[BroJoeK]

DiogenesLamp: "I can tell you this much from memory.
Federal payments to Northern Shipping companies for carrying the US Mail, gave them a competitive advantage against shippers located anywhere else."

Effecting only a handful of specialized ships out of the many hundreds required for US export/import trade.
The real reason Southern ship-building declined after 1800 was very likely the introduction of steam power and other heavy metal components which were not at the time produced in the South.

USS John Adams built in Charleston SC, launched 1799:

SS Planter steamboat carrying cotton, built in Charleston SC, launched 1860:

[DiogenesLamp]

Let me juxtapose these two seemingly contradicting statements by you, one after the other.

The real reason Southern ship-building declined after 1800 was very likely the introduction of steam power and other heavy metal components which were not at the time produced in the South.

.

SS Planter steamboat carrying cotton, built in Charleston SC, launched 1860: