I have always been convinced that Donald didn’t commit suicide,” says the bright-eyed 77-year-old grandmother, sitting by her fireside in Seaton, a south Devon coastal town. “It’s such an awful story and I suppose we will never know what happened at the end.” Outside, it’s thriller weather: grey skies, an icy swell breaking on the deserted front, and the plaintive commentary of a few stray seagulls. Clare Crowhurst recollects the terrible past calmly enough today, but 40 years ago she was known to news-paper readers as the “sea widow”.

Back in 1969, her husband, Donald Crowhurst, was the protagonist of the strangest, most disturbing story of its time, part adventure, part mystery, but mostly tragedy. He was the yachtsman who fooled a credulous press and public into believing that, after a voyage of 240 days, he was sailing home to England in triumph, apparently the winner of the Sunday Times’s Golden Globe Race, the fastest nonstop single-handed round-the-world race. Thousands prepared for his happy return. Then Crowhurst vanished. When his trimaran was found, ghosting through the mid-Atlantic under a single sail, there were clues to its last voyage in three log books, but its lone captain was missing, and when the truth came out his fate was swamped by the larger story of his hoax.

“I used to dream about it for years,” says Clare. “In fact, during June 1969, I imagined I heard the front door open and Donald calling out ‘Clare’, as he always did.”

Her second son, Simon, a young middle-aged man with a premature shock of white hair and the bright, questioning eyes of a lost boy, is also haunted by his father’s fate. He holds a chunky wooden model of the boat, and talks about the curse of the past. “I feel compelled to think about my father’s story,” he says. “He’s the Ancient Mariner, of course, but I feel like the narrator.” Simon sees it as an existential cliffhanger. “My father becomes this solitary hero in the limelight of history,” he says.

Poignant, ominous and unforgettable, the story has inspired many elegiac narratives: by the American poet Donald Finkel, the playwright Chris Van Strander, and an opera, Ravenshead. Responding to its archetypal depths, director Nic Roeg developed a film script in the 70s, though it was never made. In 1982, the French based Les Quarantièmes Rugissants (The Roaring Forties) on the Crowhurst story. In 1992, the American novelist Robert Stone based Outerbridge Reach on the strange events of that long-ago summer.

The story starts in 1968, the climactic year of the 60s: to the soundtrack of Sergeant Pepper and the Doors, tides of workers and students demonstrated against the Vietnam War; just a few weeks apart, Martin Luther King and Bobby Kennedy were assassinated; Soviet tanks rolled into Prague; and, out in space, Apollo moonshots were pitching man against the universe.

Here in Britain, the mood was nostalgic and quasi-Elizabethan. Seafaring adventure was in the air. The year before, Francis Chichester had sailed his Gipsy Moth into Plymouth to a tumultuous welcome, a media frenzy, and a knighthood from the Queen, conferred on the quayside, as if she were Gloriana herself. The Americans might hurtle upwards in their rockets, but here on earth plucky Brits still ruled the waves. The press, scenting a new audience for drama on the high seas, splashed yachting stories across its front pages. Chichester’s account of his voyage, The Lonely Sea and the Sky, became an instant bestseller. During the spring of 1968, in direct competition with the Observer’s Transatlantic Race, the Sunday Times launched a nonstop challenge, the Golden Globe round-the-world yacht race.

“Nonstop” was to be the supreme test. Chichester had broken his journey in Australia. It was widely held that neither a solo yachtsman - nor his boat - could endure the stresses and strains of single-handed sailing for months on end. However, swept up in the mood of the moment, nine sailors stepped forward to compete for two prizes. Setting off any time before 31 October, the first man home would take the honours, a Golden Globe, while the fastest circumnavigation would scoop a tempting £5,000.

The competitors came from the cream of international yachting. There were high-profile challengers, the transatlantic oarsmen Chay Blyth and John Ridgway, in rival monohulls. There were two veteran French sailors, Bernard Moitessier and Loïck Fougeron, an ex-merchant seaman, Robin Knox-Johnston, the Italian Alex Carozzo, two former naval officers, Bill King and Nigel Tetley. Finally, a very late entry, almost as an afterthought, there was the “mystery man”, an obscure West Country electronics engineer named Donald Crowhurst.

