What really happened to sailor Donald Crowhurst on the voyage that inspired The Mercy?

Or a delusional fantasist – someone whose desire to be noticed cost him his life, robbing his wife of a husband and his children of a father?

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Most likely, a little bit of all the above. Which is why Crowhurst’s life, and death, have so fascinated writers and filmmakers ever since he plunged over the side of his small trimaran during the first nonstop, round-the world yacht race in 1968-69 (a race ultimately won by the only finisher, Robin Knox-Johnston).

Here was a man who lied about his position in the race – a competition he was disastrously ill-equipped to take part in – realised his fraudulent actions would be uncovered and, rather than face the music at home, took his own life.

Or did he? Director James Marsh (a Bafta winner for documentary Man on Wire and the Stephen Hawking biopic The Theory of Everything) has another theory that extrapolates the written evidence found on Crowhurst’s boat, showing that the amateur sailor had totally lost his mind.

“Clearly, the pattern of agony you see in the logbooks suggests that he really is on a path to self-destruction, and that’s one very obvious way of interpreting what happens. The second is that it was simply an accident and he may have just slipped and fallen off the boat. And the third possibility is one that I think intrigued Colin and I more than anything else.

“We felt that in his final writings he was constructing a different version of reality for himself to enter into and he may well have believed he was going somewhere else when he stepped off the boat. But, clearly, the logbooks do suggest a huge mental crisis.”

The “Colin” in question is Colin Firth, who plays Crowhurst in Marsh’s new film The Mercy, a title that takes its name from the sailor’s maniacal final writings.

“The last words written in his logbook are ‘It is the mercy,’ which feels like a kind of idea of a release from all his torment,” says Marsh.

Both he and Firth would be the first to admit that this is a sympathetic evocation of Crowhurst’s decline and fall (his abandoned boat, the Teignmouth Electron, was found drifting in the mid-Atlantic more than eight months after he’d set off from south Devon).

Both feel that history has been unkind to him. “What you see is a man under enormous pressure,” says Marsh. “You know that he shouldn’t be going. He hadn’t prepared well enough and the boat was not fully seaworthy.”

But Crowhurst did put to sea. After struggling with faulty equipment, he fell behind in the race and, aided and abetted by his PR man back in Devon (brilliantly played by David Thewlis), began falsifying his race position.

The fact that Crowhurst was, as Marsh describes, “a good father and husband” makes the tragedy even more unsettling.

“That was what he betrayed,” says the director. “He doesn’t return to the people he loves because he can’t, and that has blighted their lives. I think some of that is the unravelling of his mind because of all those months of isolation at sea, and under the burden of these decisions that he’s made about cheating. I think he would say, ‘I’ve brought disgrace upon my family and maybe it’s better not to come back at all.’”

Crowhurst’s wife is played by Rachel Weisz. She says of her character, “I sense that Clare loved Donald very deeply and she didn’t want to stop him living out his dreams.”

The real-life Clare, now in her 80s, never remarried after her husband’s death and, remaining protective of his memory, is wary of the attention of this new film (in cinemas from Friday 9 February).

“I don’t think they’re particularly ready to welcome another telling of this tale, and who can blame them?” says Marsh. “It’s a private family tragedy that on a regular basis seems to get into the news, even after all these years. I don’t think it’s something that any of us would like if it were our family. But from what I can gather, they’ve seen the film and do regard it as a sympathetic telling of Donald’s story.”

Sympathetic it unquestionably is. But in attempting to rehabilitate the reputation of Crowhurst, is Marsh guilty of rewriting history?

“The thing is, I don’t think he was guilty of some grand conspiracy to cheat. He falls into it step by step, which is how most terrible things done by decent people tend to happen. You gradually get yourself into a situation that you can’t get out of. I really sympathise with that. I understood it from a personal point of view and wanted to give the most forgiving account of that process.

“If it were me, I would have turned back and gone straight to my family and said, ‘I’ll deal with the humiliation.’ There were contradictions that you can’t really reconcile, but that’s true of any complicated person.”

The Mercy is in UK cinemas now

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