Architecture: In full sail under a glass sky

In the National Maritime Museum, a boat is becoming part of the building.

All that remains of a grand galleon of a 200-year-old French sailing ship that survived the Battle of Trafalgar is a sliver of the stern. Crash- landed inside the new glazed courtyard at the National Maritime Museum in Greenwich, HMS Implacable, as the Navy renamed the ship after it was captured, looks suspiciously like a bit of Liberty's half-timbered facade pasted on to the Victorian buildings that house the museum. The architect Rick Mather, with the Building Design Partnership, has made a quadrangle in its heart by building a fourth wall, the captain's bridge, and glazing over 11,250 square metres with the largest clear-spanned glass roof in Europe.

This vast indoor marina without any water in it will house a fleet of beached boats when it opens in March next year. Robin Knox Johnson's little yacht, in which he sailed single-handed around the world, is anchored beside rowing-boats and catamarans. Celebrity boats include Prince Frederick's gilded barge, on loan from the Queen, which inspired Handel to write the Water Music; a Greenpeace dinghy that saw action at Bikini Atoll in the Pacific where the French conducted nuclear testing, and the 200-year- old stern of HMS Implacable. Seized after Trafalgar, the ship was used to teach sailors until the Navy scuttled it in 1949 off Portsmouth. Pig- iron ballast fell through its bows when the explosives were detonated, and it bobbed about the ocean for days. Now its figurehead of a snake- haired Medusa who turns all who look upon her to stone is fortunately under bubble wraps while Neptune Court is being built.

That glass ceiling, fritted to block 67 per cent of the sunlight, has made the beautifully lit enclosed courtyard that is known as Neptune Court. A network of steel girders made in France by Eiffel stretches like a high- wire safety net below a toughened glass roof that cleaners can walk on. The roof lattice has more struts than technically needed, but the real feat was to free the interior from a lengthy line-up of support pillars for the roof. Rick Mather, who was brought into the project late on, after the lottery Heritage fund had refused to back a scheme to land the equivalent of a flying saucer in the space, fought to stop those regimented lines of pillars being built inside the courtyard. Instead he toughened the load-bearing walls by underpinning them. The result is a soaring, uninterrupted space.

Greenwich at zero degrees latitude is where the eyes of the world will focus on the millennium. It's where Great Britain launched herself as a seafaring nation and cracked the problems of navigation. To make sure that it doesn't lose its way in the Millennium Experience, the National Maritime Museum, which already gets half a million visitors each year, is redeveloping its exhibition space. They have more than 750,000 photographs and 4,000 paintings in the world's best collection of seafaring pictures, and they want to tell the story of our maritime history with more displays in 16 new spaces, which are due to open early in 1999. A cube, a sphere and a rectangular tower will house displays on the future of the sea. The Heritage Lottery Fund gave them pounds 12m and they managed to raise a further pounds 5m, mostly from P&O - which is why you'll find cruise liners on exhibition.

"Also, they're enjoying a revival in popularity," the museum director, Stephen Riley, points out. Only six months before it opens to the public the National Maritime Museum needs another pounds 3m sponsorship, which is why HMS Implacable is taking an early photo-call. Too early - the ship's remains are still framed in scaffolding.

"Four weeks ago we started assembling the stern on site. I'd hoped to finish it yesterday." Kelvin Thatcher tries to explain the laborious business of restoration. For nearly 50 years Implacable's carvings, decorations and 16 windows in two rows, which were removed by the Navy, have been kept in boxes at the museum. Kelvin Thatcher, a Norfolk maritime conservationist and model ship builder (and one-time student of architecture), has been piecing together more than 220 parts like a jigsaw, a far tougher task than making a fiddly model of the Titanic for an Alaskan museum.

It took a year to clean the timbers, and then the workers needed to plot the curve of the stern with a scaffolding framework. Originally, copper rivets bolted the columns and windows on to a solid oak frame. Now they are fixed to the wall on a plywood backing. Just getting the curves right was difficult. Some of the curved timbers, bent in the 18th century when the wood was cut green, had straightened; others went down with the ship and timber technology today can't replicate that kind of bending. Kelvin Thatcher had to make a series of cuts to weaken the timbers to get the curves he needed. Since there is no longer any need for the stern to be watertight or to have any structural strength, this was acceptable.

"It's theatrical, so we had to do what looks right. It's not an exercise in ship-building," Thatcher explains. "I don't know anybody who could build a ship like that these days."

Ship-building has always reflected the architecture of its time, which is why the World Ship Trust preservationist group uses the stern of HMS Implacable as it its logo above the terse motto, "Implacable Never Again".