During the years of Henry the Navigator (to 1460) and after, Portuguese explorers (see Portuguese Exploration of West Africa) had slowly stretched the boundaries of knowledge down the African coast in their attempts to find a sea route to India. By 1487 Dioblo Cao (Diogo Cão) had almost reached the Tropic of Capricorn – the central coast of modern Namibia.
Diaz, commanding two caravels and a store ship, was despatched from Lisbon in 1487. He passed Cao’s point of return and battled the southerly wind and current to possibly what is now Luderitz, the only decent harbour on that inhospitable coast. There he left the store ship, and continued, finally being forced to stand out to sea to seek more favourable conditions. They instead encounted a storm which blew them south. At about 40 degrees south they found the prevailing westerly wind and headed back for the coast. After several days they turned back towards the north and found land. (Mossel Bay, South Africa.) It was 3rd. February, 1488. They had rounded the southern tip of Africa. They continued east and then northeast for a while until they reached what is now the Great Fish River. There the current was warm and from the northeast. Diaz knew that he had found the route to India.
He had a tired crew, and ships showing the wear and tear of months at sea. He consented when the crew asked to return home.
J. H. Parry, the maritime historian, considers Diaz to be “a fortunate as well as a brave and skillful navigator”. The Agulhas current at the southernmost point of Africa is a trap for the unwary mariner skirting the coast from the northwest; Diaz had avoided it with his deep-sea rounding of the Cape, and was able to note it on the way back, thereby giving valuable information to those who would follow.
Diaz was supplied with two small caravels of the type that had been used for exploration for over half a century. They were not trading vessels. Nor was Diaz a trader, diplomat or soldier. When King Manoel sent Vasco Da Gama on his historic voyage to India a decade or so later it was an expedition designed for trade and diplomacy, and able to defend itself. It knew where it was going. Diaz advised on the planning and route of that voyage.
In 1500 Diaz commanded a ship in Pedro Cabral's expedition to India - his was one of four vessels lost in a storm in the south Atlantic.
Reference: J. R. Parry; “The Age of Reconnaissance”.