They're sleek, they're fast, they're Zapcats. And they're versatile enough to roar up to the source of the Nile or race around the British coast. Isabel Knolles gets a white-knuckled grip and becomes an instant convert

A Zapcat. Just like a cheetah, the world's most popular one-design power- boat is built for speed, with twin inflatable hulls and a 50bhp engine. "You don't have to be Bernie Ecclestone to own one of these boats, and it certainly doesn't require a F1 budget," says Tony Jones, Zapcat Racing's commercial manager and a former Zapcat champion. "If you bought, ran and raced one for a year you'd still get change from £10,000."

Zap: to propel suddenly. Cat: the fastest creature in the world on land. A Zapcat has no seats, no screen to protect you from the spray, no cocktail holder, not even so much as a steering wheel; steering is by tiller. You should consider yourself lucky to have straps to hook your feet into. "It's a stripped-down, white-knuckle ride, the ultimate affordable boys' toy," Tony says.

Zapcats originated in South Africa, where since the early Eighties local crews have raced inflatable boats down treacherous rivers and along rugged coastlines. They were developed from lifesaving surf-rescue craft, and these buoyant boats are designed to cope with a variety of sea conditions from flat, calm lakes to where they can really show their true colours: large, breaking surf.

Late one afternoon in Cornwall's hub of extreme sports, Watergate Bay, near Newquay, I don the relevant paraphernalia: wetsuit; helmet; buoyancy aid. I am going to take part in a Zapcat race, and as co-pilot I have to start waist-deep in the water while Tony waits in the boat, hand on throttle. I watch the flags, screeching at Tony: "Yellow... green - go, go, go!"

I jump on and clamber towards the furthest point at the front of the boat. "Lean right out over the front, as if you're about to jump out," Tony bellows at me. All of a sudden we are hit by a wave and launched to what feels like a stratospheric altitude, although it is only about five feet. I take in the view: to my front, sets of rolling white horses; to my right and left, eight other boats all airborne too, I feel like I am part of a flock of birds taking off, then experience a sensation of falling from a great height, and let out the sort of blood-curdling death wail you would hear in a burning cattery: "Yarrrrrrrrreeeeee!"

We clear the next wave, pick up speed and then I look up to be confronted by a wall of water 17ft high. At this point, Tony tells me later, "there are three possible outcomes. 1) get up as much speed to try and make it over the wave before it breaks; 2) wait for it to break, as it's easier to get over, but this will cost time and position; or 3) nature triumphs and it breaks on us."

In the last-named scenario I, as the co-pilot, would act as a breakwater for Tony, and the wave's sheer velocity would force me to lose my grip, ending up either in Tony's lap or in the drink. Luckily for me, Tony opts for No 1, and we motor on.

It's a Force 6 day, though, with 15ft waves, and my nerves are worn ragged. But Tony deploys the boat astutely as the g forces pile up as we turn each hairpin on the M-shaped course; in fact he is so accurate with his turns I could touch the buoys in the water.

"Shift your body weight as I pull into a turn," he instructs. I need to throw my weight around the boat to improve the overall handling and speed. "Zapcats all have the same-size engine, it's the adept deployment of the co-pilot that determines the race winners," Adam "Radar" Wheeler, the race director of the National Championship series, explains to me afterwards.

Not only is racing a Zapcat physically demanding, but the grinding need to concentrate the whole time makes it mentally tiring as well. Martyn Willcox, the reigning series champion, says: "Like any professional sport you are constantly thinking ahead, looking for where you want to go and where others are. This requires unmitigated application from both team members."

The proven reliability and durability of these boats explains why they were chosen above all other vessels for a high-profile adventure that ended a couple of months ago by rewriting the geography books. In September last year, Neil McGregor, Cam McLeay and Garth Macintyre set off in three Zapcats to ascend the Nile to its farthest source and authenticate its exact length. Their journey took them 4,198 miles through five countries and lasted over six months, during which time they faced an alarming series of obstacles.

The most prominent of these were wildlife: their boats were charged by both hippos and crocodiles, which are native to most parts of the river. They also overcame rapids and water-falls, and crossed the largest swamp in the world.

When the trio faced insuperable rapids in Uganda they used a unique method of transporting their Zapcats and equipment: a FIB (Flying Inflatable Boat). The outcome of their explorations revealed that from the Mediterranean Sea to its furthest source in Rwanda, the Nile is longer by 66 miles than had previously been recorded.

Last year I presented the National Zapcat Championship series for the Extreme Sports Channel, and spent many weekends in varying weathers watching a fraternity of adrenalin junkies turn up to each venue and with unfaltering enthusiasm to risk limbs and sanity. Until I actually experienced a race myself I often questioned their motives. But now, after bundles of fun and camaraderie I hadn't experienced since my Girl Guide days, I'm going to be advertising for a co-pilot imminently.

If you want to try out the Zapcat experience, contact Vortex Racing: 0870 777 5598, The remaining rounds of the Zapcat National Championship are: 17-18 June (Brighton); 15-16 July (Glasgow); 12-13 Aug (Littlehampton); 2-3 Sept (tbc); 22-23 Sept (Watergate Bay). For more details: