Why the Portuguese, having mastered the coast of Africa, did not penetrate into the interior of the mainland, climbing the rivers?

Why the Portuguese, having mastered the coast of Africa, did not penetrate into the interior of the mainland, climbing the rivers? Why were the Spaniards able to do this in America?

African rivers (Niger, Congo, Zambezi) are crossed by rugged rapids near the mouth – this prevented the caravels from penetrating deep into the country. The great rivers of South America (Amazon and Parana) have no rapids. In addition, the Portuguese had enough funds only for construction, trading posts on the coast.

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