The razing of the Cutty Sark
On 21 May 2007, the Cutty Sark, the famous clipper kept in dry dock in Greenwich, was badly damaged in a fire. E&T reports on the progress of the legendary ship's extensive restoration.
They all thought she was finished last year. In May 2007, fire swept through the Cutty Sark in her berth in Greenwich. Fire-fighters fought strenuously for more than two hours before it was put out. But all was not as it first seemed; fortune was smiling down on the Cutty Sark, as she was undergoing a refit at the time and had been stripped down. Decks and planking had been removed and the masts, spars and rigging had been sent to Chatham Dockyard for refurbishment.
The Cutty Sark is one of the nation's treasures; a glorious monument to the age of sail, when clippers battled it out across the Indian Ocean, round the Cape of Good Hope, striving to be the first to bring a cargo of tea from China to London. Launched in 1869 in Scott and Linton's yard in Dumbarton, Scotland, the Cutty Sark had a sailing life of about 40 years, but she survived many adventures and eventually became a landmark at Greenwich.
She was built specifically as a tea clipper at a time when prudent ship owners, not yet convinced of the benefits of steam, still thought in terms of wooden ships. She was a compromise: a composite ship, with a wooden skin on an iron structure, with a wrought-iron frame and wooden planking. The decks were of teak and the ship's sides and keel were American rock elm; the rudder was English oak.
To keep her hull clean and fast, she was sheathed with Muntz metal (a brass process of 60 per cent copper and 40 per cent zinc, with traces of iron - two-thirds the price of copper) developed by Birmingham engineer, George Muntz in 1832. A layer of quarter-inch tarred felt was laid under the Muntz sheets, fixed with large copper tacks and an overlap of 1in.
The lower masts were rolled iron plates, riveted together to form tubes. The upper masts were Oregon pine; lower yards iron; while other spars were straight-grained Oregon pine.
The design of girders and diagonal bracing enabled her to be built longer and narrower than similar ships. She was also stronger and could bend on extra sails, increasing her speed from a conventional four to ten knots.
Overall she is 85.4m (280 ft) long and weighs 963t. Her main mast is 47m high; she had 11 miles of rigging and carried 34 sails, with a total area of 32,000sq.ft.
The owners of the Cutty Sark got their timing wrong. They did not foresee the effect of the opening of the Suez Canal in 1869 - which clippers could not use, nor the increasing reliability and economy of steam ships. So, after only eight years, the Cutty Sark was taken off the China run. She never became the fastest clipper on it; her best time to London was 107 days in 1871; and she carried her last cargo of tea in 1877.
She later became the fastest ship on the wool trade from Australia to London, with her best time of 72 days in 1885. By 1895, however, she was beginning to lose money and was sold to a Portuguese company for general world tramping.
In 1922, she was brought back to Falmouth by Captain Dowman, refitted and used to train cadets. In 1938, she was sold to the Thames Nautical Training College at Greenhithe, where she continued as a training ship until 1954 when the Cutty Sark Society bought her and towed into a specially constructed dry dock in Greenwich, UK.
After almost 150 years service, towards the end of 2006 the Cutty Sark was in urgent need of conservation and so the Cutty Sark Conservation Project was set up. Decay would be checked, corrosion treated, and the vessel was to be restored as far as possible to her original condition.
The Cutty Sark Trust was determined that original materials, or near alternatives, were to be used and, after completion, she should be 90 per cent original. This would make her unique among existing old ships. The Mary Rose in Portsmouth, UK and the Vasa in Stockholm, Sweden were submerged underwater for many years and much of their structure cannot be preserved.
The older HMS Victory, also in Portsmouth, has had much of its woodwork replaced but not with original; and the oldest commissioned ship afloat, the USS Constitution, in Boston, US is having its decks replaced with original wood.
But the Project did not stop there. With great imagination, they decided to lift the Cutty Sark and suspend her over the dry dock constructing a museum and visitors' centre below. What is more, visitors could look up at the bottom of the Cutty Sark and admire the dynamic lines of the hull that made her so swift.
The Project was a very complex process and involves many different, but complementary, specialisms.
First, the Trust had to carry out a thorough survey. How strong was the frame, had it deteriorated through stress and corrosion? Could corrosion be checked and the frame strengthened? Would it be feasible to lift the ship without breaking her up?
The Cutty Sark Trust turned to Greenwich University's consulting Professor Chris Bailey and Dr Stoyan Stoyanov of its Computing and Mathematical Sciences Department. They developed a programme to test the stresses on wood and metal components during the conservation.
To design the overall plan, the Cutty Sark Trust brought in architects Grimshaw, and their associates Youmeheshe, with Simon Beames as lead architect.
To put the plans into action, multi-disciplinary engineering consultancy, Buro Happold, was appointed, with associate director Jim Solomon as project leader. They found that fire damage was surprisingly minor: limited to local buckling and distortion which could be resolved by heating and bending.
The policy for corrosion was to restore the robustness of the frame. Where corrosion had weakened it, steel members were spliced onto the frame using the existing rivet holes.
Peter Mason, chief engineer of the Cutty Sark Trust, says the iron frame of the ship was found to be much corroded, particularly in the bilge areas. This is not surprising, since it was the practice to take on sea gravel as ballast and salt reacted with the iron of the ship.
