'The last time I shot one of these boats,' said Kos Evans, as she hurtled along the Thames in a rubber dinghy in pursuit of a fancy racing yacht, 'I was strapped to the top of the mast when it capsized. Not a lot of fun that. These boats turn turkey. I could easily have found myself 40 feet under.'

It is a hazard not often faced by professional photographers, going down with the rigging of ocean-going yachts. But Ms Evans, a slight, tanned, furiously energetic woman, possibly

in her late thirties (it's hard to tell and she won't), faces it almost every day of her working life.

'We were in the Solent. It wasn't particularly bad weather, the skipper just lost control, ran out of water,' she remembered. 'The first 30 feet went at an unreally slow toppling pace, but the last 10 we went pretty quickly. At the time I thought it was hilarious, then that night I woke up in a cold sweat, thinking 'Christ'.'

Kos Evans - Kos is short for Koren - has been photographing the world's yachts in their natural environment for 10 years. With 'Kos' emblazoned on the back of her anorak, her camera in a plastic bag and her packet of disposable nappies to keep her lenses dry, she has become a familiar sight on the yacht-racing circuit. Particularly since she is one of the few photographers prepared to shoot pictures from atop a mast, where the most dramatic pictures are to be had.

She was in London briefly last week, taking promotional snaps for a boat sponsor, before heading off to Uruguay to join the penultimate leg of the Whitbread Round The World Race, during which she is chronicling the progress of the leading American yachtsman Dennis Conner's boat, Winston, named after the cigarette company and currently in fifth place.

'Boats and water are fantastically photogenic,' she said, training her telephoto lens across the murky depths under Tower Bridge. 'But I found that if I got up the masthead, I was presented with a whole range of better angles.'

She first climbed a mast when she was working at the Boat Show as a promotions girl, to supplement her grant as a photographic student. She had been brought up in a sailing family, yachting from the age of three, so when a bet was laid that no one would dare to climb the tallest mast at the show, she shimmied up with ease.

'Generally I get hoisted up in a bosun's chair. But on a square-rigger mast, you have to climb up with nothing holding you on. At the top, you sort of wedge yourself into the shrouds. It's bloody uncomfortable.'

Moreover boats are notoriously unwilling to stay still as their masts are scaled. Racing yachts can keel over at angles of more than 45 degrees.

'It really whips up there, sails and ropes fly. I get cut and bruised a lot. And I'm always scared. You try to tell yourself it's not going to go all the way over when you're up there, but you sound pretty unconvincing. Once I took some shots up the mast of The Endeavour, the biggest yacht in the world. It was 175ft, that's the equivalent of a 10-storey building, and it was swaying around like crazy. I thought I may be getting a little old for this.'

Recently Kos has started to explore new, less hazardous angles on a boat, like looking up the legs of the crew as they undertake a tricky manoeuvre. But yacht owners and sponsors of big races seem unwilling to let her retire from the masthead, anxious that nobody else could make their sport look so heroic.

'Sponsors seem more and more aware of how important photography is to them,' she said. 'A boat can look wonderful in a photograph. And their product name gets seen all the more. Now they are painting them brilliant colours, having amazing sails produced. It is a commercial world we sail in.'

But still, occasionally, the odd eccentric individual will hire her services to make them look the business on the water.

'I was with Simon Le Bon on his boat Drum when I was called off to do some work in Holland,' she said, as the dinghy, in her pursuit of the ultimate picture, came perilously close to colliding with the yacht in the Thames. 'Which was just as well. It sank while I was away. Knowing my luck, if I'd been on it I would probably have been on top of the mast at the time.'