Strong circumstantial evidence supporting piracy as a cause of the Civil War.

[Solvedit]

I think I see your problem. There were never a lage number of pirates historically. What we know of the stories of them occured over a few hundred years. You are thinking they were all ocuring once and therfore had to be tens of the thousands of pirates if you count all the land support traiders, the small nations that supported the practice, etc.. At any given time in the classic era of piracy, there were possibly up to a very few thousand individuals. The famous, known of ones were possibly between 10 and 50 at a time period.

You may be conflating the existence of piracy with how many pirates were caught red-handed on the high seas or left enough credible witnesses that they could be convicted and hunted down.  

There may have been that few big name, full time pirate captains at a time.  A few hundred?  There were probably that many on some of the larger crews.  Especially if they let the pirating life corrupt them a bit and they got slack with whatever their normal job was, which is possible. 

Just because there aren't that many Al Capones or Baby Face Morans or Bugsy Siegels doesn't mean everyone else obeys the law.  It is possible people with working boats were able to switch over to pirating from time to time.  Suppose they could row out to raid becalmed ships while normally using their craft for fishing or some other work. 

Suppose they had oceangoing craft hidden upriver.  Such oceangoing pirate ships were already stripped down.  If the pirates removed the cannon, the ships probably required very little water to float.  Perhaps the pirates could drag them up shallow rivers and either remove the masts or tilt the ship on its side.  

Steam power meant such oceangoing pirate ships could not escape as easily if the steam powered navy ships came out to check the suspected pirate coves in calm weather. 

If there were occasional pirates, it would have been more difficult to prosecute them than capturing a pirate on the high seas.  The authorities may have favored a strategy of destroying the ships, docks, and shore facilities of places which harbored pirates who were evading capture. 

Exploding shell guns were introduced in the 1820s.  They were longer ranged than many previous naval cannon in addition to firing an exploding, powder filled shell.  This refinement made shore bombardment more effective because they could blast a ship or a dock or an onshore building into splinters instead of holing one or two of the planks covering the hull or deck.  

If some of the people practicing piracy were barely scraping by and supplementing a meager income through piracy, then the exploding guns served another purpose: they kept the pirates from reusing the cannonballs because the cannonballs would be blown to shards.  This factor would have vastly reduced occasional pirates' firepower.  

Such pirates need not have been supporting a small number of people onshore.  They may have been good at making each coin pass through as many hands as possible.

They could have been swept into the Southeast in successive waves, not merely after the development of the screw propeller.  The Royal Navy had a major campaign to clear piracy off the Atlantic in the 1710s for example.