Simon Crowhurst was eight when he saw his father for the last time. He remembers going out with the rest of the family in a motor boat, to the yacht his father was sailing around the world. “On the deck of the Teignmouth Electron, he kissed us goodbye. I think he said something like, ‘Look after your mother’, and then he was off,” Simon says, nearly 50 years later. “I remember seeing the sail getting smaller and smaller, and disappearing over the horizon. We didn’t think then that we wouldn’t see my father again.”

Simon was nine when he learned his father had died. “My memory was two nuns coming down the drive and speaking to my mother, and my mother taking us to the bedroom I shared with my younger brother, Roger. She sat us down on the bed, said the boat’s been found and he’s not on it. Then she broke down in tears.”

The tragic story of Donald Crowhurst’s last voyage is well-known. On 31 October 1968, the last day the rules allowed, he set off in the Sunday Times Golden Globe Race, a competition to be the first person to sail nonstop single-handedly around the world. He was underprepared and underfunded; his 35ft boat was unsuitable and leaky, no match for the monster waves of the Southern Ocean.

He never reached the Southern Ocean. After being beset by problems and slipping farther and farther behind his competitors, Crowhurst was faced with an impossible choice: to continue to his death, or to return in humiliation and financial ruin (he had staked everything, including his house, on the race). He found a third way: he began to report false positions, fabricating a voyage he wasn’t making, pretending to circumnavigate the globe when in fact he never left the Atlantic.

His yacht would rejoin the race on the return leg – not to be the first to arrive, or to record the fastest time, but to save face. And his house.

Robin Knox-Johnston was first in, and first to circumnavigate the world alone. Another sailor, Nigel Tetley, looked as if he was going to come next and claim the prize for the fastest time, but then he sank (and was rescued). Suddenly, Crowhurst, the only other competitor left in the race, was going to be fastest, claim the Golden Globe and become a national hero. And his log books would come under serious scrutiny.

But Crowhurst never did come home. The Teignmouth Electron was found drifting and abandoned. On board were the log books that charted his actual voyage, together with records that showed how he had made a false speed claim, and indications that he had worked on a false navigational account. They also contained evidence of his psychological state, a rambling, scribbled, crossed-out philosophical interpretation of the human condition. The last page contained the words: “I have no need to prolong the game”; and “It is finished – It is finished IT IS THE MERCY.”

His story has since captured the imagination: there have been books, documentaries, an art installation, an opera, a nod to it in Blackadder, even. And now a new feature film, The Mercy, starring Colin Firth as Crowhurst.

Simon, now 57, understands the obsessive levels of interest in his father’s story. “People can identify with a situation where they feel their options are closed and they’re led into a course of action they wouldn’t normally have anything to do with,” he says, sitting at the kitchen table of his Cambridge home.

I know Simon Crowhurst, who works for the earth sciences department at Cambridge University. He is married to my cousin Alex, which is why he has agreed to talk to me, and why he wouldn’t speak to a journalist who turned up at the door shortly before my visit.

That, incidentally, is how Simon’s mother, Clare, 85, found out there was to be a film with Firth playing her husband: a journalist knocked on the door to ask what she thought about it. She was to be played by Kate Winslet, though Rachel Weisz now plays her opposite Firth.

If it was up to the family, no feature film would ever be made, because it tells the story of events that have affected them deeply, in a range of ways. Because it is disturbing to see the death of your father or husband portrayed on the screen. Because it is a family tragedy, and it has been taken out of their hands. One detail that upset them was that there are three Crowhurst children, rather than four; a brother was cut.

Simon and Alex have written a play about the experience – just for themselves, as a way of letting off steam. It’s very funny, particularly a bit about Firth spending a day with Simon’s mother and sister on an appeasement mission. (Of course it worked: he’s Colin Firth.)

But Simon sees the film-makers’ point of view, too. “Here’s an interesting story, they’ve acquired the film rights, why can’t they tell it? It’s out there, or some of it. But to my family it’s different. It appears intrusive, a kind of violation.”

He’s also generous about the film, and about Firth’s performance. “It captures some of the emotions. Other things are more complex and difficult to put across. In particular, the way my father’s thoughts became very confused.”

This is what you see when you open Crowhurst’s log books, as Simon has. They’re extraordinary artefacts, and to be shown them is like a strange voyage in itself, into the soul of Donald Crowhurst, steered there by his son.

Some of it is routine, functional: sunsights and calculations of positions – both real and imaginary. But then he reports a sudden increase in speed, a new sailing record – 243 nautical miles in a day – a record that never happened. The deception has begun. (Actually, that speed record did sort of happen: later, when Crowhurst was heading back to Britain, he sailed almost exactly that distance in a day. That was typical of him, Simon says. “I think he always hoped and believed that he could make things true eventually, even if they weren’t true at the time.”)

When Crowhurst is delivering his insights on the meaning of life, the universe and everything – that through imagination, humans can become godlike and translate themselves into another dimension – the writing becomes wild and erratic. “In that extreme psychological state, what came to his mind appears to be the Solution, in big flashing neon lights,” Simon says.

Does he understand what his father was trying to get across? “I think there’s a kind of coherence to it, but most of that relates to how he sees his situation at the end of the voyage. His guilt – a lot of it is, by implication, his sense of guilt. And sins – the sin of concealment, of not being where you’re supposed to be, hiding, which is what he’d done.”

How does he feel, reading it now? “I feel very sorry for him. He’s actually somewhere in the margins of the philosophy of madness, you know, sailing over the edge of the world.”

The final page of the log – “It is finished IT IS THE MERCY” – has been torn out, though Simon has the page. He thinks it was the captain of the ship that found the Teignmouth Electron who ripped it, because he felt it would be unbearable for Crowhurst’s wife and children to see.

One of the disturbing things about the new film is that Simon worries his own memories are being overwritten. “You wonder which is true, the memory or the reconstruction.” Does he think about it a lot? “Certainly in times of stress: is this what my father was feeling? Is this what was going on in his head when he was making these decisions that ended so badly?”

And what does it tell him about the meaning of life? “That many questions are still open. On the face of it, we’re a chemical disruption on the surface of a small rock in an obscure part of the galaxy. But, in a way, the profundity of human suffering makes you think: is that all it is?

• The Mercy is released in the UK on 9 February.