In order to sail the South Atlantic and round the tip of Africa, Portuguese sailors had to confront two powerful ocean flows: the Agulhas and Benguela currents.
The warm Agulhas runs south and west from the Indian Ocean pushing against the near-freezing waters of Antarctica, before meeting the cold Benguela current off the Cape of Good Hope.
The second swiftest current in all the world's oceans, the Agulhas is deadlier than the swiftest current (the Gulf Stream) for two reasons. First one of its branches surges through a narrow passageway between Madagascar and Mozambique on the east coast of South Africa (downward arrow on map). Furthermore its waters rush from north to south--the opposite direction from which Portuguese ships needed to travel in order to round the tip of Africa.
In nearly a thousand years of crossing the Indian Ocean, neither the Arabs nor Persians nor Arabs nor the fifteenth-century Chinese Star Fleet had ever navigated the Mozambique channel, even sailing with the the Agulhas Current.
To sail against the Agulhas Current is even tricker than sailing with it. Only a very narrow band of water northward through the current (up the east coast of Africa to reach the Indian Ocean) boats had to tack back and forth in a very narrow band of water--in which submerged sharp rocks abound--and modern shipping trawlers with sophisticated navigational instruments still wreck themselves today.
Gale force winds (up to 180 kilometers per hour) are common in the Spring (September through November). Even more frightening are the deadly changes that occur when the winds shift direction. When the winds begin to blow from the West and Southwest (the opposite direction from the current), monster waves (up to five stories high) are known to occur. There is no way to survive such rogue wages, for even the largest vessels plummet to the ocean floor without a trace.
After Bartholomew Dias successfully sailed the treacherous intersection of the Benguela and Agulhas current (the Cape of Good Hope), it took three separate Portuguese voyages between 1486 and 1497 to learn to navigate successfully through the Agulhas current, travelling in the opposite direction. Each separate fragment on the map represents a separate voyage through the Mozambique channel sailing against the Agulhas current.