8-Inch Rodman (left) and 9-Inch Dahlgren (right) displayed at the US Army Field Artillery Museum in Fort Sill, Oklahoma. Photo by Flickr User “rcaustintx”. Used with permission.

The Photos in this post were taken by Flickr User “rcaustintx” in 2019. They are used with his permission. Many, many thanks! Please visit his Flickr photo stream for many, many photos of historic, cultural, and military artifacts with excellent and informative captions. https://www.flickr.com/photos/9681508@N03/

The US Army Field Artillery Museum at Fort Sill offers the unique opportunity to directly compare an 8-Inch Rodman to a 9-Inch Dahlgren. In presenting these two cannon side by side, the visitor to the museum is able to compare and contrast the approaches taken the US Navy and the US Army in producing smoothbore shell guns which were also capable of firing solid shot in the Civil War era. As can be seen in the photo stream linked above the Field Artillery Museum houses a fascinating collection of 18th to 21st century artillery. I can’t wait to visit!

Briefly to compare the 8-Inch Rodman and the 9-Inch Dahlgren:

The US Army 8-Inch Columbiad, Pattern 1861 (also known as the “8-Inch Rodman Gun”):

Was designed by US Army Officer Thomas J. Rodman after experiments in the 1850s to address address weakness found in the US Army Pattern 1844 Columbiads.
Is thickest at the breech and thins quickly towards the muzzle. This shape dispensed with the multiple “reinforces” and muzzle swell of traditional designs.
Rodmans (and Confederate Columbiads which imitated their shape) are distinctive for having a “mushroom knob” shaped breech - really a grove for the rope of lifting tackle. This solved the problem of small cascabels breaking of when used to lift the entire weight of the gun. (The Big Guns, pg. 75).
Weight of the barrel as manufactured is about 8,500 pounds
Is manufactured using a hollow casting process where water is run through what will become the barrel as the molten cannon cools. This process, described in greater depth in the post about the 20-Inch Rodman, produces a stronger cannon.
Fired an explosive shell weighing 50 pounds or a solid shot weighing 65 pounds. Both shot and shell were fired with a service propellant charge of 10 pounds. (Source.)
Was designed to be mounted on an iron barbette carriage as at the museum or a somewhat shorter version for use in casemates (examples at Fort McHenry). All Rodman guns were manufactured with very short trunnions designed for iron carriages.
Was elevated using levers and a sawtooth ratchet system to hold the elevation like earlier Columbiads. Later 8-Inch Rodmans, and most 10-Inch and 15-Inch Rodmans used a lever-post-socket system which relied on neutral preponderance and the friction of the trunnions on the carriage to hold the elevation once set (The Big Guns, pg. 75-76).
Was designed to be fired using a brass friction primer.
Was part of a system that included 8-Inch, 10-Inch, 13-Inch, 15-Inch, and 20-Inch Models - though only the 8-Inch, 10-Inch, and 15-Inch models were produced in numbers.
The accompanying sign notes that this 8-Inch Rodman was cast at Fort Pitt Foundry in 1861 and was mounted at Fort Sandy Hook.
See other posts about Rodman Guns.

The US Navy 9-Inch Dahlgren Shell Gun of 9,000 Pounds

Was designed by US Navy Oficer John. A. Dahlgren in the 1850s after experiments to address weaknesses found in the 1846 32-Pounders. Dahlgren also felt that the Navy had erred in selecting the relatively small 32-Pounder (6.4-Inch bore) as its standard shipboard gun.
Dahlgren developed the distinctive “soda bottle” shape of his shell guns based on information about the force of the propellant charge being primarily near the breech of the gun. Dahlgren would accuse Rodman of copying the shape of his design.
Weight of the barrel as manufactured is typically just over 9,000 pounds.
Dahlgrens were cast solid and bored out in the traditional manner of cast iron cannons. However, Dahlgren required that foundries cast his cannons barrels nearly cylindrically - creating the distinctive “soda bottle” shape by turning the cast guns on a lathe.
Fired an explosive shell weighing 72.5 pounds with a propellant charge of 10 pounds or a solid shot weighing 90 pounds with a propellant charge of 10 or 13 pounds (Ordinance Instructions for the United States Navy. 1866).
Was designed to be mounted on a two-wheel version of the traditional wooden naval carriage for broadside use. During the war it was also used on pivot mountings. As originally manufactured, all 9-Inch Dahlgrens had long trunnions for wooden carriages. Post-war, Dahlgrens were mounted on iron carriages. Some but not all had their trunnions shortened.
Was elevated using an elevating screw which ran through the cascabel. The breeching rope also ran through the cascabel. Unlike the Rodman designed to be used on a stable fortress ashore where friction may suffice to keep a cannon in place, the Dahlgren is designed to be used on a ship rolling and pitching at sea.
Was designed to be fired with a hammer mounted to a locking lug cast on the breech of the gun (the things that look a bit like “ears” at the breech). The Navy did not use metal friction primers until after the war as it was thought that the metal friction primers would become a hazard on the enclosed spaces of a ship’s gun deck.
Was part of a system that included 9-Inch, 10-Inch, and 11-Inch guns. (Dahlgren guns of 15-Inches, 8-Inches, and 6.4-Inches (32-Pounder) were produced during the war.)
See other posts about Dahlgren Guns.

Additional Information about the 9-Inch Dahlgren

The Dahlgren is displayed on a hand-crafted reproduction front-pintle barbette carriage of the type used by the US Army on land fortifications. Historically Dahlgrens were unlikely to be mounted on such carriages, but it matches that of the 8-Inch Rodman and makes a very visually appealing display at the entrance to the museum!
According to posts on the Museum’s Facebook page, the Dahlgren was installed in its present location in 2012. (Photos on Facebook: One Two Three)
The curator of the museum noted that marked weight of the Dahlgren is 9,111 pounds. The Dahlgren was cast in 1855 at Tredegar Foundry. It is thought that it may be US Navy Registry Number 50 - though that registry number is now faint and obscured by past corrosion.
The sign at the museum states that the Dahlgren was carried aboard USS Washington from 1855 to 1875. This is likely not the case as the 74-Gun USS Washington was broken up in 1843 and no ship of that name was in the US Navy during the Civil War.
The weight of 9,111 pounds matches an 1855 Tredegar Dahlgren in the Registry in the appendix of The Big Guns that saw service aboard USS Wabash (which was commissioned in 1856 and saw active service through 1875 - before being used as a receiving ship, at Boston, until 1912). The registry notes that this gun was a “rusted former bollard”. The corrosion on the breech of the Fort Sill Dahlgren is similar to that noted on other former bollards such as at Fort McClary or the Washington Navy Yard.
USS Wabash was one of the five heavy steam frigates built for the US Navy around 1855 (Minnesota, Merrimack, Roanoke, and Colorado were her sisters.) These large ships were designed to carry a main battery of 24 9-Inch Dahlgrens on their gun deck with a mixed battery of 8-Inch and 10-Inch guns on the spar deck. By 1863, Wabash carried 42 9-Inch Dahlgrens on both decks. Wabash saw a very active career during the Civil War from the capture of Port Royal in 1861 to the capture of Fort Fisher in 1865 (Warships of the Civil War Navies, pg. 27). If this Dahlgren was aboard Wabash, it may have been witness to these events.