For the past week, Britain has been in the thrall of an extraordinary sporting challenge. Within 24 hours of the start of the Velux 5 Oceans, a three-stage solo round-the-world yacht race starting in Bilbao, four sailors were forced to return to port, so horrific were the conditions off the coast of Spain. It was a reminder, if one were needed, that a solo circumnavigation of the globe remains the Everest of sporting achievements. Sir Robin Knox-Johnston - who, at 67, is the Velux 5 Oceans' oldest competitor - called the Bay of Biscay's 40ft waves and high winds "boat-breaking".

He should know. In 1969, without the global positioning systems, or support crews, or the corporate sponsorship with which modern single-handers put to sea, Knox-Johnston became the first person to complete a non-stop solo circumnavigation on his boat Suhaili, winning the inaugural Sunday Times Golden Globe Race in the process. "In 1969, my only advance weather system," said Knox-Johnston last week, "was a barometer from the wall of a pub".

But as remarkable as Knox-Johnston's feat was, it will forever be eclipsed by the plight of one of his fellow competitors in that first race: Donald Crowhurst, a 36-year-old English businessman, who went to sea in a leaky boat and committed suicide in the Atlantic 243 days later. Now, splicing harrowing onboard audio and visual footage of Crowhurst with interviews of the sailor's friends and family, a film, Deep Water, tells the story of Crowhurst's last voyage, and his descent into madness.

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A former RAF pilot with a small, ailing electronics business called Electron Utilisation, Crowhurst was, at best, an enthusiastic weekend sailor. He was also married and a father of four children. So what convinced him he should go to sea for nine months is anyone's guess. But not only was Crowhurst determined to enter the race, he was determined to win.

To understand Crowhurst's peculiar obsession with competing in this gruelling race, one needs to know that in 1968, Britain was in the grip of sailing fever. The previous year, Sir Francis Chichester had achieved the then monumental feat of sailing around the world, on his own, punctuated only by a stop in Australia. In the era of the space race, when the possibilities of human endeavour seemed limitless, the world lapped up the heroism of Chichester's achievement, and 250,000 people lined the south coast to cheer him home.

The Sunday Times, which had reaped the rewards of sponsoring Chichester's journey, was looking for a way to continue tapping into the appetite for maritime derring-do. The Sunday Times Golden Globe Race was born. Using the clipper route, from Britain, through the Atlantic and round the Cape of Good Hope; through the Indian and Pacific Oceans; round Cape Horn and back to Britain, the competition was billed as a test for the world's greatest yachtsmen.

But, to encourage entrants, no evidence of sailing experience was required, and competitors were allowed to set off any time before 31 October. A trophy for the first man to complete the course, and a separate prize of £5,000 for the fastest time, would be awarded. Out of the nine men who set off in 1968, only one, Knox-Johnston, finished.

Crowhurst's bid to win the Golden Globe always looked precarious. In the months before the race, the businessman had become convinced that by coupling a new, triple-hulled boat design with his own technological innovations (a self-righting mechanism in case of capsize, for instance), he could win. With the financial support of a local businessman, Stanley Best, Crowhurst bought and developed his vessel. But Best's money came with a proviso: if Crowhurst failed to finish the race, he would have to pay for the boat himself.

To make matters more complicated for Crowhurst, he fell into the hands of one of Fleet Street's more manipulative press officers, Rodney Hallworth, who knew Crowhurst was journalistic manna. Hallworth sold him to the press as a plucky, English amateur with incredible prospects of bringing home the big prize. All the while, the deadline for starting the race, 31 October, was creeping round, and Crowhurst was struggling to make his boat, the Teignmouth Electron, seaworthy.

The day before his voyage began, Crowhurst made last-minute preparations on the Electron, then retired to a hotel with his wife, Clare. That night, he broke down in tears. The boat, he knew, was not ready. But he also knew it was too late to pull out. Hallworth would not allow it. Best would want his money back. The family would be ruined.

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Crowhurst set sail the next day with unsorted provisions and vital equipment strewn across the deck of the Electron. And, even at this early stage, calamity loomed. He was forced to return to harbour in the first minutes of his journey when an anti-capsize air-bag got caught in the rigging.

Over the next two weeks, Crowhurst made slow progress. As Knox-Johnston and the Frenchman, Bernard Moitessier, who had set off weeks earlier, neared New Zealand, Crowhurst was languishing in the North Atlantic. At least he was afloat. Four men - John Ridgway, Loick Fougeron, Bill King and Alex Carozzo - retired before they reached the Indian Ocean. Of the four who remained, one Englishman, Chay Blyth, who set off with no sailing experience, retired in East London, South Africa. Another, Nigel Tetley, was making good time ahead of Crowhurst.

