USS Neptune has been the name of more than one United States Navy ship, and may refer to:
USS Manhattan was a single-turreted Canonicus-class monitor built for the Union Navy during the American Civil War. After commissioning in 1864 the ship was assigned to the West Gulf Blockading Squadron and participated in the Battle of Mobile Bay. At the end of the battle, Manhattan took the surrender of the Confederate casemate ironclad ram Tennessee. She bombarded Fort Morgan during the Siege of Fort Morgan and later blockaded the mouth of the Red River until the end of the war.
The ship was placed in reserve after the end of the war and Manhattan was only occasionally recommissioned before being sold for scrap in 1902.
The ship was 223 feet (68.0 m) long overall, had a beam of 43 feet 4 inches (13.2 m) and had a maximum draft of 13 feet 6 inches (4.1 m). Manhattan had a tonnage of 1,034 tons burthen and displaced 2,100 long tons (2,100 t). Her crew consisted of 100 officers and enlisted men.
Manhattan was powered by a two-cylinder horizontal vibrating-lever steam engine that drove one propeller using steam generated by two Stimers horizontal fire-tube boilers. The 320-indicated-horsepower (240 kW) engine gave the ship a top speed of 8 knots (15 km/h; 9.2 mph). She carried 140–150 long tons (140–150 t) of coal.Manhattan's main armament consisted of two smoothbore, muzzle-loading, 15-inch (381 mm) Dahlgren guns mounted in a single gun turret. Each gun weighed approximately 43,000 pounds (20,000 kg). They could fire a 350-pound (158.8 kg) shell up to a range of 2,100 yards (1,900 m) at an elevation of +7°.
USS Neptune (1863) was a large steamer, with powerful guns and a large crew, acquired by the Union Navy during the American Civil War.
She served the Navy primarily, as an armed escort vessel in the West Indies, for Union Navy and commercial ships traveling through that area on their way to and from California.
Neptune, a wooden steamer, was purchased by the Union Navy at New York City 17 July 1863 from William P. Williamson, of the Neptune Steamship Co. She was delivered to the government at the New York Navy Yard 3 September, and commissioned there 19 December.
Assigned to the West India Squadron, she steamed from New York City 9 January 1864 and was employed principally in convoying the California steamers through the West Indies.
At the end of the Civil War, Neptune returned to New York City, where she decommissioned 31 May 1865. She was sold at auction at New York City 12 July 1865 to John Henderson.