The Portuguese also sailed down the west coast of Africa, going beyond Cape Bojador in 1434, a feat theretofore considered to be impossible.
The Portuguese failed to sail down the west coast of Africa beyond Cape Bojador. 
mnli hypothesis: The Portuguese failed to sail down the west coast of Africa beyond Cape Bojador.  premise: The Portuguese also sailed down the west coast of Africa, going beyond Cape Bojador in 1434, a feat theretofore considered to be impossible.