USS Napa was a Casco class

Casco class monitor

The Casco-class monitor was a unique class of light draft monitor built on behalf of the United States Navy for the Mississippi theatre during the American Civil War. The largest and most ambitious ironclad program of the war, the project was dogged by delays caused by bureaucratic meddling...

 twin-screw

Propeller

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

 light draft monitor

Monitor (warship)

A monitor was a class of relatively small warship which was neither fast nor strongly armoured but carried disproportionately large guns. They were used by some navies from the 1860s until the end of World War II, and saw their final use by the United States Navy during the Vietnam War.The monitors...

 built during the American Civil War

American Civil War

The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...

 for operation in the shallow inland waters of the Confederacy

Confederate States of America

The Confederate States of America was a government set up from 1861 to 1865 by 11 Southern slave states of the United States of America that had declared their secession from the U.S...

. These warships sacrificed armor plate for a shallow draft

Draft (hull)

The draft of a ship's hull is the vertical distance between the waterline and the bottom of the hull , with the thickness of the hull included; in the case of not being included the draft outline would be obtained...

 and were fitted with a ballast

Sailing ballast

Ballast is used in sailboats to provide moment to resist the lateral forces on the sail. Insufficiently ballasted boats will tend to tip, or heel, excessively in high winds. Too much heel may result in the boat capsizing. If a sailing vessel should need to voyage without cargo then ballast of...

 compartment designed to lower them in the water during battle.

Construction

The single-turreted Napa was built by the Harlan & Hollingsworth  Co., Wilmington, DE, and launched 26 November 1864. Though the original designs for the Casco-class monitors were drawn by John Ericsson

John Ericsson

John Ericsson was a Swedish-American inventor and mechanical engineer, as was his brother Nils Ericson. He was born at Långbanshyttan in Värmland, Sweden, but primarily came to be active in England and the United States...

, the final revision was created by Chief Engineer Alban B. Stimers following Rear Admiral

Rear Admiral

Rear admiral is a naval commissioned officer rank above that of a commodore and captain, and below that of a vice admiral. It is generally regarded as the lowest of the "admiral" ranks, which are also sometimes referred to as "flag officers" or "flag ranks"...

 Samuel F. Du Pont's failed bombardment of Fort Sumter

Fort Sumter

Fort Sumter is a Third System masonry coastal fortification located in Charleston Harbor, South Carolina. The fort is best known as the site upon which the shots initiating the American Civil War were fired, at the Battle of Fort Sumter.- Construction :...

 in 1863. By the time that the plans were put before the Monitor Board in New York, NY, Ericsson and Stimers had a poor relationship, also Chief of Naval Construction John Lenthall had little connection to the board. This resulted in the plans being approved and 20 vessels ordered without serious scrutiny of the new design. $14 million US was allocated for the construction of these vessels. It was discovered that Stimers had failed to compensate for the armor his revisions added to the original plan and this resulted in excessive stress on the wooden hull frames and a freeboard of only 3 inches (76.2 mm). Stimers was removed from the control of the project and Ericsson was called in to undo the damage. He was forced to raise the hulls of the monitors under construction by nearly two feet and the first few completed vessels had their turrets removed and a single pivot-mount 11 inch Dahlgren

Dahlgren gun

Dahlgren guns were muzzle loading naval artillery designed by Rear Admiral John A. Dahlgren USN, mostly used in the period of the American Civil War. Dahlgren's design philosophy evolved from an accidental explosion in 1849 of a 32-pounder being tested for accuracy, killing a gunner...

 cannon mounted. These same few vessels had a retractable spar torpedo

Spar torpedo

A spar torpedo is a weapon consisting of a bomb placed at the end of a long pole, or spar, and attached to a boat. The weapon is used by running the end of the spar into the enemy ship. Spar torpedoes were often equipped with a barbed spear at the end, so it would stick to wooden hulls...

 added as well.

Torpedo vessel conversion

As a result, Napa was converted to a torpedo vessel, 25 June 1864 and turned over to the government upon her completion 4 May 1865. Never commissioned, she was laid up at League Island

League Island

League Island was an island in the Delaware River, part of the city of Philadelphia, just upstream from the mouth of the Schuylkill River, which was the site of the Philadelphia shipyard, which eventually became the Philadelphia Naval Shipyard, now known as the Philadelphia Naval Business...

, PA

Pennsylvania

The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...

, until 1875 when she was broken up by John Roach at New York. While at League Island, her name was changed twice: to Nemesis, 15 June 1869; and back to Napa, 10 August 1869.