In 2013, Robert Redford starred in All is Lost, a film about one man’s desperate attempt to survive when he gets lost at sea. It’s an impressive piece of work: Redford is the only member on board, the film’s only cast member and you count the number of sentences spoken on your hands. All this, and it manages to grab your attention for a tense 100 minutes or so.

The Mercy bears many similarities to that earlier film, except it has a lot more words, many more cast members and very little tense edge-of-your-seat drama.

It follows small-time businessman and amateur sailor, Donald Crowhurst, played superbly by Colin Firth, and his doomed attempt to compete in the Sunday Times Golden Globe Race in 1968 for the first single-handed journey around the world.

A quick Google search will confirm that Crowhurst and his homemade trimaran very much get lost at sea. So the film’s job isn’t to build any will-he-make-it suspense but to show us exactly how Crowhurst unraveled. The Mercy does just that – but you wish the film were brave enough to get ruffled along with him.

For all Crowhurst’s cosmic desolation – the sailor waxed lyrical with quasi-religious fervour about 'the sin of concealment' in his infamous logbooks – The Mercy feels very assured.

The film is good at giving us a picture of the pressures that pushed the doubtful Crowhurst out to sea – his supremely mouthy publicist (David Thewlis) and a particular pliable local businessman (Kevin Stott) who bankrolled his ambitions being the main perpetrators.

There also are moments when director James Marsh – who also put together the glowing Stephen Hawking biopic The Theory of Everything – really drives home Crowhurst’s isolation on the creaky boat.

But we keep on hopping back to the cosy huddle of Crowhurst’s Devon hometown to check in on his friends and family in the sort of choppy back-and-forth that makes you wonder if it wasn’t devised just to make sure Rachel Weisz, playing the sailor’s wife Clare Crowhurst, can justify her place on the poster.

You wish that the film would just stick with the dreamer, as he battles the sea, himself and his outdated Victorian belief in great deeds and great men. And Colin Frith’s performance is easily strong enough to carry our interest. As it stands, it carries most of the film anyway.

There is a real gem of a film lurking in the depths somewhere here but The Mercy is happy to dwell on the surface. As such, it’s a perfect film for a wet afternoon – which is exactly what the films feels like.