Columbus was a sailor, and any experienced sailor of his time (at least one with a modicum of reasoning ability), would have known that the world was round. The human eye can perceive an object—the complete object—at a maximum distance of 12 miles, on land as well as on water. Beyond that point the object, whether coming or going, begins to appear (or disappear) as it follows the curvature of the earth. If the earth were flat, a lookout in the crow’s nest would see an approaching vessel in its entirety at first sight—a very tiny object, of course, but a vessel complete with its tall masts under full sail, and the vessel would increase in size as it drew nearer to the viewer.
Conversely with a round earth, the lookout would first see the tip of the tallest mast, and more of the masts and sails would be discerned as the vessel drew nearer (or drew nigh, as some would say), and finally would be seen as a whole sailing ship, complete from topmast to the waterline and from bow to stern.
Obviously the earth is not flat—it is round. We know that—we routinely sail, fly and walk around it, and Columbus knew it in 1492. We know that as he plotted his course he severely underestimated the circumference (as in round) of the earth in his efforts to prove that a ship could reach East-Asia (the Indies) by sailing west. If true, the trip could be completed in far less time than required by the current method of traveling eastward through Arabia on the overland trade route, and the new route would allow Spain to participate in the lucrative spice trade.
That underestimation landed him in the Bahamas Archipelago in North America, on an island he named San Salvador. Believing the island to be the East-Asian mainland (the Indies), he named its inhabitants “Indios.”
None of this in any way diminishes Columbus’ accomplishments. He discovered the New World, and proved that a vessel could go east by sailing west, thus improving Spain’s commercial trade efforts. He successfully made the round-trip across the Atlantic in vessels which may have been state-of-the-art at that time, but in which few modern sailors would choose to make the journey.
We know that Columbus began his journey with four ships—the Nina, the Pinta and the Santa Maria, but only three completed the voyage (it’s not well known, but one sailed over the edge!)
Just kidding! However, I believe he may have begun the voyage with four ships, but one had some serious problems and returned to port.