Compared with the field, Crowhurst was hopelessly inexperienced, at best a Boy’s Own hero, at worst a fantasist. Occasionally described as a “businessman”, Crowhurst was British, but really an orphan of empire, born in the India of the British Raj in 1932, where his father worked as a superintendent on the railways. After Independence in 1947, the family had returned with their meagre savings to England, but discovered that life in the suburbs of Reading was not an idyllic homecoming. The climate was brutal; money was tight; almost at once Crowhurst senior dropped dead from a heart attack. Like a character from Dickens, young Donald was forced to leave school early and train as an apprentice at the Royal Aircraft Establishment (RAE) in Farnborough.

Restless, broke and ambitious, a fish out of water, Crowhurst drifted from a commission with the RAF into the army, but was forced to resign after a rowdy evening involving a stolen car brought him before Reading magistrates. Eventually, he married Clare O’Leary from Killarney, moved to the West Country, and started a small computer business, Electron Utilisation Ltd.

An obsessive tinkerer, Crowhurst had invented the Navicator (a radio direction-finding gizmo that is now commonplace in any weekend sailor’s arsenal), which he believed would make his fortune. Crowhurst was scarcely more than an enthusiastic amateur sailor, but when the Sunday Times’s Golden Globe Race was announced, its £5,000 prize money (the equivalent of £65,000 today) seemed a heaven-sent way to stave off impending bankruptcy, until sales of the Navicator took off.

Impetuous, charming and headstrong, a self-confessed “romantic” in search of fame and glory, Crowhurst persuaded a local caravan dealer and millionaire, Stanley Best, to sponsor his entry, and commissioned a Norfolk boatyard to build a trimaran. From the moment of Best’s involvement, the Crowhurst story takes on a darker hue. He hired a publicist, Rodney Hallworth, a provincial hack and former crime reporter for the Daily Mail and Daily Express, who fed Crowhurst’s fantasy life and persuaded him to headquarter his race campaign in Teignmouth. Race fever took hold. Crowhurst mortgaged his house and his business against the sponsorship. He was Icarus, with an overdraft.

It was a desperate gamble. Time was running out. Competitors had to set sail before 31 October and some had already left. The “mystery man” was coming into the race with an untried boat, seriously unready and ill-equipped.

“I don’t think,” says Simon Crowhurst carefully, “that my father realised how badly things could go wrong.”

On its first sea trial, from East Anglia to the West Country, Crowhurst’s yacht, the Teignmouth Electron, underperformed so badly in the Channel that a three-day trip took two weeks. Now there was no time to equip and provision the vessel properly for the race. Up against the departure deadline, Crowhurst was faced with a stark choice: set sail with a dodgy boat or withdraw from the race and face humiliation and bankruptcy.

So, in the afternoon of 31 October 1968 - the last possible moment - after an embarrassing false start, Crowhurst set out from Teignmouth. “Look after your mother,” he whispered to his son, a strangely prophetic command. Simon remembers the departure well. “We were watching from the shore. I don’t think any of us quite knew what was going to happen next.” It was the beginning of Crowhurst’s career as the Ancient Mariner. Few could have anticipated how cursed, and literally fabulous, his voyage would become.

To sail round the world in the 60s was to embark on a voyage of the ages. There was no GPS, satellite communication, or internet: just a fuzzy radio link, and perhaps a morse code transmitter. The lone sailor was a speck on the ocean, relying on sextant calculations. Simon Crowhurst believes that this is part of the lasting appeal of his father’s story: one man against the elements, a man on the edge of oblivion, risking all. “It’s a story that people remember, and that’s a consolation,” he says. “It’s a story that tells you something about what it means to be human.”

As the Teignmouth Electron slipped down the Channel on the long leg to the Cape of Good Hope, the first act of the Crowhurst drama was concluded. All the elements of tragedy were in place: a curious public; a hungry media machine; and a weekend sailor heading into dangerous water. Worse, and grimmer still, it was only once he was properly at sea that Crowhurst’s secret fears were realised. His boat, so hastily assembled, was a dud.

