“Divergent” trilogy captain Shailene Woodley turns in a highly seaworthy star performance as she battles an onslaught of Category 4-level flashbacks in “Adrift.”

This is a fact-based drama of what happened to Tami Oldham (Woodley) when she set sail in 1983 from Tahiti to San Diego with her fiancé, Englishman Richard Sharp (Sam Claflin). Their craft was a 44-foot yacht, and their adversary was Hurricane Raymond, which they met a few weeks into their planned 4,000-mile trek. The hurricane whipped up 40-foot waves and 140-knot winds. Clearly, this story was destined for the movies.

Woodley looks, moves and responds like someone who knows her way around a craft on water, as opposed to an actress who recently completed a crash course on faking nautical skills for the camera. Good thing, because most of the picture depends on her and Claflin (from the “Hunger Games” series), who, ironically, while doing adequate work on the sailing scenes, is less than convincing when the characters are on land.

The rest of the movie consists of somewhat hammy digital effects work, and on scenes set months earlier, on Tahiti, from the couple’s recent past. These are meant to illustrate the depth of their love and the inevitability of their shared journey, although as written by Aaron Kandell, Jordan Kandell and David Branson Smith, they also serve to get the audience thinking: We’d like to get back to the crisis, if you don’t mind.

Here we get into spoiler issues and questions about the story’s adaptation from Oldham’s 2002 memoir that inspired the movie. Icelandic-born director Baltasar Kormákur begins the narrative with an injured Tami jolted awake aboard the boat, the battered yacht now on calm seas. Richard is nowhere to be seen; soon enough, however, she spies him clinging to a dinghy, and rescues him.

“Adrift” works on two timelines: As the flashbacks move ever closer to the hurricane itself, the present-tense action progresses, ticking off the days and weeks of the yacht adrift, ultimately revealing certain truths about Tami’s predicament.

It makes for a fairly gripping and refreshingly small-scale disaster movie. But there’s a “but.”

The “but” is everything designed to get us interested in these two before the heavy weather. The breezy courtship sequences feel stiff; the writing’s generic in the extreme. On their first date, Richard tells Tami that sailing alone is full of adversity and loneliness and seasickness. “If it’s not fun,” she wonders, “then why do you do it?”

His answer may well be close to what the real Richard said (the “intensity” of it, the feeling of being “reborn,” etc.), but Claflin’s exertions don’t convince. He’s a talented actor, but too often, especially in these bashfully smitten getting-to-know-you scenes, it’s as if he’s being paid per ingratiation.

Uncharacteristically, Woodley struggles a bit in these scenes, as well, though her instincts are generally terrific (and have been, ever since “The Descendants,” in which she played George Clooney’s tetchy elder daughter).

Movies like this one, whether set on water (“All Is Lost,” with Robert Redford) or in a slot canyon (“127 Hours,” with James Franco), reveal to us the worst of what an adventurer sometimes faces, alone or otherwise. These stories are overwhelmingly male; this one, for once, is female-driven.

They never quite got the script right, but director Kormakur toggles well enough. And Woodley sees it through.