Colleen Blair, 36, from Aberfeldy, is hoping for a suitable weather window this week to make the cold water crossing from Lewis to mainland Scotland, which is renowned as one of the roughest in Britain.
She will leave Stornoway to swim 25 to 27 miles to the north west coastline, depending on where she reaches the mainland.
The experienced swimmer, who has already swum the English Channel, is taking on the challenge to raise money for the Royal National Lifeboat Institution (RNLI).
Ms Blair, an asthma sufferer, will take an inhaler with her. She will follow the rules of open swimming and will not be allowed to wear a wetsuit during the crossing, during which she may encounter whales, dolphins and porpoises.
She will not be allowed to touch the support boat travelling with her and will be thrown chocolate drinks, chocolate bars, creamed rice and carbohydrate drinks to sustain her.
She said: “One of the difficulties that open-water swimmers face, apart from the tides, is the temperature and generally swimmers do struggle in the cold.
“Swimmers do the English Channel, which I have done, where the sea temperature can be as high as 18C compared with The Minch where the water temperature will be much colder, perhaps up to 14C when I do my swim. That is why The Minch is such a challenge, to be done in just a swimming costume and cap.”
But she added: “I became hooked on swimming at a young age and now I am at the stage where I look for different challenges.”
Explaining why she had chosen to raise money for the RNLI, Ms Blair said: “Our sport of open-water swimming is growing and there are more and more of us. However, I do not think people appreciate how dangerous it can be and therefore it is very important to us to know that the RNLI is out there, providing support on the water for anyone who gets in difficulty.’
Last year she visited the RNLI lifeboat station at Lochinver in the Highlands to meet the charity’s volunteers and talk about her plans. A Lochinver crew member, Andrew Stewart, is providing her support boat.
Richard Smith, the RNLI’s public relations manager for Scotland, met Ms Blair there.
He said: “This really is going to be some achievement by Colleen if she can swim her way through The Minch in such cold water.
“The psychological barrier of overcoming the cold, and also the physical demands, will be a big challenge for Colleen – but she is no stranger to challenges and the RNLI is delighted that she wants to raise money for us. We wish her the best possible luck.”
Ms Blair will swim between two to two-and-a-half miles an hour and her anticipated time is 15 to 18 hours, although it could take up to 24 hours depending on the sea conditions.
Weather permitting, she will aim for 54 strokes a minute with her front crawl.
Ms Blair, the duty manager at Breadalbane Community Campus, Aberfeldy, trains in Loch Tay where the temperature is particularly cold at 12C or under. She also trained in Spain in May, covering 24 hours over four days in the sea.
Her achievements at sea include being the first person to swim the Pentland Firth from Hoy to the Scottish mainland; swimming the North Channel, 23 miles between Blackhead in Northern Ireland and Portpatrick in Scotland; swimming 44 miles around Manhattan Island, New York, and also swimming a similar distance around Jersey.
• To make a donation visit the fundraising page http://www.virginmoneygiving.com/ColleenBlair