In late August 1861 combined forces of the United States Navy and Army captured the fortifications at Hatteras Inlet, N.C. This toe-hold on North Carolina’s coast became a threat to the state and to the nascent Confederacy. Walter S. Brewster of South Carolina wrote to Governor Henry Toole Clark of North Carolina and offered a plan to sink United States naval forces off the North Carolina coast. Brewster’s plan involved floating “bombs,” utilizing the ocean tides, and ordinary objects such as rope and logs. The letter was accompanied by a diagram showing how to sink vessels blockading the North Carolina coast (or any Confederate coast).
Note the coloring of the vessel in the diagram: black and white pattern of US naval vessels.