Hidden shallows

Tourists attractions have to keep coming up with bigger and bolder ideas to pull in the crowds. And this rule applies to the established heritage sites just as much as the garish modern theme parks. One particular trend can be seen in the presentation of historic ships in dry docks, which with a bit of ingenuity can be given the appearance of floating on water. The famous tea clipper the Cutty Sark in Greenwich is set to have its dry dock covered with a wave-like steel and glass canopy next year. But the Cutty Sark will be pipped to the post by an equally venerable ship, Brunel’s SS Great Britain in Bristol, which will be presented in an aqueous setting. It is due to be “relaunched” next week after the completion of a £11.3m programme of conservation and restoration.

Although described as a relaunch, the SS Great Britain will not be leaving its dry dock at the Great Western Dockyard. Casual observers will be tricked into thinking it is floating on water by the dry dock being capped with glass, which in turn will be covered with 50 mm of water. This illusion is a by-product of an ambitious and unusual attempt to halt corrosion in the SS Great Britain’s wrought iron hull. The idea is to create a dry environment around the hull to halt the corrosion. The glass lid simply stops moisture getting in from outside. “It sounds a perverse thing to do, to put 50 tonnes of water over an area you want to keep dry, but that’s what we have done,” explains Chris Jofeh, a director of engineer Arup, which is part of the team working on implementing this solution. Although the idea is elegantly simple, actually getting it to work has proved more challenging.

The SS Great Britain desperately needed restoring. Back in 1843 it was the most advanced ship of its day. Not only was it the largest ship in the world, but it also had an all iron hull and was the first ship to have a screw propeller. The Cutty Sark was launched 26 years later but looked like a product of an earlier age. Conceived as a luxury transatlantic liner, the SS Great Britain performed a number of passenger roles before ending up in the Falkland Islands as a floating coal store. Eventually it was dragged up on a beach and left to rot until 1970 when funds were raised to tow it back to where it was originally constructed in Bristol, and rebuilt as a visitor attraction.

Paradoxically, taking the ship out of water has caused the hull to corrode badly. It is full of holes, and there are many areas of flaky rust. The problem is that the wrought iron hull below the water line is ingrained by chloride ions that came from salt water when the ship was at sea. The combination of the chloride ions, moisture trapped in the metal and abundant oxygen from the air has caused it to rust away. The challenge faced by conservators is to stop the corrosion without any physical intervention. “The idea is to do nothing irreversible – that’s the cardinal rule these days,” says Jofeh.

The plan is to stop the corrosion by reducing the relative humidity around the hull to 20%. Two huge dehumidifiers, each the size of a site hut, will be used to dry the air. The first is located in the dry dock and will supply air via ductwork to a series of nozzles at the base of the hull. Dry air will be blown over the hull with the moist air extracted at the top and returned to the dehumidifier.

The second dehumidifier keeps the inside of the hull dry and is located within the ship. “It came in one metre cubes and was lowered into position using an old style block and tackle and rebuilt as a complete unit,” says Jason Hunt, contractor Bluestone’s project manager. A void between the hull and areas open to the public has had to be created to direct the air from the bottom to the top of the ship. Creating the void has been made easier because the interior of the ship has been newly recreated. The void has been hidden by building dummy cupboards at the edges of the boat in some areas, and in others using the packing cases the glass plates E E came in to create what looks like boxes of ship’s supplies. And all this has been achieved without breaking the conservator’s cardinal rule. “In two-and-a-half years not a single piece of the historic boat has been cut out,” says Hunt. “It’s taken a lot of head scratching and careful engineering to fit the dehumidifier and all the ducting in.”

The glass cap will keep the dry air surrounding the ship’s exterior in the dry dock. The panels are supported on a grid of primary steel beams, with secondary glass beams used to make the structure appear more transparent. These are supported by a steel structure attached to the dock wall, and columns around the boat. This structure has to be independent of the boat because of differential movement. “The ship could be up to 150 mm longer on a hot day, and the dock walls also flex by up to 50 mm because of the water pressure,” says Hunt. “If that was pressing on the glass it would break immediately.” The solution is to leave a small gap between the glass and the dock walls. The water is kept in by bridging the gap using a flexible reservoir liner.

Water from the harbour is used to cover the glass. It is a closed system – the water is filtered, circulated over the glass from bow to stern and returned to the other end of the boat to repeat the cycle. Ensuring the glass cannot break is crucial to the success of the corrosion control strategy. “The last thing we need is a torrent of water in there,” explains Jofeh.

He says two different load scenarios were considered. The first is someone high up in the rigging dropping a 1.5 kg hammer. The second could potentially happen during a wedding party – these are held on the boat. “A nasty load scenario is a drunken 100 kg rugby player being thrown from the side of the boat into what people think is a dock full of water,” says Jofeh. Should this unfortunate event actually happen, it will at least be definitive proof that you can successfully make old ships look like they are back at sea.

Project team

Concept architect Julian Harrop Associates

Concept engineer Jane Wernick Associates

Architect Alec French & Partners

Project manager Capita Symonds

Sstructural engineer Arup

M&E engineer WSP contractor Bluestone

Glass and steel plate Space Decks