The story was
Sail On! Sail On! by Philip José Farmer (published 1952). It takes place on Columbus’ first voyage. Some technology is more advanced, thanks to research carried out by the Baconian Order, so the ship is equipped with a radio operated by Brother Sparks. They believe that the world is round and are encouraged to sail further when they spot seagulls flying over the ship. Though it is a bit disquieting that the birds have no legs. In the end they sail too far and are swept over the edge, with Brother Sparks desperately transmitting a warning back to Spain.