Ever optimistic, before departure he had calculated that, however late he set off, the superior speed of his trimaran would enable him to overhaul the other competitors and record the fastest circumnavigation. He had never done much more than cruise up and down the south coast in a small sloop at weekends, but with impressive self-belief he had estimated that the Teignmouth Electron could be made to sail some 220 miles per day.

After a fortnight at sea, Crowhurst had not averaged more than 130 miles a day, and had barely passed Cape Finisterre and the coast of Portugal. More alarming than his boat’s underperformance, it had sprung a leak. He wrote in his log, “This bloody boat is just falling to pieces!!!” As well as the terror of the seas, waves as high as a 12-storey building, merciless winds and the terrible apprehensions induced by solitude, Crowhurst was now battling a more insidious, mental terror: the fear of not winning the all-important £5,000.

Crowhurst’s solution to his predicament was a version of the truth that he, alone, could verify. On 10 December, after about six weeks at sea, he cabled Rodney Hallworth with the astounding news that he had just sailed, in one day, a record 243 miles. To himself, he described his false record as “a game”. As the remorseless logic of the hoax corrupted his relationship with reality, this game became a matter of life and death.

Now the media side of this strange tale kicks in. Hallworth had only one concern: to hype his client’s story. In these early days of modern media relations, flogging the hell out of a scrap of news, unsourced, unverified and over-exaggerated, was all in a day’s work for the publicist. All at once the “mystery yachtsman” became the record-breaking “lone sailor”. Francis Chichester was privately sceptical and referred to Crowhurst as “the joker”. He could never have anticipated how audacious the joker’s prank would become.

The race was still front-page news. As Crowhurst struggled to get the Teignmouth Electron to make headway, the Sunday Times ran a story, “The Week it all Happened”, describing how Carozzo, Fougeron and King had been forced to retire from the race from which Blyth and Ridgway had already withdrawn, while Robin Knox-Johnston battled mountainous seas off New Zealand after a horrendous capsize.

There was nothing to report about Crowhurst, trailing at the back of the pack, but this did not stop his press agent parcelling out his client’s progress with teasing hints of more record-breaking feats. Hallworth’s public faith in the yachtsman he called “my boy” was part of his charm as a PR man.

On Fleet Street, indeed, only the Observer yachting correspondent, Frank Page, evinced any disbelief about the progress of the Teignmouth Electron, sceptically describing “a typically forthright claim from Donald Crowhurst, currently lying a poor fourth in the race”. The truth of his situation was infinitely worse. Even with the trade winds of the mid-Atlantic, he was making painfully slow progress south and had barely crossed the equator.

The log books tell the true story. In parallel with the fake co-ordinates of Crowhurst’s record-breaking voyage, pages of meticulous fabrication, is the record of a man dawdling about the South Atlantic in a leaky boat, slowly going out of his mind.

Christmas came. While her skipper was claiming to be “somewhere off Cape Town”, the Teignmouth Electron was actually sailing past Brazil weeks behind the race leaders, a deception that would be impossible today. Crowhurst spoke to his wife, but he was vague about his location and did not confess the truth of his predicament. Soon after this, blaming a broken generator, he shut down all ship-to-shore communications.

Simon Crowhurst remembers that he and his brothers used to trace their father’s progress by sticking pins into a map of the world. Slowly, through January, February and March 1969, this comforting ritual faltered, and stopped. Things were bad at home. Clare Crowhurst was now drawing the dole. Her youngest son, Roger, was suffering nightmares in which his father stood staring at him from the doorway of his bedroom. Simon says that, “The sense that something was badly wrong began to grow at the back of our minds.”

Out on the ocean, a terrible race continued to take its toll. Bernard Moitessier, having sailed past Cape Horn, decided that he preferred the solitude of his boat to the strain of la vie normale. The Frenchman cabled his wife an enigmatic au revoir and changed his course to begin a second circumnavigation. He would finally make landfall in Tahiti. Now in a field of three, Crowhurst was still lying last.

