After the devastating fire in 2007, the Cutty Sark has undergone a complete makeover. We take a look at the engineering behind the legendary ship's revival.
The Cutty Sark holds a special place in the heart of British shipping enthusiasts, being one of the finest and fastest examples of a 19th century tea clipper. However, its condition by the late 1990s, after 50 years in dry dock, was so serious that the Cutty Sark Trust ordered a comprehensive survey from Three Quays Marine Services. The survey found that immediate action was needed to prevent the ship from deteriorating beyond repair: “the wrought iron was actively corroding, had become very thin in places and in others completely rusted away. The timber planking was also found to be in need of attention to prevent further decay.”With no water to provide resistance, the hull of the ship was sagging under its 963t weight, and the concern was that she would disintegrate if no measures were taken to stop the deterioration. The Trust drew up a conservation plan to preserve as much original fabric as possible, to use specialised anti-corrosive paints, to restore hull planks, replace supporting struts and cover the ship at the waterline with a glass canopy.
The new Cutty Sark
The Cutty Sark, then, was to receive a complete makeover. Any plans to refloat the ship on water or to keep it as it was on dry dock struts were scrapped.
Grimshaw was appointed as the architect for the project, and one of its directors, Chris Nash, dreamt up the innovative plan of lifting the ship 3m in order to solve the problem of sagging. It would also revolutionise the way in which people looked at the ship, offering the opportunity of passing under, looking up and enjoying her sleek clipper lines from beneath. There was also the commercial attraction of using the space below the ship for restaurants and functions on the dry-dock floor.
To meet these challenges, the Cutty Sark Trust first turned to universities. Dr Iwona Beech and the late Dr Sheelagh Campbell of Portsmouth University carried out tests to reduce corrosion and restore corroded sections where possible. At Greenwich University, consulting Professor Chris Bailey and Dr Stoyan Stoyanov of its computing and mathematical sciences department developed a programme to test the stresses on wood and metal components during the conservation.
Before progress could be really made though, an incident occurred that could have brought the project to a premature end.
On the morning of 21 May 2007 the ship caught fire, and burned for several hours before the London Fire Brigade could bring the flames under control. Fortunately, the ship had been largely dismantled and much of her timberwork was in storage as part of the conservation process. However, the timber planking of the tween deck, which was planned to be reused, was lost in the fire and a small proportion of the hull planks were also damaged. In addition, there was a considerable amount of distortion to the ironwork, mostly limited to the horizontal plating of the tween and main deck.
Structural engineers Buro Happold took over after the fire and a team, led by its conservation engineer Jim Solomon, devised a plan to strengthen the ship’s frame.
The structure consists of 12 metal frames, cradles or bulkheads linked together by a longitudinal connecting metal strake beam of 500x40mm plates at the top just below the tween deck and yoked at the bottom to a newly fabricated keel plate that is bolted to the existing keel throughout the ship.
The frames, built by SH Structures Ltd, are made of horizontal cambered compression members with internal diagonal tie rods fixed to the keel plate. When the plates had been welded, they were hidden beneath the ship’s planking.
When completed, the new skeleton had to be lifted on to supports. These hollow steel compression struts, capacity 50t, were coupled to node points located at the end of each of the cradle frames where they protruded through the hull. The lower ends of these external supports were connected to plates cast into the steps of the dry dock.
The steps, though, had begun to crumble and had to be dug out and strengthened with grouted steel piles before the supports could be connected and the ship lifted. A series of upper ties, or arms, provided lateral wind load restraint; these ties are bolted to reinforced concrete tension piles at the top of the dry dock.
The connection nodes are also used as a secondary support to the glass canopy. Structural steel grade for the members are of S355 J2 to BS EN 10025: hot rolled products of structural steel came from mills in the UK.
Lifting the ship
The ship was lifted at intermediate frames using 24 200Te SWL cylinder jacks, one under each node point. The whole process took two days. Consultants AV Technology (AVT) installed a system of 96 strain sensors to monitor loads in the ship’s metal frame and the supporting props and tie rods that held up the vessel.
Continuous monitoring of loads was achieved by a PC-based data-logging system with real-time displays of loads in all critical locations and with automatic alarm settings to provide text message alerts if any of the loads exceed preset alarm thresholds. Wireless technology was used to transfer data from the data logger on the ship to the real-time display on a PC in the control room.
Neil Parkinson, AVT’s technical director said: “The commissioning of a reliable instrumentation system to provide real-time monitoring of load levels in critical areas was a key part of the lifting operation, enabling the jacking process to be properly controlled to ensure that structure of the ship was not damaged and that the operation remained stable.”
Build by numbers
As the ship was being stripped for restoration, each plank was numbered as it was removed for accurate reconstruction later. Where timber had deteriorated it was replaced with equivalent wood; the intention was to replace like with like where possible.
The tween – the internal deck – was rebuilt using new Douglas fir and caulked traditionally with hemp, which was easier to install and maintain because it was forced into place to provide a tight deck. The main deck was of composite construction, with plywood lower layers and new teak upper layer. The original lower rock elm planks and the upper teak planks were repaired and reinstalled on the ship. The original rule was rock elm below the water line and teak above. Elm does not normally survive for long out of water. Rock elm is the hardest and heaviest of all elms. It is very strong and can be comparatively free of knot and other defects.
One of the biggest afflictions from which the old Cutty Sark suffered was corrosion. The latest measures to counter this came from Leighs Paints, which supplied aluminium and zinc phosphate primers for protection in a marine environment. The paint used was the glass flake epoxy Epigrip M922. Epigrip K267, a MIO (Micaceous Iron Oxide) and Resistex C137V2, a special finish coating, were used for the finish. All old work was painted white and all new work became grey.
To prevent the growth of marine organisms that would, over time, slow the ship down, the Cutty Sark’s bottom was originally sheathed below the waterline with Muntz metal plates laid on layers of felt and bitumen.
However, it was eventually decided to use brass covering because it is more readily available in large quantities. Today’s visitors, as they pass under the ship, can still be dazzled by shining brass panels on the ship’s bottom, 1200x400x0.6mm thick and fixed on with brass nails.
The glass canopy
Seele was instrumental in designing the 90m-long, 20m-wide, 1,500m2 curving glass envelope which slopes away from the ship at an angle of 9°. The height varies from 1.9m above deck level in the middle of the ship to about 2.90m at bow and stern.
To handle the multi-curvature form, Seele developed a free-form gridshell based on triangles with a side length of about 2m. The load-bearing construction consists of steel hollow sections bolted together via six-way nodes. Concealed point fixings within the joints of the triangular insulating glass unit – every single one unique in shape – fix the glass to its support. The entire construction is self-supporting.
The second supports are the peripheral steel members that also support the Cutty Sark in its raised position. The glass has a bluish solar coating to lessen solar gain and indicates flowing water.
When the ship was built in 1869, her sea-going life was expected to be 30 years; she is now 143. Whenever fortune was going against her, something turned up. She survived the opening of the Suez Canal, which allowed steam ships to seize the tea trade. When her wool run days to Australia were over, she was turned into a tramp ship, taking any commercial cargo. When she was no longer making money, a sentimental British captain bought her and turned her into a training ship, in which role she survived until she became a museum piece in Greenwich.
When a ship survives and her contemporaries cross the bar, such as with HMS Victory, USS Constitution and the Vasa, public opinion will not let them go. Money is raised; they survive. The current refit of the Cutty Sark prolongs her life for just 50 years; then the decision to refit her will start all over again. *