Defining "discovery" as the initiation of sustained contact between Eurasia and the Americas, I doubt you could delay it more than a decade or two without a very early POD. The Earth is already known to be spherical (this was known to the ancient Greeks, and is well known among both sailors and anyone with a decent education in Europe in the late 1400s), so a western route to Asia is a widespread idea, the only debate being whether it's short enough to be more practical than the Cape of Good Hope route (Columbus was wrong about this, badly underestimating the Earth's circumference and badly overestimating the size of Eurasia, but he's not the only one). Beyond that, shipbuilding and navigation technology are good enough that ships are already traveling deep into the Atlantic for other reasons: Portuguese ships bound for the Cape of Good Hope often detour deep into the South Atlantic to
take advantage of wind and current patterns (Brazil was discovered in 1500 by a
Portuguese fleet taking this route that got blown off course and sighted signs of nearby land), and
English cod fishers had been fishing in the Northwestern Atlantic since the 1480s (and indeed probably had been making landfall in New England or Nova Scotia to process their catch).