Winds in the Age of Sail

The captain of a steam ship naturally chooses the shortest route to his destination. Since a sailing ship is usually pushed by winds and currents, its captain must find a route where the wind will probably blow in the right direction. Tacking, i.e. using contrary wind to pull (sic) the sails, was always possible but wasted time because of the zigzagging required, and this would significantly delay long voyages. The early European explorers were not only looking for new lands. They also had to discover the pattern of winds and currents that would carry them where they wanted to go. During the age of sail winds and currents determined trade routes and therefore influenced European imperialism and modern political geography. For an outline to the main wind systems see Global wind patterns.

Pilotage or cabotage, in one sense, is the art of sailing along the coast using known landmarks. Navigation, in one sense, is the art of sailing long distances out of sight of land. Although the Polynesians were able to sail the Pacific (with great difficulty) and people regularly sailed north and south across the Mediterranean, before the time of Columbus nearly all sailing was coastal pilotage.

East Asia: A Chinese or Japanese sailor who sails east finds only thousands of miles of empty ocean and a few tiny islands. The Kuroshio Current tends to push his ship northeast into the westerlies and towards North America. There are records of unlucky Japanese fishermen being blown to North America, but no records of any who sailed home. It is easy to sail south and link up with the Indian Ocean trade. North China had few ports and little coastwise trade. South China has a number of good ports but the country inland is hilly or mountainous which restricts trade.

Indian Ocean and the Monsoon Trade: There are no barriers to trade along the coast between the Red Sea and Japan. Local coastal routes soon were linked and extended to Indonesia. By about 850 trade was mostly in Arab or Muslim hands. This trade brought Hinduism and later Islam to Indonesia. A great advantage in the Indian Ocean is the monsoon which blows south in the winter and north in the summer. An Arab wishing to go to Africa or Indonesia would go south on the winter monsoon and return north with the summer monsoon. In Africa this trade extended about as far as Mozambique at the southern limit of monsoon winds. Further south was a lee shore with no trade goods that could not be obtained further north. It is not clear how far south trade and geographic knowledge extended, but there is said to be a Chinese map of the thirteenth century showing Africa in roughly its true shape and there is a Venetian report from the mid-fifteenth century of a Chinese or Javanese junk seen off the southwest African coast.

...
Wikipedia