The older and/or sadder among you will remember an episode of Round the Horne in which Kenneth Williams plies the seven seas as Captain Ahab in pursuit of a giant waterfowl called, if memory serves, Moby Duck. The show's denouement – "Aargh!" "Quack." "Aaaaaargh!" "QUACK" – still rings in my ears.
Which brings us, inevitably, to the Loire. Sail the river's estuary from Nantes to Saint-Nazaire just now and there is every chance that you will share Ahab's vision – if not, one hopes, his end. For the Loire is currently patrolled by a 25m-long duck, called Canard de Bain or, in plain English, Rubber Ducky.
It is, as you would expect, a work of art. So what is it doing plying its duckly way past Saint-Jean-de-Boiseau and Cordemair?
It's like this. For a thousand years, the Loire estuary was one of the richest parts of France, servicing a wine-soaked hinterland and building great ships like the Normandie and France. The roof of Nantes' cathedral is higher than Notre Dame's, its 18th-century terraces more elegant than the Left Bank's. Then came the Second World War, which saw both Nantes and Saint-Nazaire badly bombed and the estuary between them blocked with scuttled shipping. While the region struggled to recover, Korea and Taiwan started building ships more cheaply than the French. The economy of the Loire went into steep decline, and with it Saint-Nazaire and Nantes.
Thus the duck. Like other towns in the same boat – Bilbao, for example – Nantes has seen her future in art. Her Musée des Beaux-Arts is one of the finest in France, and its hip new ex-industrial establishments would fit quite happily into Hoxton or Hackney: a one-time banana warehouse; the wacky Lu biscuit factory punningly re-christened Le Lieu Unique.
Now Jean Blaise, the man behind Paris's famous art all-nighters, the Nuits Blanches, has decided to export Nantes' artistic good fortune downriver. To this end, he has launched an art boat. Over the summer it will make a daily two-and-a-half hour trip from Nantes to Saint-Nazaire, taking in a variety of specially commissioned artworks, of which Rubber Ducky is one. The project is known, with devastating French directness, as Estuaire.
Which is why I am standing, shivering slightly, on the Quai des Antilles, the city's run-down dock area. To my left is a work called Les Anneaux by the French sculptor, Daniel Buren. Not quite "the world's most famous contemporary visual artist", as Estuaire's publicity material gamely maintains, Buren is a hard-hitter nonetheless. Unfortunately, Les Anneaux hasn't been switched on yet, which means that it looks less like the serried neon haloes of the press photograph and more like 18 steel hoops. The unseasonally icy wind has also filled my eyes with tears, which means I can only focus on the curious object floating toward us across the Loire.
This turns out to be Blaise's boat, decorated for the occasion by a local art collective called La Valise. Your first sight of the craft might put you off boarding it. It has been covered in mirrored tiles that apparently allude to a Paco Rabanne dress but look like the ones that are designed to protect space shuttles. Actually the boat is perfectly nice, windproof, and serves coffee of a strength to make your eyes bulge. In any case, I would board the Titanic for the chance of glimpsing an 80ft duck. We putter off in search.
The voyage does not begin well. (The story has a happy ending, though, so read on.) This is the boat's pre-launch trial run and the first four artworks aren't ready yet, meaning that I will miss seeing an aquatic dot-matrix sign, an interactive fountain and a machine for emptying the Loire. Capitaine Cat, by Alain Séchas, is up and running, or rather spouting: another fountain, this time issuing from the three masts of a scuttled Norwegian tanker called Antarktis. The cats' faces look disconcertingly like a comic-book take on the Crucifixion.
On we plough, in search of the ever-elusive duck. Before we get to it, though, the boat veers to port to show us Misconceivable by the Austrian artist, Erwin Wurm. In 1892, the Canal de La Martinière was dug to bypass a tricky mudbank in the Loire. By the time its riverside lock was finished, newly invented steam dredgers had rendered the project pointless. Accordingly, for over a century the Canal has served as a graveyard for defunct shipping. Wurm has balanced a single-masted sailing boat on its lock, bent in the middle like a porpoise so that it looks as though it is leaping the gate to sprint off downriver. The work is sweet in its way, like an artistic version of Free Willy. But it is not a duck.
