The Big Brother of the Rodman Family: 20-inch Rodman, Part 1

Time to look at the biggest gun made during the Civil War and the largest of the Rodman family – the 20-inch Rodman Gun.

Fresh off the 15-inch Rodman prototype’s tests, and while the nation was rushing to war, Thomas J. Rodman first proposed a 20-inch gun on April 17, 1861:

The entire success which has attended the manufacture and trial of the 15-inch gun, leaves no doubt of our ability to make reliable guns of even greater diameter of bore than 20 inches, and to maneuver and load with facility, and without the use of machinery, guns of that caliber.

A 20-inch gun, one caliber thick, 210 inch length of bore, and 20 feet total length, would weigh about 100000 lbs.

A solid sphere of iron, 20 inches diameter, would weigh about 1000 lbs…. The ordinary service shell need not e over 3.5 inches thick; would weigh about 725 lbs., and contain about 38 lbs. of powder making the total weight of the loaded shell about 763 lbs…..1

Rodman went on to suggest a 100 pound powder charge for these projectiles.  He estimated the force of the solid shot would equal the combined force of six 10-inch solid shot.  Likewise one 20-inch shell would equal the destructive power of seven 10-inch shells.  From those estimates, Rodman drove the point home –

And the destructive effect of such shells, as compared with 10-inch shot, upon iron-clad ships and floating batteries, would be in a much higher ratio; their whole crushing force being brought to bear upon a single point at the same time….

I would point out that Rodman’s concern about defeating ironclads was likely with an eye cast across the Atlantic.

The Army didn’t act upon Rodman’s proposal for two years.  In April 1863 Fort Pitt Foundry received an order for a 20-inch Rodman.2

In the May 1864 American Journal of Science and Arts, a corespondent offered a wealth of details concerning the gun’s casting. As I’ve already described the Rodman method at length, let me highlight one detail not mentioned in Rodman’s earlier experiments. Describing the core inserted into the mold when casting –

The core barrel is a hollow cylinder of iron 17 inches in diameter, closed at one end, open at the other.  It is about one inch thick and is grooved longitudinally or fluted, like a column in architecture, its entire length, the furrows being  1/2 an inch apart.  Around this is wound a rope of the size of ordinary bed cord, and over this a layer of clay 3/4 inch thick is evenly spread and the whole dried.  The object of the grooving is to allow the free escape of gas.  The rope prevents the clay from filling up these grooves.3

Casting took place on February 11, 1864 using three furnaces, and a couple backups.  In about 20 minutes some 85 tons of iron poured into the mold.  After two hours of the prescribed water cooling, foundrymen added more molten metal in order to better form the “sinking head” of the gun.  After about fourteen hours of cooling, the men removed the core. But unlike the smaller Rodman’s this gun was too hot for direct water cooling.  Instead the gunmakers used a continuous blast of air.  Not until February 23 was the gun finally pulled out of the casting flask.  Yet the gun remained warm, and required two more days of cooling.  After that the gun went to a lathe to machine off the excess metal and smooth the exterior.

As seen on the chart below the 20-inch gun dwarfed the other Rodmans in all respects.

Stephen C. Lyford inspected the gun in August 1864, giving it registry number 1.  The gun then went to New York for trials.  And it’s still there today:

20-inch Rodman #1

(Special thanks to my friend Bill Coughlin who provided the photo and posted the nearby marker to HMDB)

I’ll turn to the trials, and then discuss the other 20-inch Rodmans produced by Fort Pitt after the Civil War.

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  1. Rodman’s proposal appears in Reports of Experiments on the Properties of Metals for Cannon, and the Qualities of Cannon Powder; with an Account of the Fabrication and Trial of a 15-inch Gun (Boston: Charles H. Crosby, 1861), pages 307-308.
  2. Edwin Olmstead,  Wayne E. Stark, and Spencer C. Tucker, The Big Guns: Civil War Siege, Seacoast and Naval Cannon (Alexandria Bay, NY: Museum Restoration Service, 1997), Appendix C169, page 265.
  3. Account of the casting of a gigantic (Rodman) Gun at Fort Pitt Foundry, The American Journal of Science and Arts, May 1864, pages 296-301.
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