France, Aug. 27 -- We set sail from Belle-Île-en-Mer, a rocky island about 20 kilometres off the coast of Brittany in western France, on a 20-metre monohull sailing boat called Whirlwind.

Aboard are a captain, a first mate and four amateur navigators, who are spending four days training with Philippe Posth.

Wearing a yellow sailing jacket, with greying hair and a deep voice, Posth has the look of a sea dog. But he's not a professional and describes himself as a "recreational" sailor.

Posth is first and foremost a shopkeeper, selling calculators, specialist books and sextants - the essential instrument for astronomical navigation, which has been his passion for 25 years.

"The sextant is an optical measuring instrument that measure...