By Martin Redfern
BBC World Service
That landmark of maritime Greenwich, the Cutty Sark, is in danger of total collapse, say her custodians.
How the ship may look after restoration, on a new kevlar, steel and glass canopy
Rotten wood and corroded metal are placing the entire ship in jeopardy, they told the BBC.
In the Discovery programme on BBC World Service this week they describe an ambitious restoration plan, involving computer modelling and electro-chemical techniques to halt the corrosion.
"She was the fastest ship of her day, a grand ship, and a ship that will last for ever," wrote Captain Moody, first Master of the Clipper Cutty Sark.
She was indeed fast, holding the record (prior to construction of the Suez Canal) for the fastest passage from Sydney to London - a remarkably speedy 67 days.
And she was certainly grand, with the sleek lines of a luxury yacht rather than the cargo vessel she was. But it now seems that, without help, she will not last forever.
The Cutty Sark was launched in 1869 at an interesting time for naval architecture. Iron was beginning to play an important part in ship construction, replacing the all-timber construction of the past.
Soon, steel plate would make it possible to build entire hulls out of metal. But for a few years, ships had a composite structure. The Cutty Sark is one of only three such composite ships left in the world, with a frame of wrought iron clad in wooden planking.
Since 1954, Cutty Sark has remained in a special dry dock at Greenwich, visited by generations of school kids.
But 50 years out of water has done the Cutty Sark no good.
The waterproof caulking between her timbers has deteriorated and without the all-round support of water she has begun to settle.
Worst of all, her iron frame is badly corroded. Many bolts are missing altogether and some of the iron struts look more like lace work than structural components.
Richard Doughty, chief executive of the Cutty Sark Trust is particularly worried about the stern of the ship, which is built out into an elegant overhang.
"Our fear is that the stern will drop and there's a risk that the planks of the hull could come away and you could have a total collapse like a house of cards.
"We are afraid that, if we don't act soon, there could be a catastrophic collapse of the ship, perhaps in four or five years time," he said.
The Cutty Sark Trust has applied to the Heritage Lottery Fund for support in a major restoration project. The first stages are already underway.
Professor Chris Bailey at the University of Greenwich is developing a computer model of the ship, a so-called finite element model, that should show up the stresses in each component and reveal the critical areas.
As Chris Bailey says: "we can try a dismantling the ship in different ways in the computer to see if it collapses. With the real thing we have no second chance.
Simply removing and replacing all the damaged components would be the easy option, says Richard Doughty. Many parts have deteriorated badly and, he says, "if we were to replace them all we would end up with a replica rather than a restoration."
The biggest worry is the iron frame. Even if critical parts were replaced, corrosion in the rest would continue.
Over the years, first sea water and then London rainwater has seeped through the deck, washing salt down to the iron frame.
The grain structure of the wrought iron means that it is porous to chloride ions in salt so they are within the metal, no amount of washing will remove them and the corrosion will continue.
The answer may be electrolysis. In a bath of an inert solution such as sodium carbonate, an electric current can draw the chloride ions out.
For that to work, the iron frame itself becomes the negative electrode and a stainless steel or titanium positive electrode or anode draws the negatively charged chloride ions towards it.
Peter Lawton of the Hampshire County Council Museums Service has already tried the technique on a World War One ship, the M 33.
That was a major project but comparatively simple as the vessel was all steel. When he tried it on a test sample of Cutty Sark iron and timber, it produced a stinking black slime. This could have been caused by microorganisms from the wood that became active as the conditions changed.
Kilogram of salt
Cutty Sark Trust CEO Richard Doughty and BBC WS Discovery presenter Georgina Ferry
So scientists from Portsmouth University were called in to help.
Under the direction of Dr Sheelagh Campbell, they have now devised a process in which a small amount biocide is added to the electrolyte. They have tested it in a small area towards the stern of Cutty Sark and successfully removed half a kilogram of salt.
It would be a near impossible job to perform electrolysis on the entire iron frame of the Cutty Sark.
The plan is to concentrate on the most critical, complex and corroded part along the keel. Even that would involve filling the hull up to the lower deck with electrolyte and, to balance the pressure, probably flooding the bottom part of the dry dock with Thames water.
The alternative would be to remove all the planking which would simplify the electrolysis but be a real test for Chris Bailey's structural model.
Once restoration is complete, the ultimate plan is to display the Cutty Sark in a striking new way. Instead of sitting in the dry dock supported by numerous props and stays, it would be suspended in a Kevlar cradle on a cantilevered steel frame disguised to look like the surface of the sea.
From above it would appear to be sailing high seas once more, whilst underneath visitors could walk beneath the hull to a new exhibition area.
Richard Doughty and his colleagues are hoping for more than £20 million to make all this possible and preserve our maritime heritage.