The Rodman
Queen of the Smoothbore Cannons
Article by Scott S. Sheads and Anna von Lunz

Fort McHenry National Monument and Historic Shrine has the largest surviving collection of Rodman guns in the United States. While the names Dahlgren, Parrot, and Napoleon are synonymous with artillery, few come close to the Rodman, the last of the smoothbore muzzle-loaders to guard the American coast during the last half of the 19th century.

Thomas Jackson Rodman, an Army ordnance officer who graduated from West Point in 1837, developed the Rodman gun. Rodman's commission in the Ordnance Department enabled him to study cannon casting methods at the nation's leading foundries.

In February 1844, Rodman was one of several officers and dignitaries who witnessed the firing of the "Peacemaker," a large gun aboard the U.S.S. Princeton in the Potomac River near Washington, DC. The gun exploded upon firing, killing the Secretary of War and several others.

Original Rodmans
Rodmans at the fort were last fired on July 4, 1903

This experience prompted Rodman to investigate his own theories about gun casting. Discovering that current manufacturing processes produced structurally-weak guns, Rodman thought that casting solid guns and then boring them out caused structural stresses. Cooling guns from the outside caused the cannon to develop strata of different densities, making the tube more susceptible to bursting.

Rodman developed a method of casting the gun barrel around a hollow core or pipe, sealed at the bottom. The mold stood muzzle-up and a small pipe was lowered into the core. When a foundry worker released molten iron into the mold, water flowed into the smaller pipe. This water filled the inner core and ran off at the gun muzzle, away from the casting. Hot coals heaped around the guns exterior insured even cooling. As the iron cooled outward, each successive layer compressed down on the top of the layer under it. The result was a firm, tight casting without any dangerous cracks or air fissures. Rodman also reduced the stresses placed on the bore from gunpowder by increasing the diameter of the powder grains. This produced more uniform pressure on the shot, increasing its velocity during its passage through the bore.

After several unsuccessful attempts to convince the Army to try his process, Rodman applied for a patent in 1847. In 1849, he signed a production agreement with Charles Knapp of the Fort Pitt Foundry in Pittsburgh and continued to experiment on the process for the next several years. After a decade of experimental castings, the War Department ordered Rodman to cast a 15-inch smoothbore. Called the "Lincoln Gun," it was the largest gun produced in the U.S. and passed all trials at Fort Monroe, Virginia. The U.S. Army subsequently adopted the Rodman process.

Rodmans at Fort McHenry

Rodman cannons as seen today at Fort McHenry.
See them at Stop 15 on the Virtual Tour.

In late 1864, the Army began replacing Fort McHenry's former Model 1842 seacoast guns with Rodmans. On June 30, 1866, five 15-inch Rodmans were mounted. In the next decade, several 8-inch and 10-inch guns were also mounted. In 1865, the Army breveted Rodman with the ranks of lieutenant colonel, colonel, and brigadier general for his meritorious service and placed him in command of the Rock Island, Illinois Arsenal, where he served until his death in 1871.

Rodman's new process revolutionized coastal armaments in the U.S. These smoothbore guns lost favor, however, as the superiority of rifled barrels became evident against older forts. As Rodman perfected his process, rifled artillery was being cast that made all smoothbore cannons obsolete. A pointed projectile could travel further and with greater accuracy, easily penetrating walls and thus making coastal forts "fortresses of the past." After the war, many Rodmans were converted to muzzle-loading experimental rifles by inserting 8-inch rifled liners. Fort McHenry's Rodmans were last fired on July 4, 1903, nine years before the Army deactivated the Fort in 1912 and 13 years before it became a national park.

With 14 Rodmans in its collection, the largest in the U.S., Fort McHenry NMHS faces the challenge of interpreting and preserving these mammoth guns for the public. The park presents a 30-minute program on one of the five mounted 15inch Rodmans, which includes a sketch of Rodman's life, a review of his metallurgical achievements, and a discussion of the guns' operation and significance.

Fort McHenry has collected copies of primary and secondary archival materials on the Rodmans from the National Archives and the Rock Island Arsenal Museum and assembled them as the Historical and Archeological Research Project. To preserve the guns, every few years the park blasts the guns with walnut shells to remove old paint and applies a new layer. Because Fort McHenry lies 200 miles inland, salt corrosion is less of a problem than at other Rodman sites. Despite 120 years of exposure to pollution, this fine collection has weathered the elements.

For more about the Rodmans, see Civil War Armaments.
For details about the Fort McHenry Rodmans, see the Table of Statistics.

Scott Sheads Anna von Lunz
Scott S. Sheads
Historian at Fort McHenry
Anna von Lunz
Museum Specialist at Fort McHenry

The authors would like to thank Bob Healy, park ranger at Salinas Pueblo Missions NM and Don Steiner, park ranger, Fort Washington Park for their assistance.

The original article appeared in CRM magazine Volume 20. No. 14, 1997, Cultural Resources, National Park Service, U.S. Department of the Interior, Washington, D.C.

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