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I think I know where the misunderstanding comes from. This might help. It's just that the outlines in this diagram are extremely symbolically drawn. I personally have never seen such a concavity on any hull, neither on period plans nor on shipwrecks, nor in the works on naval architecture, but maybe I haven't seen enough yet...
Taken from Castro Filipe, Fonseca Nuno, Santos A., Vacas Tomás, Reconstrução Virtual de uma Nau Quinhentista.
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