And then I see Jean Blaise make that Gallic palms-upward gesture that fills the non-French with fear. It seems that Canard de Bain is not any old 25m duck. It is a techno-duck, fitted with an anemometer on its head. If the wind hits 60kph a computer inside the duck instructs it to deflate. The wind is indeed whistling past us like a freight train at 60kph et donc... pas de duck. Likewise La Maison dans la Loire, Jean-Luc Courcoult's exact replica of a house in the village of Lavau-sur-Loire: the piece was apparently marooned by the weather in Saint-Nazaire harbour. I mentally un-write my opening paragraph – the duck's majestic plumage glistening in the dying sun, etc – and try to stop my lower lip wobbling.
But all is not lost. Felice Varini finishes his contribution to Estuaire just before we dock at Saint-Nazaire. The Swiss artist has covered various dockside roofs, walls and chimneys with red rhomboids. Through a veil of gloom, I try to feign interest in these as he urges us up the side of a concrete submarine pen built by occupying Germans during the war. Then, suddenly, I gasp: stand in just this spot on the pen's roof and the disparate rhomboids suddenly line up to form a pattern of interlocking red triangles, flattening out Saint-Nazaire's perspective entirely. It is amazing. Then Blaise, visibly mournful at the way things have turned out, suggests we visit Courcoult's La Maison dans la Loire in the harbour. Not quite the same thing as seeing it moored off its doppelgänger in Lavau, perhaps, but not bad.
The house is lovely. Even tethered to a dock, it has the surreal feel of a Dalí painting. Every crack in its plaster, every smudge of lichen is faithfully reproduced. Courcoult has even arranged for smoke to waft from the chimneys, so that you picture an elderly French couple pottering about behind its blurry windows, him in a string vest, her in a flowered pinny. Like most of the works in Estuaire, the house relates to its site. The one-time port of Lavau, marooned by silt, is now half a kilometre from the Loire. Courcoult's bobbing house-boat turns back the clock, and then some.
Musing on the capriciousness of time and geography, my eye is caught by a flash of yellow. Lying on the dock just down from the house is what seems to be a vast sou'wester dropped by a careless giant child. Closer inspection reveals it to have a baleful eye and what, even in crumpled form, is obviously a bill.
It is The Duck.
Even in deflated form, I feel a sense of triumph. Later, back at Nante's Lieu Unique, I talk to Canard de Bain's maker, a passionate Dutch skinhead called Florentijn Hofman. He is best known for making inflatables of what he describes as "slightly overweight animals". One of these, a rabbit in a square in Rotterdam, holds what the artist cheerfully points out is a sex aid. The duck, he says, is an ambassador of peace, its role in life "to calm down world tensions". A 25m inflatable Henry Kissinger. I can't wait to do Estuaire again.
The writer flew to Nantes with GB Airways (0870 850 9850; www.ba.com) from Gatwick; Aer Arann (0870 876 7676; www.aerarann.ie) also flies from Bristol and Cardiff; and Ryanair (0871 246 0000; www.ryanair.com) from Stansted, Bournemouth and Nottingham.
Trains run to Nantes from London Waterloo and Ashford via Paris (Rail Europe: 08708 371371; www.raileurope.co.uk).
To reduce the impact on the environment, buy an "offset" from Equiclimate (0845 456 0170; www.ebico.co.uk) or Pure (020-7382 7815; www.puretrust.org.uk)
Abbaye de Villeneuve, Nantes Sud (00 33 2 40 04 40 25; www.abbayedevilleneuve.com). Doubles from €90 (£64).
The Loire Estuary Project 2007 (www.estuaire.info) runs to 1 September. The art boat river cruise can be taken in either direction between Nantes and Saint Nazaire. A €30 (£21) pass grants access to the Loire cruise and seven of the participating venues.
Musée des Beaux-Arts de Nantes (00 33 2 51 17 45 00), €3.50 (£2.50).
Le Lieu Unique, Nantes (00 33 2 51 82 15 00; www.lelieuunique.com).
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