Once upon a time I was heavily into wooden warships of the early 19th Century. Allow me to help with the "frigate" designation."
These ships were the heavy cruisers of their day with a gun rating of between 32 and 50 cannons depending on the nation and ship class. The American "super-frigates" had two classes - 36 guns and 44 guns (the latter including the famous USS Constitution). These warships typically had three masts.
By the Civil War, frigates, as a class, were being replaced by large sloops of war. Sloops had been classed as smaller vessels earlier on, even below brigs (20-25 gun warships with two masts - light cruisers if you will). The surviving USS Constellation in Baltimore was a sloop of war and not a frigate despite it having the three masts typical to the class. Ships of the line (the battle ships of their day), were also being replaced in many navies as being too large and too expensive to operate. Modern cannons packed more punch than the masses of older guns from before and, as such, faster ships with fewer cannons came into popularity. Ships of the line carried, depending on their class rating 60 to over 100 guns. They also had three masts.
The CSS Tennessee was an ironclad and not a sailing vessel at all. It was built hull up as such a warship.
Tom's point about the number of merchant vessels being cut off in Mobile when Farragut captured the outer passes is on target. As part of the flags group that recently discussed the flag in Iowa, there was nothing present to us that proves conclusively that it was the same flag flown in 1861 on Morris Island. This flag could well have come from one of those vessels.
The question has also been posed as to how Culpepper's Battery, considering all of the Citadel grads in CS service, got to claim this flag over all other grads. There is no paper trail to back this claim. I think, like Tom, that a copy-cat flag was made should this prove to be the flag of the battery. The captor only claimed to get the flag "in Mobile" and not specifically at Ft. Blakely.
There is much research work to do regarding the flag in Iowa.