While it's conceivable that at some point, a Roman galley may have been blown across the Atlantic (although, given prevailing winds, it seems highly unlikely), that's a far cry from knowledge of the Western Hemisphere. Ptolemy was the greatest geographer of the classical world, and he had no knowledge of the Western Hemisphere. The Greek, Krates of Mallos, who lived several hundred years beforehand, predicted that North and South America would exist, but not because of any knowledge of them, but because he thought that the world would look funny if there weren't continents in the Western Hemisphere to make the world symmetrical.