Then he came up with the narrative twist that changed everything. On 10 April 1969, Crowhurst broke radio silence with a typically ebullient message, claiming to be heading back up the Atlantic, having cleared Cape Horn.”What’s new ocean-bashingwise?” he asked. “It was as if,” in Simon’s words, “he had come back from the dead.” Hallworth hammered out an excited press release. Across Fleet Street, a frisson of spring fever sent the Teignmouth Electron “rounding the Horn” and Crowhurst into serious contention for the £5,000 prize.

Ahead of him in the race were just two boats, Robin Knox-Johnston’s battered ketch, Suhaili, and Nigel Tetley’s trimaran. Knox-Johnston was almost home, but Tetley looked most likely to be the winner of the prize for the fastest circumnavigation. With a message that now seems richly ironic, Hallworth cabled Crowhurst: YOURE ONLY TWO WEEKS BEHIND TETLEY PHOTO FINISH WILL MAKE GREAT NEWS STOP. The stage was set for the denouement of this “seafaring classic”.

Crowhurst’s plan relied on Tetley’s two-week lead. His deception - the circumnavigation that never was, the fake log books, the whole hoax of his nonexistent voyage - depended on not winning. It was essential, having survived undiscovered, that he should come in last. He would be the plucky small-town loser who had flown the flag for weekend sailors everywhere taking on the world’s most gruelling endurance test and making it home to his loved ones...

This was the kind of hogwash in which Rodney Hallworth specialised. If Crowhurst sailed into Teignmouth, behind Robin Knox-Johnston and Nigel Tetley, as seemed inevitable, no one would give his phoney log books a second glance. He could slip ashore and resume civilian life as that quintessential British hero, the nearly man. He reckoned without Tetley’s British naval bloodymindedness, a determination to win that would shortly prove disastrous. To keep ahead of the Teignmouth Electron, now reportedly coming up fast behind him, the ex-naval commander piled on the canvas, ploughing through a gale in the mid-Atlantic to maintain his position as race leader.

In the storm, Tetley sustained more damage. Finally, off the Azores, just 1,000 miles from home, his trimaran began to sink. Air-sea rescue plucked him to safety from a life raft on 21 May. Now Donald Crowhurst - the last man afloat now that Knox-Johnston was home - was going to take the £5,000 prize for the fastest circumnavigation. The de facto winner, he would come home to face the inevitable scrutiny of race officials and yachting correspondents.

Crowhurst’s lies had helped sink Tetley, now - in June, the final month of the race - the same lies returned to drive him to the edge of a breakdown. “He went downhill after he heard the news of Nigel Tetley,” comments Simon Crowhurst, sadly.

On board the Teignmouth Electron, the Marconi transmitter had finally conked out. Crowhurst could receive incoming news, but he couldn’t communicate with the outside world. He was alone with the self-inflicted fiction of his voyage. On a boat clogged with the weeds and jellyfish of the Sargasso Sea, his imagination was driving him to the brink of madness.

Simon, reflecting on his father’s last days, says, “It’s a psychological maelstrom that can drag you down.” In particular, he is unnerved by Crowhurst’s final record, in the ship’s log books. “I’m wary of the log books,” says his son. “My wife doesn’t like me thinking about them. They have a bad effect on me.”

The log books, which had begun as a mundane record of a circumnavigation, had become the disturbing repository of a cumulative lie, the painstakingly contrived details of a false voyage. Now, in these final weeks, they became a more terrible document: the record of a mind at the end of its tether, 25,000 words of confessional philosophising and deranged speculation about the nature of the cosmos in which he, Donald Crowhurst, saw himself as the son of God. “It is finished,” he wrote on the final page. “It is finished. IT IS THE MERCY... I will resign the game.” It was 1 July 1969.

At this point, a bizarre hoax becomes the stuff of myth as much as literature. On 10 July 1969, the Royal Mail vessel Picardy, steaming through the mid-Atlantic towards the Caribbean, encountered a yacht, drifting under a single sail, like the Marie Celeste. The Teignmouth Electron was cluttered and untidy, with dirty dishes and filthy bedding, but of her crew there was no sign. Baffled and frustrated in his search for the missing yachtsman, the captain of the Picardy hoisted the trimaran on board, sailed on and began to read Crowhurst’s three log books...

