Digital model aids ship relaunch

By Monise Durrani BBC Producer, Click On

The Cutty Sark was an impressive ship in its day

Those dedicated to restoring London's iconic Cutty Sark are using computer models to help rebuild it safely and to decide once and for all whether it was the fastest ship of its day.

The Cutty Sark, a Scots-built tea clipper which in its day was one of the fastest vessels in the world, is a much-loved London landmark.

But a visitor to Greenwich today will not see those famous masts because the ship is currently nestling under a layer of scaffolding as it receives a massive conservation overhaul.

At the moment the iron frame of the original Victorian ship is exposed.

The planks have been removed and the iron is receiving a treatment similar to that used on the Forth Rail Bridge - another Victorian structure that exists in a marine environment.

Once the conservation work is complete and the wood has been replaced, the plan is to lift the ship, raising it in the dry dock where it currently sits, to create a visitors' centre underneath.

One chance

The team has to get the details right

From beginning to end, the process has been a virtual one as well as physical.

"We've been using computational technology to build a digital model of the ship," explains Greenwich University computer scientist Chris Bailey.

"Once we've got the iron frame and hull of the ship built into the computer, we can answer all the 'what if' questions - how to lift the ship, how to support it, before you do it in real life, where you only have one chance to get it right."

Using a combination of information from the original Victorian designs of the ship, and modern measuring and imaging techniques, the Greenwich team have created detailed computer models of the Cutty Sark.

They then built a replica of the ship's hull, which was used to calibrate and confirm the information in the computer models. The digital models could then be used to predict stresses as various elements of the conservation project took place - for example, what would happen as planks were removed, or as the ship was lifted.

An image of the ship on a computer screen shows a mix of blue and red - the blue where there is low stress; red where the stress is high - allowing the engineers to tell at a glance where problems could arise, and to virtually test out different methods of stripping or moving the ship.

Sensors

The use of the models will continue when the project is complete and the Cutty Sark is reopened to the public. "Part of our commitment is to preserve the original Victorian fabric of the ship, to arrest degradation so that she will last at least 50 years before another major maintenance is needed," explains Peter Mason, the ship's chief engineer.

In order to do that, the Cutty Sark Trust and the Greenwich University team have created sensors which will be placed around the ship - small squares of iron and wood which mimic the material of the ship, and will give an indication as to how the Cutty Sark itself will wear and weather as time goes by.

Information from these sensors can be incorporated into the computer models, and used to predict - and, it is hoped, prevent - any further damage.

This sort of predictive technology is commonly used in hi-tech applications such as aerospace, but this is the first time it has been used in the heritage sector. "It's very exciting," says Chris Bailey.

"And it means you can do small repairs and ensure no major work is required - so it's very cost effective."

Virtual race

We're going to take the digital model of the Cutty Sark, and build one of Thermopylae, and we're going to race them Chris Bailey, computer scientist

There is one further way in which the computer modelling is an aid to the Cutty Sark Trust - not only can they help ensure its future, but they are casting new light on its past.

The Cutty Sark is remarkable for its unique hull shape. "She was made for a tea merchant called Jock Willis who wanted to make a killing on the tea trade," explains Peter Mason.

"There was a price premium for whoever got the crop back from China the fastest. So he commissioned a young designer called Hercules Linton, who created a ship with a very radical shape - with a very sharp bow, totally different from other ships of the time".

The Cutty Sark's great rival was a ship called Thermopylae, with a more conventional bow shape. It was said that the Thermopylae was faster in light winds, and the Cutty Sark in strong winds. In a race between the two in 1872, the Cutty Sark lost, suffering a broken rudder.

Historians have long wondered what would have happened had it been able to continue unhindered. So now, the plan is to put the two ships back in the water - virtually at least.

"We're going to take the digital model of the Cutty Sark, and build one of Thermopylae, and we're going to race them," says Chris Bailey.

"We're using a technology called computational fluid dynamics - which is used in ship designs today. We'll be able to see the flow vectors around the hull, see the magnitude of flow around the vessel, the vortexing behind, and understand the drag under different sea conditions."

Using the models, they will be able to explore the weather and circumstances which suited each vessel. "There's always been debate over which ship was the best - perhaps we'll be able to answer that," adds Chris Bailey.

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