, replacement.

The University of Portsmouth is looking at using electrolysis to halt corrosion, a process that worked well on the all steel frame of another historic ship, the Minerva.

However, this process may be less effective on the Cutty Sark's composite frame. If electrolysis does not work, the only other option is to dismantle the Grade I listed structure - plank by plank or one iron frame at a time - to clean the tightly knit surfaces before replacing members in situ.

So state of the art research technology is being used to discover exactly how much residual strength is left in the ship.

For this the trust approached sChris Bailey, professor of computational mechanics at University of Greenwich's school of computing and mathematical sciences.

A grant of £135,000, 60% from the Department of Trade & Industry Knowledge Transfer Partnership Scheme and 40% from the Cutty Sark Trust, has funded Bailey and his research team's development of a finite element analysis (FEA) programme that can calculate stress for any point in the ship.

This allows the conservation team to see the impact on the remaining frame of removing any element in the structure.

The programme will show behaviours in the structure while the element is removed. It will indicate an optimum programme for removing members to reduce stress and shifting loads within the composite frame.

Bailey's team has two years to develop and analyse the data needed to build the virtual map of Cutty Sark.

'Our first task is to undertake a detailed survey of the entire ship, looking at all the design records and actual geometry of the frame itself, so we can understand how the ship was put together, ' explains Bailey.

'We will also survey and capture the regions of severe corrosion. Combining this information with the university's own data on the performance under stress and strain of wood and iron will give us a damage map of Cutty Sark.

'Areas where the rust is more pronounced could well affect the behaviour of the vessel, ' he adds. 'Wood behaves differently to iron under both stress and strain. Iron that has been degraded by corrosion behaves differently to iron that has just been cast.'

Bailey adds that the FEA technology being deployed at the Cutty Sark is regularly used in complex structural mechanical materials analysis projects for the electronics and aerospace industries, looking at the properties of any material from ceramics and fabric through to plastics and glass.

The difference here is that for the first time the subject matter is a historic structure. Analysis will be split and run on a number of processors simultaneously to produce the highly detailed finite element mesh that will demonstrate the actual ship's structure. 'The analysis software links into an optimisation tool which allows us to find optimal values for the conservation plan, ' Bailey adds.

'Combining the FEA results from the damage map with the university's own data on the performance under stress and strain of both wood and iron will give an accurate value on stress predictions in the ship's structure at any one point.

'It will allow the conservation team to capture the changes in the structure that could come about when things are moved, with sensitivity analysis showing the magnitude of any change in the ship's equilibrium before it takes place for real.'

The impact over time of removing a member will be presented - treatment could cause a piece to be out of place from a day to a week or more - and the programme will be used to help conservators understand the transient nature of supporting the ship during this process, propping and supporting in the best places to prevent a local or major collapse.

A computer on board the ship will assist conservators on a daily basis, working with the virtual model and the real thing side by side to identify the least risky option for each individual component.

The University and Cutty Sark Trust plan to use the technology immediately to help preserve the ship. From 2006 onward they plan to market the system worldwide as a facility for the careful preservation of other threatened historic marine and land based structures.

High and dry

The endangered 130 year old Cutty Sark has sat in drydock at Greenwich for 50 years, docked down on a continuous concrete plinth.

The last surviving example of an extreme clipper, Cutty Sark was built to Lloyds Rules in Dumbarton in 1869, for the lucrative China tea trade. The construction of these fast ships varied: some wood, some iron. A few clippers, including Cutty Sark, were built to a strong composite design combining wood and iron.

The ship has a wrought iron flat plate keel, laid over a wooden keel and swept up at the ends inboard of a wooden stern and stern posts. It has a wrought iron framework of frames and reverse frames, floors, box keelson, side and main and 'tween deck beams, decks stringers and mast partners, underneath a wooden skin.

Teak was used for the main deck and topside planking; American Rock Elm for bottom planking, keel and false keel, with the 'tween deck planked in yellow pine. Yellow Muntz metal - basically brass with a higher than normal zinc content - was used for fastenings.

This composite construction, which gave the ship the great strength it needed under full sail, is now the reason why Cutty Sark is in such deep trouble. Iron members are in a serious state of decay, rusted through in some places. Invasive corrosion can be seen all over the ship, which is now in such a parlous state that there are fears it may not survive the proposed conservation process. Freshwater seeps in through the leaky main deck and topsides, running into the bilge, causing electrochemical decay in the wooden keel. All the floors are heavily corroded.

The invasive build up of progressive rust action can be seen everywhere, particularly at the meeting points between timber and iron, and iron and iron, forcing the materials apart and causing rivets to shear, and metal bolts to fail.

The structure is being forced apart everywhere, in an escalating process of degenerative corrosion.

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