And how long would it take? A year… or more? There were more questions than answers as Knox-Johnston slipped his lines on June 14th 1968 and headed south, one of nine sailors trying to be the first to lap the planet non-stop and alone.
Initial progress was slow as Knox-Johnston was suffering from a bout of jaundice as he left. But he and Suhaili, the ketch he built himself, clawed their way south towards the Southern Ocean where the young naval officer was expecting the real tough test of his boat, his skill and his nerve to begin.
Suhaili was sighted by the occasional ship, but for eight and a half months Knox-Johnston was effectively out of communication with the rest of the world. He was unable to share the stories of 25 metre Southern Ocean waves, of self-steering damage that meant that Suhaili needed to be hand-steered or balanced precariously, or of the knockdowns that threatened to sink the tiny 32-foot ketch.
But after nearly a year at sea, after a bout of appendicitis that threatened to put an end to his attempt, Knox-Johnston arrived in Falmouth.
Sir Robin Knox-Johnston had sailed alone around the world without stopping in 312 days. Nine sailors set out to become the pioneering first, but the British adventurer was the only one to complete the course. It was a moment in history of truly monumental proportions.
But Robin Knox-Johnston's life of achievement did not stop there. In 1992 he teamed up with Peter Blake, who had been a watch captain on the Whitbread Round the World Race entry Condor skippered jointly by Knox-Johnston and Leslie Williams, to try and sail around the world in less than 80 days. In the spring of 1994 ENZA arrived at the finish having set a new time of 74 days and 22 hours.
And when most other people are fine-tuning a comfortable golf swing, Sir Robin, as he had become in 1995 after the ENZA trip, aged 67, set of to sail alone around the world once more… as an entry in the Velux 5 Oceans Race. And, after overcoming countless technical problems and frequent batterings from the elements, Sir Robin Knox-Johnston and Saga Insurance arrived at the finish in Bilbao in fourth place. It was one of the most heroic stories of the race and completed by a man with a genial smile but who, underneath, is one of the toughest adventurers alive today and whose first - sailing solo and non-stop around the world - will never be erased from the history books.