Colleen Blair, from Aberfeldy, is hoping for a suitable weather window this week to make the cold-water crossing, which is renowned as one of the roughest in Britain. She will leave Stornoway, on the isle of Lewis, to swim to mainland north- west Scotland, a distance of 25 to 27 miles, depending on the point she reaches land.
The experienced swimmer, who already has an English Channel crossing under her belt, 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, where she may experience close encounters with whales, dolphins and porpoises.
She will not be permitted to touch the support boat travelling with her and will be thrown chocolate drinks, chocolate bars, creamed rice and carbohydrate drinks to help sustain her.