The Cutty Sark, the world’s last surviving tea clipper and a relic of the great age of sail, usually stands proudly overlooking the Thames in Greenwich, but her years on the sea, and standing in dry dock, have taken their toll.
In November 2006 an ambitious £25m conservation project was undertaken, started with £11.5m from a Heritage Lottery grant. Undeterred by the dramatic fire on board the ship in May last year, the conservation continues and will help to conserve the fabric of the ship, including the timbers.
Launched in Dumbarton in 1869, the Cutty Sark is of composite construction, with an iron frame to which wood planking and outer sheathing is fixed to complete the hull. In earlier ships, such as HMS Victory, the frame would also be made from “compass timbers”, selected for their shape. In later ships, both the frame and hull sheathing were constructed using ferrous materials. This is one of the reasons why the Cutty Sark is unique and offers a challenge for conservation, since we are faced with very different materials.
The Cutty Sark was moved to the dry dock in Greenwich in 1954 where extensive restoration was undertaken and she was opened to the public in 1957. Although removing the ship from water through dry-docking reduced the risk of her hull planks wetting from outside, the deck was still exposed to the elements, including rain. The ingress of rainwater into the hull and its movement towards the lower and less well ventilated parts of the ship have led to corrosion of the iron frame and fixings and localised decay of the insides of the planking, in spite of ongoing maintenance over many years.
Lottery funding will conserve the fabric of the ship
The corrosion of the Cutty Sark’s iron frame has been accelerated by chloride ions. These are present in sea-water and remained on the inner surfaces of materials in the hull long after her final voyage. The conservation strategy involves removing these chlorides through electrolysis. Since this is carried out in solution, it was important to isolate the wood during treatment, as when wetted it has the propensity to move and decay.
For this reason individual planks have been removed from the hull. This is challenging, since many are several metres in length, up to 250mm thick and 300mm in depth. In addition, it is important to maintain their shape on removal. A system has been developed to release individual planks into strongbacks – forms shaped to retain their curvature during the conservation process. Fortunately, Ian Bell, technical manager at Cutty Sark, and the team at Cutty Sark Enterprises have a great deal of shipwright skills, which allow for the careful removal and handling of the planks. This team has taken on most of the conservation works themselves, with their intimate knowledge of the ship a great advantage.
The location of the planks on the ship is recorded and their condition assessed. In most cases the planks are in remarkably good condition, due in part to the fact that they have been manufactured from teak, a highly durable species. Also, the outside of the planks is coated with a layer of tar and felt and finally overlaid with a layer of Muntz metal sheathing. This sheathing prevented attack by marine wood borers when she was at sea, while the felt and tar provided waterproofing.
Where any significant degrade has occurred, in line with good conservation practice, an appropriate repair is carried out, where possible using a reversible resin repair system. The conservation plan does not attempt to remove chlorides from the hull planks, but to isolate these from the iron frame through specialist inert washers.
The fact that the conservation team was on schedule with the removal and conservation of the planks meant that very few were damaged in last year’s fire.
To extinguish the fire, water had to be introduced into the hull, resulting in wetting of some of the remaining planks and parts of the wooden keel. To avoid problems with decay, it was essential to lower the moisture content back below a nominal 20%, the threshold above which wood decay may occur.
Normally meters that measure the electrical resistance of wood are used to assess moisture content. However, for many timbers on the ship, the high levels of chlorides and corrosion by products at their surface, means this type of meter produces erroneous readings.
For this reason, small samples were removed to measure moisture using the proven oven-dry method in TRADA’s laboratories. The results were reassuring: the moisture contents of the planks were too high only near their inner faces, so will be likely to dry down quickly during conservation works over the next 18 months through passive drying. This is something we will carefully monitor at intervals over the next year.
More problematic is the pitch pine and rock elm keel, where the moisture content is now too high in places. The large dimensions of timbers making up the keel and the high moisture content throughout its profile in certain positions mean it will require treatment with a diffusible wood preservative to reduce the risk of decay.
The Cutty Sark is Britain's last surviving tea clipper
The keel cannot be removed from the ship and this offers a further conservation challenge. Its very position, at the bottom of the ship in the dry dock, is one of the main problems. Just as water in a building progresses downwards through gravity, affecting timbers in cellars, any water that has entered the hull of the ship over the years has ultimately wetted the keel. Of course, once the conservation project is completed, the interior of the ship will be kept dry and ventilated to ensure that no further degrade of the hull timbers will take place.
Work on the ship has been challenging but rewarding. It is exciting to be given the opportunity to stretch wood science skills, draw on expertise from experts in timber repair and remediation to solve conservation problems and preserve an important part of our heritage.
Keywords: Cutty Sark TRADA