The mystery of Crowhurst’s disappearance made him famous worldwide, though not in a way he would have wanted. There were reports of Crowhurst sightings from Cape Verde to Barnstaple. Simon recalls the British media staking out the family home in the hope of news about the “mystery man”. For the Crowhurst family, the reality was more tragic.

“At first,” he remembers, “we were told he had just disappeared. Then one day two nuns came to the house. My mother said: ‘The boat’s been found, but he’s not on it.’” The children huddled upstairs in a bedroom. “We knew something was very badly wrong,” Simon recalls. Clare, who had so bravely held the family together for months, began to break down.

Two days later, the log books began to yield their secrets. The air-sea rescue was called off. Simon, his brothers and sister were left to puzzle over a new mystery. Why was no one looking for their father any more? For years after, Clare Crowhurst could not bring herself to discuss the loss of her husband, or his embarrassing hoax. A great, and painful, silence descended. Accident or suicide? This is just one element of the Crowhurst mystery.

To extract maximum publicity from the sensational story of the “Missing Yachtsman”, the Sunday Times sent one of its top correspondents, Nicholas Tomalin, to interview the captain of the Picardy, inspect the Teignmouth Electron and collect whatever papers had been found on board. Instead of a thrilling front-page story, they got the embarrassing tale of the amateur yachtsman who had fooled Fleet Street. Tomalin turned an awkward moment into a sensational scoop. With co-author Ron Hall, he now raced against the clock to unravel the mystery of the log books and publish The Strange Last Voyage of Donald Crowhurst, widely regarded as the definitive account.

Simon Crowhurst, who works as a research technician in the Earth Sciences department of Cambridge University, wonders if he should not make a pilgrimage to see the Teignmouth Electron, still beached amid weeds and driftwood on the dunes of Cayman Brac in the Caribbean, and said by the locals to be haunted. He feels the curse of the past. “When I was a small boy, I was excited by my father’s story. Then it became quite visceral, upsetting and exciting. When I was about 16, I read the Tomalin-Hall book. That was a bizarre experience. At first there was a terrible revulsion. I didn’t talk to anyone. I just absorbed it.”

There is another dimension to this tale, rarely explored. Having spoken at length to Simon, I went to visit his mother, Donald’s widow, Clare, at her seaside home on the Jurassic coast, some 20 miles from Teignmouth, for a very rare interview.

“I definitely think about Donald every single day,” she says, almost before I am inside the house, a gloomy, cluttered Victorian pile at the end of a terrace behind the Seaton seafront. “No, I don’t talk to him,” she says. “I genuinely feel that that’s it - there really is nothing left.”

All this comes out in a rush, but, once the conversation settles down, Clare concedes that she “used to be angry with Donald”, as well as angry with herself. “It was a terrible thing to do to the children.” Could she have worked harder to stop her husband from sailing? “You know, I never thought he would raise the money. Then he was so full of excitement. Of course I wish I’d said, ‘Don’t go.’ But at the time I thought he was doing the right thing - I was not being brave, but being loyal to his dream, as a wife.”

Her main regret is that she did not take more control of the story. “If I’d had my wits about me, I’d never have released the log books.” She has consistently set her face against publicising the story. “Nic Roeg [the film director] used to buy me dinner regularly. Roeg thought he was very charming. But I couldn’t agree.” She has wanted to keep the tragedy to herself, at a considerable cost. “I’ve lived on very little money these 40 years,” she says, sadly. “I’ve muddled through. I still feel as if I’m muddling through. There are moments when I do feel extraordinarily happy, but then I feel guilty about it.”

She has known some other terrible moments. Ten years after Crowhurst disappeared, her eldest boy, James, was killed in a motorbike accident. Now, in her 77th year, Clare Crowhurst seems at peace.

“There it is,” she says, having shown me the famous log books. “I still feel as if it could all have been yesterday, or last week.” Has she never thought of emigrating to Australia (where she owns property) or remarrying? “After it happened, I was just another mum, really. I was pursued for a while by one or two locals, but I really wasn’t interested. Something died with Donald.”

• A personal detail was amended on 17 July 2019.