Do you have a source? Because all I could find was this.... "most slaves were brought in by British ships..."
"...Since most slaves were brought in by British ships, and virtually all were purchased from the British on the coast of Africa, a ban on the trade was an important part of the colonists' general policy not to trade with Britain.[/B]...."
U.S. Constitution and Acts - The Abolition of The Slave Trade
I think one needs to see a couple of benchmarks here, mostly economic ones. BOTH Britain and the US banned the Atlantic slave trade practically simultaneously and the navies of both went after "blackbirders" from 1807 onwards. In that sense the US abolishment of slave trade only impacted Britain for a few years.
Despite being outlawed (and both navies were prone to blow ships out of the water in mid-ocean, slaves and all, if they could not catch them) the trade however continued. Primarily the gauntlet kept being run to the West Indies and Brazil, Venezuela etc. by ships of all nations, British, American, Spanish, Dutch etc.
All of them now having to rely on something not so important before the navies started their hunt, speed. And this, to outrun warships, was achieved by newer hull design, often copper sheathed to prevent seaweed growth and thus considerably reducing if not indeed eliminating "drag".
A favorite was the design of the Baltimore clipper. Need one say more?
Now what shipyards did the South have compared to the North? How many vessels did the later confederate navy have compared with the North? What sailing tradition compared to New Englanders? And finally, what money after trading its cotton, sugar and tobacco with Europe for exchange goods needed at home that could not be produced there? Money that is for getting ocean worthy Clippers built?
The US sailing potential, experience, construction capacity, design advance, in short the whole technology lay in the North. As did the financiers.
Before and after the Revolution and before and after the abolition of the slave trade.
Ships so built were not Northern in the manner of being commissioned, owned and finally run by any Northern State, they were run in private enterprise. The profiteers being Northern Moneybaggers. By no count ruling out that many a politico turned a blind eye in exchange for a cut, if indeed not investing into the voyage as was custom world wide. And washing his hands of it all if a US vessel actually called home to discharge a by then illegal (after 1807) cargo.
American ships had only a small portion of the Atlantic slave trade at all times, the post 1807 pirate runs included, but they were neither commissioned nor financed nor run (let alone built) in the South. The South couldn't or wouldn't hack it. No need. Exceptions in their negligence notwithstanding.
Of course they got on the bus (boat, rather) by 1861 but then it was too late.