20-Inch Rodman Gun displayed at Fort Hamilton, Brooklyn, New York. Photo by another photographer and used with permission.

Please note: The recent photos of the Fort Hamilton 20-Inch Rodman were taken by another photographer and are used with his permission. Many thanks!

Thomas Rodman’s hollow casting technique allowed the manufacture of massive cast iron ordnance. Over 300 15-Inch smoothbore guns were produced in Rodman’s “Pattern 1861” design for the US Army and 100 15-Inch Dahlgrens were also produced for the US Navy. Beginning in 1864, a total of six 20-inch smoothbores were cast - two as seacoast guns for the US Army in the “Pattern 1861” design and four for the US Navy in a Dahlgren shape for use in the monitor USS Puritan. Both of the US Army Rodman guns survive while none of the four Dahlgrens are known to survive. At least one of the Dahlgren type 20-Inch guns was sold to Peru at some point after the American Civil War (photo below).

The Fort Hamilton 20-Inch Rodman is marked “No. 1”, Fort Pitt, and 116,497 - its registry number, place of manufacture, and weight in pounds as completed.

In Artillery and Ammunition of the Civil War, Warren Ripley writes of the 20-Inch Rodman, “Mounted at Fort Hamilton, New York, the piece was fired only a few times since no target could be found sufficiently tough to resist the impact of 1,080 pound solid shot propelled by 100-pound service charges. It was fired four times after mounting and four more in 1867. The latter practice, with 125 pounds of powder, gave a range of 6,144 yards and with 200 pounds, 8001 yards. Elevation in both instances was 25 degrees” (Ripley, pg. 80).

For comparison, the much more common 15-Inch Rodman weighed about 50,000 pounds, fired a solid shot weighing 440 pounds, and was fired using a service charge of 50 pounds of black powder.

A contemporary news magazine article gives the following account of the manufacture of the Fort Hamilton 20-Inch Rodman:

The Monster Rodman Gun Just Cast at the Fort Pitt Foundry - Frank Leslie’s Illustrated Newspaper of April 9th, 1864

We are enabled to present our readers with a view from a photograph by Dobbs, of a monster cannon of the Rodman pattern just cast at the Fort Pitt Foundry, Pittsburg. The engraving shows it as it is in the rough, and its immense size may be seen by comparison with other cannon of different calebres, and with Major Rodman, a man of six feet height, who stands beside this specimen of his artillery. The cannon was cast on the 11th of February, and is intended for New York harbor. Its length is 25 feet, and the maximum diameter 5 feet 7 inches. The gun lying before it is an eight inch, and on it are a three-pounder on the middle with a smaller cannon on each side.

Major Thomas J. Rodman, the inventor of the gun, is a United States ordnance officer, appointed from Indiana. He graduated at West Point in 1841, and was appointed to the arm in which he shows such scientific ability. He was appointed Captain in 1855, and a Major during the last year.

The gun, notwithstanding its size, was cast in 22 minutes from the time of tapping the furnaces - so perfect and accurate were all the arrangements. The amount of metal was 85 tons, or 173,000 pounds. The metal was melted in three furnaces, which were charged at five o'clock in the morning. The furnaces were capped at half-past twelve, and the seething metal ran through three conductors to a reservoir near the mould. This was set perpendicularly in a mammoth pot, and held in position by huge chains suspended from two large cranes worked by steam. When the reservoir was sufficiently charged, the metal was let into the mould, which was filled in 22 minutes from the moment of tapping the furnaces. As the metal settled, the mould was filled from the furnaces. The work was all under the superintendence of Mr. Joseph Kage. The gun, when finished, will be reduced to 112,000 pounds. Its length will be 20 feet 3 inches; the bore being 17 feet 6 inches; the maximum diameter 5 feet 2 inches; the minimum 2 feet 10 inches. It will carry a spherical shot of half a ton, and a shell of 750 pounds. It was cast on the hollow core principle of Major Rodman. This core is an iron cylinder, half an inch thick, through which a stream of water is kept running.