Because of the blockade, Union ships were allowed to board merchant ships bound for the South and seize the vessels and their cargo as prizes. However, if the ships were en route to a neutral destination, they could not be seized. Consequently British ships began heading for Nassau, Bermuda and Havana. From there, they sailed to Charleston, Savannah and Wilmington, only three days and five or six hundred miles away, at much less risk of exposure to Union warships.
Another advantage of going to ports like Nassau was that cargo could arrive in large, deep-draft freighters from England and then be transferred to the small, fast, shallow draft, dark-colored, low-profile boats known as "blockade runners." Piloted by experts and departing the Bahamas only at night, the blockade runners could elude the Union ships, which had to remain well offshore in deeper water.
During the early stages of the war, Union forces concentrated their blockading efforts on the larger ports of Charleston, Savannah and New Orleans. Wilmington, the largest city in North Carolina at the time with about 10,000 residents, flourished as a result of the highly successful efforts of the blockade runners. With good rail service into Virginia, and General Lee's forces, Wilmington was instrumental in keeping the Confederacy operating.
Wilmington turned out to be a very difficult port for the federal ships to blockade. The city is about 25 miles up the Cape Fear River from the Atlantic Ocean, and access to the river is tricky, hence the Cape Fear name. Even though the Union vessels eventually numbered about 40, they were unable to intercept most of the sleek, dark blockade runners slipping through the treacherous waters on very dark nights without lights.
Federal ships attempting to enforce the blockade at Wilmington had to contend with several obstacles. The two entrances to the river were through shallow channels widely separated by Smith's Island and Frying Pan Shoals extending for miles into the ocean. Consequently, the Union had to maintain two blockades, and it took several hours going around the shallow shoals to get from one blockade to the other. Confederate signal stations on shore signaled to the incoming blockade runners which entrance was more lightly guarded.
As the war progressed, the blockades closed down all the Southern ports except Wilmington, which became the sole lifeline for supplies to the Confederacy. However, in January 1865, Fort Fisher fell to a major Union assault, and the city of Wilmington was captured and held. The supply line was cut, and the fate of the Confederacy was sealed.
Overall, the Wilmington blockade runners were highly successful, with three-fourths of them getting through during the course of the war. Only about 130 vessels were sunk, captured or wrecked.