In April 1863, the C.S. Congress authorized a uniformed Volunteer Navy, many of whom were British. The Confederacy had altogether eighteen commerce-destroying cruisers, which seriously disrupted Federal commerce at sea and increased shipping insurance rates 900%. Commodore Tattnall again unsuccessfully attempted to break the Union blockade on the Savannah River in Georgia with an ironclad in 1863. Beginning in April 1864 the ironclad CSS "Albemarle" engaged Union gunboats for six months on the Roanoke River in North Carolina. The Federals closed Mobile Bay by sea-based amphibious assault in August, ending Gulf coast trade east of the Mississippi River. In December, the Battle of Nashville ended Confederate operations in the western theater.