It intrigued me as well. I only discovered it since January of this year when I noticed certain things did not make any sense. That's when I started looking deeper.
Were there no ports in the South that could have been used?
Sure. *IF* the South was independent, they could be used. If the South was not independent, it was economically unfeasible to use them.
Did trade have to go through northern ports?
The way the laws and the power structure of the day were jiggered, virtually all Southern trade was controlled by New York. Much of this jiggering of the law to favor New York/New England shipping interests was created by "The Navigation act of 1817."
It did a whole lot of things to create a defacto monopoly of trade by the New York/New England shipping interests. I'll get into a couple of details.
The most significant thing it did was bar foreign registered or foreign crewed ships from carrying trade between US ports. Only American ships who had a predominantly American crew, were permitted to carry cargo from one US Port to Another US port.
It also set heavy penalties on the usage of foreign ships or American ships with foreign crews to carry cargo into US ports.
The American shipping interests, all headquartered in New England/ New York area, set their prices such that they were slightly less expensive than paying the penalties of using foreign ships or foreign crews.
The South didn't have that many ships of it's own, and seemingly couldn't hire Northern ships outside the control of a sort of guild that acted very much like a Labor Union which didn't tolerate "scabs."
These are but a couple of examples of how through law, certain powerful interests had gained control of the vast bulk of shipping, warehousing, insurance, banking and so forth in the United States.
If the South wanted product shipped to Europe (Southern products accounted for 3/4ths of the value of all products shipped to Europe.) they were compelled to use New York shipping and other companies, with their commensurate 40% skimming of all profits from the sales of the Southern produced products.
Independence would have netted the Southern producers an instant 40% profit for their goods sold abroad, not to mention the loss of Import tariffs, the vast bulk of which (3/4ths) were paid by the value obtained through Southern products shipped to Europe.
In other words, the South was an enormous cash cow for both New York/New England *and* paid for 3/4ths the cost of all Federal government.
Here is a map I post often to illustrate the point.
3/4ths of all the money earned which are represented by those coins, was earned from Southern products being shipped to Europe.
This map is what put me on to what really happened. I had learned from other sources that the vast bulk of all trade value came from Southern products, so why was all the import value ending up in New York? Shouldn't it have been showing up in the South in order to balance the Production/Payment trade equation?
That the map was showing the vast bulk of the money ending up in New York told me immediately something didn't make sense.