Actually Columbus thought the world was pear-shaped. (or, as he described it "like a ladies' breast"). The figures for the circumference of the world were off by a considerable margin, so Columbus, did what any person would have done in the face of a failing theory: invent a new one to explain the old. The earth's circumference was just fine, he reasoned -- It the
shape of the earth that baffled the great minds before him. He just happened to find a path that took him to the "stem" of the pear. Up and down the stem took quite a bit of time - and that's why he sailed for so long without seeing land.
Personally, I think he was going loopey.
Nevertheless, Columbus did know about the trade winds and he was able to return to the same island he went to first (even though he didn't know what it was, he did know how to get ther.)
Of course, the Vast Round Earth Conspiracy has hid the fact that the San José fell of the edge of the world while the other three ships tacked violently.