But if Crowhurst's slow times were worrying, they were as nothing compared to the anxiety he felt about the state of his boat. The Electron had started to leak, and her skipper faced a dilemma. Should he continue into the Southern Ocean, where his boat would almost certainly sink? Or should he return home to face shame and financial ruin?

In the end, Crowhurst did neither. Instead, he started to radio a series of incredible positions and speeds to Hallworth, who, in turn, embellished the lies and sold them to Fleet Street as fact. Suddenly, Crowhurst seemed to have a genuine chance of scooping the prize for the fastest circumnavigation.

The truth was that the difference between Crowhurst's real and stated positions was growing by the day, a discrepancy he kept track of by recording two logbooks. But this ruse only exacerbated his problems. Because of the frailty of the Electron, Crowhurst could not enter the perilous Southern Ocean. Neither could he return home, where ignominy and bankruptcy awaited. All the while, his wife, his four young children, and the rest of the world thought he was sailing into the record books.

Faced with an insoluble problem, Crowhurst did the only thing he could think of; he stayed put. Bobbing around in the Atlantic off Brazil, Crowhurst scrupulously filled out his fraudulent logbook, and cut off all radio contact with the world for three months. At one point, he was forced to pull into an Argentine fishing port to make vital repairs to his boat, an action that in itself would have been enough to disqualify him from the race. Still, Crowhurst kept up his pretence.

His fellow competitor, Moitessier, disillusioned about competing in a commercially motivated event, had rejected the idea of finishing the race at all. Instead, he simply to kept on sailing, eventually dropping anchor in Tahiti, after one and a half laps of the globe. Knox-Johnston arrived back in England to a hero's welcome. He had won the trophy for coming in first. But he had made slow time, a leisurely 312 days. In the eyes of the world, the race was now on between Tetley and Crowhurst to see which man would win the race for the £5,000.

The thought of winning terrified Crowhurst. He knew that if he came home in the fastest time, his logbook would be subject to scrupulous checks by the Golden Globe judges and the press. He determined on making a slow journey across the Atlantic, so Tetley would win the prize, and he could come in a dignified, unheroic second. Re-establishing radio contact with Hallworth for the first time in 12 weeks, Crowhurst confirmed he would not be able to catch Tetley. Still, his family and friends, who had been fearing the worst for Crow-hurst, were relieved that he was, apparently, safe and well.

Crowhurst's scheme was looking good. In May 1969, he began to make for home, his faked journey matching his real one for the first time in months. Then, disaster. Tetley, thinking Crowhurst was hot on his heels, had pushed his boat too hard, and sunk in the Azores. Crowhurst was going to win the prize.

The news of Tetley's sinking affected Crowhurst profoundly. He drifted deep into depression, refusing to sail, and took to his logbook. As he lolled in the mid-Atlantic, Crowhurst wrote a 25,000-word treatise on time travel and divinity. He counted down his remaining hours on Earth, believing death would not only be "the mercy" but that it would transform him into a "cosmic being". On 29 June 1969, after 243 days at sea, Crowhurst made one last entry into his logbook. His self-allotted time had come. This was "the mercy" he had been praying for. His boat was found 12 days later, with logbooks recording his genuine position and grainy sound and video recordings unharmed. It has since been assumed Crowhurst took the logbook of his fraudulent positions with him as he threw himself overboard. Back in Britain, Knox-Johnston was awarded the £5,000 for the fastest time, which he donated to the Crowhurst family, and Tetley was given a consolation prize. For unknown reasons, Tetley committed suicide a year later.

Deep Water's co-producer, Al Morrow, said: "What I felt about Donald was that although not all of us would go to sea, the situation was one that any of us could have got ourselves into. You tell one tiny little lie, and that turns into another lie, and suddenly there's no way out. The other thing that really struck me about the story is that being on your own for nine months at sea is such a unique thing. You have no one to speak to. He must have been so lonely."

Her fellow producer, Jonny Persey, added: "I recognise [Crowhurst's story] could arouse feelings of anger. He could have done the right thing any number of times. He had those opportunities, but every decision he made was wrong. It was very hard for his family to contribute to this film, but they have come to a point where they understand what happened, and recognise the complexity of what was going on with Donald."

The first screening of Deep Water, with the creative team and the family, was a charged occasion, says Morrow. "I was terrified," she admits. "The family had been so supportive through the process, but they had to trust us with the direction the film was going in. Clare's daughter, Rachel, walked out a few times in the screening, because she found it too emotional to sit through."

Another person in whom Deep Water will strike a resonating chord is Knox-Johnston, himself an interviewee for the film. Unable to attend any preview screenings, he has taken a DVD of Deep Water on his Velux 5 Oceans boat, Saga Insurance. When he finds a spare two hours away from racing, he will watch Crowhurst's story, all alone at sea.

Deep Water is released on 15 December