The problem was the wind. The current was difficult too. The coastal winds of west Africa and the steady northward current running along its shore pushed back against the Portuguese caravels slowly exploring southwards.

For sixty years, the Portuguese, under Prince Henry the Navigator and King John II, had mounted a consistent and continuing effort to find a new route to the east. The only way to do that was to go around Africa. But according to Ptolemy, the foremost geographer of the classical world, the Indian Ocean was landlocked. There was no direct sailing route to it.

However, the Portuguese, perched on the edge of the known world and with the ocean as their western border, did not believe this. Having invented the caravel, a lateen sailed ship of unparalleled manoeuvrability and seaworthiness, they had already discovered the Azores and Madeira, islands unknown to antiquity.

Now, with the encouragement of Henry and John, a succession of Portuguese expeditions made their way south, following the west coast of Africa, and planting stone columns on prominent headlands to mark the southern limit of their sailing. In 1486, Diogo Cão discovered the Congo River. In 1486, he made it as far south as Namibia. But the coast of Africa seemed to stretch on without end and the winds along the coast of Namibia and Angola blew against their further progress. What’s more, the ships had to contend with the north-flowing Benguela current too.

But having reached so far south, King John was determined to find the route to the Indian Ocean he and his geographers were certain existed.