The navigation act of 1817 made it unprofitable for foreign ships to go to Southern ports. That's why getting out of the deal would have been such an economic boon to the Southern port cities. It would have also reduced shipping prices between ports because north eastern owned shipping companies could no longer gouge because they had a monopoly.
Especially since there was no demand for the imported goods.
The navigation act of 1817 made it unprofitable for foreign ships to go to Southern ports.
Then who took the cotton from Southern ports to Europe? U.S. ships?