My thanks to those who responded to my question. Particularly helpful were the response from Freddy the Pig and the Wikipedia link provided by watchwolf49. Bonzer’s point about Shakespeare’s Globe was also a good one, even though this was a century after Columbus.
I wonder if ordinary seamen really all accepted that the world is spherical. The point about sails appearing first is a strong one if it is pointed out to you, but not everyone would think of it on their own.