US Navy warships.
During the US Civil War, the US was called: Federal forces; Union forces; Yankees; Yanks; Blue Coats; Northerners; etc.
Based on the shortage of Union warships that were prepared to do battle, the Navy began a rapid program to commission warships. By the middle of July 1861, the US Navy had 82 warships fit for duty.
blockade runners
ironclad ships
They were actually called blockade runners.
During the US Civil War, the Union navy had a number of responsibilities. This included blockading Southern seaports, transporting troops and supplies, bombarding Southern forts that guarded their seaports and protecting Union cargo ships from the Southern warships. They also engaged in battle directly with Souther warships.
In terms of blocking as it pertains to the US Civil War, the Union attempted to blockade the important ports of the Confederacy. The blockade was successful to a degree, however, with no modern radar or ship to ship communications between Union warships, under the cover of night many privateers were still able to slip past blockading Union warships. To this day, the effect of the blockades with regard to their effectiveness remains under debate by historians. There is no doubt, however, that the Union blockades were not as efficient as the Union would have liked them to be.
The Confederate Secretary of the Navy was Stephen R. Mallory. He knew the South had no war fleet to take on the Union warships attempting to blockade Southern ports. With that said, Mallory's initial idea was a sound one. He envisioned small and fast Confederate warships to attack the Union's large cargo fleet. Doing damage in that sphere might force a number of Union blockaders to withdraw and defend the Union's cargo fleet. Later Mallory would produce more ways to thwart the Union blockade.
The Union navy fleet that forced the surrender of New Orleans in 1862 was led by David Farragut and David Dixon Porter. They commanded a fleet of seventeen warships and nineteen mortar boats. They bombarded Rebel forts guarding New Orleans and after the forts fell, the city surrendered.
a flotilla of warships
The collective nouns are a fleet of warships, an armada of warships, a flotilla of warships.
"Fleet" can be used for both warships and peaceable merchant ships. "Convoy" requires some warships, but may involve merchant ships being protected by the warships.
The most notable naval battle in the US Civil War was the Battle of Hampton Roads on March 8th and 9th of 1862. In this battle it was the first time in naval war history that two ironclad warships fought each other. The battle was important as Union warships were sent to Hampton Roads to support General McClellan's Peninsula campaign. The Confederate ironclad the CSS Virginia engaged Union warships and basically either sunk them or set them afire. The Union had sent their new ironclad, The USS Monitor to challenge the Virginia. The battle was basically a draw.