Another wrought iron structure that is troubled by corrosion problems caused by a salt atmosphere is the Forth Railway Bridge. There, a policy of abrasive blasting has been used together with preservative paint. The Trust has followed a similar programme of grit blasting.
It was decided to lift the ship's 15ft, 7t stern counter by crane and place it in a purpose-built electrolysis tank for three to four weeks. This would help to reduce chloride levels and the risk of corrosion to this exposed part of the ship.
Other areas that could not be removed - winches and deck furniture for example, were enclosed in localised tanks with water and electrolysis carried out.
Strengthening and lifting
Jim Solomon and his team worked out a plan to strengthen the ship's frame before it could be lifted.
Cleveland Bridge UK was commissioned to fabricate, supply and install 12 inverted triangulated frames, or cradles, which have been positioned athwartships throughout the ship below the tween deck. The frames are to be made of horizontal cambered compression members, with internal diagonal tie rods fixed to a fabricated steel keel plate that is bolted to the existing keel throughout the length of the ship.
The frames will be tied together using a steel strake plate which runs the length of the hull on both sides. When the plates have been welded, they will be hidden beneath the ship's planking. A node point at each frame projects externally through the hull at a level just below the tween deck of the ship where it intersects with the principal support members.
When they have been welded, the plates will be hidden beneath the Muntz metal, once it has been replaced. The members will be attached to each node point and comprise a hollow compression strut, which will ultimately support the load of the ship once raised. A series of ties will provide lateral load restraint. The lower ends of the external members will be connected to plates cast into the existing dry dock and to concrete piles respectively. The connection nodes will also be used as a secondary support to the glass canopy.
There is a lilypad deck, a new open grid floor, propped along the ship's centre line on circular hollow section columns, with perimeter beams that are supported by tie rods attached to the main intervention nodes. Access to this deck will be via a steel bridge from the dockside and connected at the ship's hull to universal beams cantilevering from the new hatch framework.
Structural steel grade for the members are of S355 J2 to BS EN 10025: hot rolled products of structural steel and will be brought in from mills in the UK.
The ship will be lifted at intermediate frames using 12 number 200Te SWL cylinder jacks that will be connected via temporary steelworks to the node point that projects from the hull at each frame. Bracing between the jacks and props will be required to provide stability throughout the operation, and can be removed when the structure has been finally lowered onto the new external support members and the props removed.
Throughout the lifting operation and load transfer process, a series of strategically placed strain gauges will continually monitor and feedback the strains within the existing wrought iron structure to ensure that it does not become overstressed at any point.
Decks and masts
The aim is to use as much as possible of the original, salvaged deck material in the main deck. A supply of secondhand teak, ethically sourced, has been bought for the re-laying of the main and tween decks.
The main deck's water tightness is critical, and caulking (applying a sealant) will, therefore, be very important. Bitumen-impregnated hemp will be used rather than oakum (old hemp rope treated with tar), as was used in the old days. If tradition is followed, planks will be laid starting toward the stern of the ship and moving forward and outward.
The masts will be treated and painted in Chatham Dockyard and returned when appropriate. The rigging will be treated in a similar way. There is also a plan to have a perforated sail, through which the wind will pass without moving the ship.
The glass canopy
Trusts governing old ships in dry dock are turning to the use of glass canopies for protection. The SS Great Britain in Bristol, UK has a glass canopy around it to help keep it in good condition. But the Cutty Sark has gone one step further. Its glass canopy, of structural glass with plastic sheeting to add tensile strength, will stretch from the ship's side to the dock - it will not only help keep the ship in good condition, but will also allow light to penetrate under the ship so that visitors will be able to view the lines of the Cutty Sark.
Access to the area will be by a uniquely designed staircase. Simon Beames, of the architects Heshemeyou, says its design was taken from research carried out at Greenwich University, where a 3D model of the flow of water along the hull was created. An interpretation of this flow will be incorporated into the design.
The staircase is to be constructed in a wood laminate of engineered timber, applying today's technology rather than the technology of 1869, which was used for the original timber-iron construction.
The curved walls and steps of the staircase will be created by machining 50mm slices of engineered timber laminate that is 160mm thick, up to 24mm long, and 4m wide. The sheets will be bent and shaped into shelves and steps. The contours of the timber will echo the planks of teak and rock elm of the hull.
At the bottom of the dock you will find yet another innovation - ground source heat pumps. Their function is to provide light and heat for the area. The principle of these pumps is to use a ground loop buried several feet deep to transfer energy from the ground.
Lengths of pipe are buried in the ground and filled with a mixture of water and antifreeze, which is pumped around the pipes, absorbing heat from the ground. The process is similar to that of the refrigerator, where heat is extracted from the inside to keep food chilled. An evaporator absorbs heat using the liquid in the ground loop; a compressor drives the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit; and the condenser heats up a hot water tank, which feeds the distribution system.
The plan was for the ship to open to the public in 2009, but the fire has caused a 14-month delay. It has also meant an increase in costs, but significant contributions from the Heritage Lottery Fund and Sammy Ofer have put the Cutty Sark Conservation Project back on target for a reopening in the spring of 2010.