'Where shall we go?" asks Hilary Lister, laughing. The motor boat that towed us out of Portsmouth Harbour has just cast us off, and now she's in charge. A very noticeable change occurs immediately. Suddenly she's happy. And free. Sailing does that to her, she says. "It's given me my life again."

She means this quite literally. Hilary is quadriplegic, paralysed from the neck down by reflex sympathetic dystrophy, a degenerative condition of the nervous system. It started with shooting pains in her legs when she was in her teens, which doctors first dismissed as growing pains. As her condition has worsened, she has had to give up her life bit by bit - her mobility, then her career as a biochemist, a secondary career as a clarinet teacher, her independence. She admits she's been very close to overdosing on painkillers.

Then, a couple of years ago, a friend took her sailing for the first time. "As soon as they put me in a boat, it was like having another dimension," she says. That time she was just a passenger, but immediately she started to plot. Without knowing anything about sailing, or even if it was possible to design a boat she could steer, she hatched a plan: "I had this mad dream to sail across the English Channel."

Last month Hilary, now 33, realised her dream. She set off from Dover, a support boat nearby but alone in her 27ft boat Malin, steering and controlling the sails by sucking and blowing into plastic straws. Six hours later, exhausted but whooping for joy, she reached Calais. It was the first time a quadriplegic sailor had crossed the Channel, the longest solo voyage ever by a quadriplegic; more importantly, Hilary had done exactly what she'd set out to do.

Today, she's taking me for a sail in the Solent. It's her first time in the boat since the Channel crossing and there's a champagne cork, a reminder of her triumph, lying on the floor of the cockpit.

The motor boat peels away, its engine note fading. "Those first few moments are magic," she says, and asks where I would like to go. Shall we head down towards Cowes? I suggest France. She's tempted. There may not be time, though. It's three times further across the Channel here. Tomorrow she's going into hospital for an operation on her spinal column. This is not an operation that will cure her in any way, but it may make the pain she experiences constantly less severe. She will be in for a couple of weeks. We head across the Solent towards Ryde on the Isle of Wight.

So what is it about sailing then? "You have a freedom on water you don't have anywhere, well I don't. It's hard to explain what it's like being stuck in a wheelchair. Here I'm the boss. As well as steering, I can chose to sail flat, or go faster. It's wonderful to have choice again."

By her own admission, Hilary is both a fiercely independent person and very competitive. She used to do loads of sports - swimming, hockey, rugby, canoeing, all sorts. Sailing has given her some of her independence back. I ask her about the competitiveness. "I always try to sail the best way, the fastest way possible. I'm now a goal-orientated person. Sailing across the Channel was a major goal."

The next goal is to sail around Britain. It's still not much more than a dream, though. She needs to find a bigger boat for a start. And after that a circumnavigation of the globe? "I guess so," she says, not entirely seriously, though I wouldn't be surprised if she did.

Hilary lives in Faversham in Kent with her husband Clifford and their new chocolate labrador Lottie. She met Clifford while she was at school - he was a music teacher there, and at that time Hilary was still able to play the clarinet. For many years they were just friends, but they married during Hilary's final year at Oxford University. Their home always has at least one carer there. Sailing is the only time Hilary can be alone, although there's always a support vessel nearby. I tell her I'm sorry to be there, ruining her moment. "No, it's OK," she says. "Because it's me taking you somewhere, not you taking me. And that's a huge difference."

She even lets me have a go - suck on this pipe to go left, blow to go right. And on the other pipe, sucking pulls the sheets in, blowing lets them out. It takes a bit of getting used to, but our wobbly wake straightens out as I get the hang of it, and Hilary says I'm quite good. She misses teaching, she says.

The "sip-puff" system is taken from Hilary's old electric wheelchair. I was expecting the boat to be incredibly hi-tech but, as she says, it's actually quite Heath Robinson. Another friend of hers, Matt Debicki, who does lots of work sailing with disabled people, connected up the sip-puff pipes to two electric motors, also taken from old wheelchairs. The motors are connected to two rotating drums with lines around them. One moves the tiller from side to side, and the other is connected to both the jib and main sheets. The battery will last about 10 hours if the boat isn't sailed too aggressively. I think Hilary struggles to get 10 hours out of it.

The boat is a Soling, a dy boat normally sailed by three people. A chair from a rally car has been screwed into the cockpit and chocked up on wooden blocks to get the angle right ("I was quite impressed I was able to get my backside into a rally driver's chair," she says). All the bits cost about four or five hundred pounds, she reckons. And that's it. Hilary is lowered into the rally chair, a bungy strap securing her head if the sea's choppy, to stop her falling forward. Then the sip-puff system is hung around her neck, baseball cap placed on her head if it's sunny, and she's off.

The sails have been reduced in size to make them manageable. She thinks they've been reduced too much. She can be quite withering about all the safety precautions that surround anything to do with disability. "God, it would be so much worse if a disabled person drowned," she says sarcastically.

Today there's only a light breeze, and our progress towards Ryde is quite gentle. Hilary is a very unflappy sailor. At one point a large ferry appears to be bearing down on us. "I think we'll tack," she says calmly, then does so. There was a lot more wind last week for her Channel crossing. "At times we were surfing down the waves. And there was that humming sound you get when everything is just right. It was wonderful."

The photographer in the motor boat wants to get a picture of Hilary sailing alone. They come alongside, and I climb out from where I've been perching behind her, and get into the other boat. Again the motor boat peels away. And again I think I notice a change in Hilary.

I know she said she was perfectly happy about me being there. Now, though, she really is alone, the wind on her face, and this is what she wants. She's in pain - usually it's just there she says, but sometimes it's unbearable, like a knife being driven into her. She has pain-relief patches, and pills for when it gets too bad, the same pills she was going to overdose on and which she says she will use if it gets too much. Right now though, alone in her boat, sailing south into the sun, she looks incredibly happy.

· To make a donation to sailing for the disabled visit www.hilarylister.co.uk