Columbus' crew may have been fearful and unruly, but not because of any fear of an edge to the earth. They were concerned because they were sailing south - into the "torrid zone" - where they thought the heat might evaporate the water to render the seas too shallow for their ships. They also may have been concerned that a westward voyage to reach the Indies was impossible because they didn't have enough food and water to cover such a distance (and they were correct - they didn't have enough provisions for a voyage to Asia and would in fact have died had they not bumped into the Americas).
Link
Worth a read.