The Rodmans of Fort McHenry
Three types of Rodman Gun are depicted in this photo from Fort McHenry: Nearest the camera is a 15-Inch Smoothbore. The next two are 8-Inch Smoothbores. The next three are 8-Inch Rifles converted from 10-Inch Smoothbores. Finally, in the distance another 8-Inch and a 15-Inch can be seen.
The 8-Inch Rodman is not a small cannon. It only appears so when placed next to a 15-Inch Rodman. Left and right are 8-Inch Rodmans. At center is a 15-Inch Rodman. At Fort McHenry near Baltimore, Maryland
Again, the 8-Inch Rodman is not a small cannon. It is heavier - perhaps by about 1,000 pounds - than this French 36-Pounder. This type of cannon was the largest available to the American defenders of Fort McHenry in 1814.
Taken in isolation, the US Army’s 8-Inch Rodman would be an impressively large and heavy piece of artillery. Weighing 8,500 pounds, it was a bit more massive than the largest cannon used to defend Fort McHenry in 1814: the French 36-Pounder. However, Thomas J. Rodman’s innovations in the casting and manufacture of heavy cannon allowed a similar weight gun to be far more effective. The 8-Inch Rodman could fire a shot weighing about 65 pounds - nearly double the 36-Pound shot of the old French guns - with a greater propellant charge. The 8-Inch Rodman was also very capable of firing exploding shells.
The 10-Inch Rodman was much larger still - weighing about 15,000 pounds it could fire a 128 pound shot or roughly 90 pound shell. And both guns were dwarfed by the 15-Inch Rodman which weighed about 50,000 pounds and fired a 440 pound shot or 360 pound shell.
All three types are on display at Fort McHenry. Next to the massive 15-Inch Rodmans, the 8-Inch Rodman appears diminutive. It is not. It only looks so by comparison.
The 10-Inch Rodmans present are actually all guns which were converted in the 1870s into 8-Inch Rifles. I’ve described this conversion process in a previous post. Three are muzzle insertion conversions. One is a breech insertion conversion. (Additional information on the conversion may be found below on this post.)
The carriages of the three 8-Inch Rifles (converted 10-Inch Smoothbores) are interesting as they had a hydraulic cylinder, rubber bumpers, and gears added - again showing the attempt to modernize these ancient muzzleloaders in a world of steel breechloaders.
Weighing nearly 50,000 pounds and capable of firing a 440 pound projectile, the 15-Inch Rodman is a massive gun.
The three cannon at right are 10-Inch Rodmans that were converted in the 1870s to 8-Inch Rifles by boring out their barrels to 13.5 inches before inserting an 8-Inch rifled sleeve.
The 8-Inch rifled sleeve of this converted cannon may be seen here.
One of the four 8-Inch Rifle conversions is of the “Breech Insertion” type. This was an attempt to create a stronger cannon using a sleeve that was a greater diameter at near the breech. They can be distinguished from the muzzle insertion type by the square shaped cascabel which is where the sleeve was threaded into the existing gun.
The rifling of the breech-insertion conversion may be seen. At left are two of the 8-Inch rifle conversions. In the left background are two 8-Inch smoothbore Rodmans and a 15-Inch Rodman.
Two 8-Inch and one 15-Inch Rodman guns at Fort McHenry showing the lever-post-socket elevation arrangement (missing the lever) of later Rodman guns.
Note the gearing and rubber bumpers on the carriages of the three rifle conversions. Not shown is a hydraulic cylinder beneath the cannon.
A muzzle-insertion 8-Inch Rifle conversion with the breech-insertion conversion in the background.
Two 15-Inch Rodmans
From left to right: Three 10-Inch Rodmans converted to 8-Inch Rifles, two 8-Inch Smoothbore Rodmans. One 15-Inch Smoothbore Rodman. In the background are the remaining portions of the destroyed Francis Scott Key Bridge.
This interpretive sign shows an image of the Rodman guns being fired near the very end of their service with the US Army in 1903. It also notes the 1888 additions of the hydraulic cylinders and rubber bumpers on the carriages for the 8-Inch converted rifles.
The Star Spangled Banner yet waves on the parade ground at Fort McHenry
The 8-Inch Rifle Conversion of the 10-Inch Rodman
The "Manual of Heavy Artillery Service" by John C. Tidball (published in 1880) includes a cutaway drawing of the 8-Inch Rifle conversion of the 10-Inch Rodman. The construction matches the similar conversion undertaken by the US Navy of the 11-Inch Dahlgren. The two piece construction of the wrought iron sleeve (later steel) can be seen with the designations "A Tube" and "B Tube". The muzzle shows the sleeve, locking ring, and cast iron tube which gives the impression of concentric rings.
The conversion was made by boring out the original 10-Inch Smoothbore to 13.5-Inches before inserting the 8-Inch rifled sleeve. The conversion increased the weight of the piece from around 15,100 pounds to 16,000 pounds. The conversion did make use of similar carriages.
While the original fired an approximately 128 pound shot or (about) 100-pound shell with charges usually not exceeding 20 pounds of powder, the converted rifle could fire a 180 pound projectile with a 35 pound powder charge. Tidball does note that the carriages originally built for the 10-Inch Rodman cannot stand too many full power discharges. He therefore recommends the smaller 25 pound charge when full powder is not needed. As can be seen on the range table page, the rifle is credited with being able to penetrate 7.42 inches of iron armor at 1,000 yards.
Regarding the conversion of these Rodmans, Tidball says,
"All heavy cannon in the U. S. land service are made of cast-iron; those pieces having greater calibres than that of the siege gun are cast hollow, being cooled from the inside upon the principle introduced by Rodman.
The want of ductility in cast-iron is unfavorable to its endurance under high vibratory strains; and as the ballistic power demanded of ordnance has greatly increased of late years, cast-iron is no longer much used for forming the parts immediately about the bore of heavy rifled guns, some other metal being substituted, the molecules of which accommodate themselves more readily to new positions when under strain.
It has been found that cast-iron guns are greatly improved by tubing them with some ductile and strong metal, as low steel or wrought-iron. A large part of the energy that the powder gas exerts on the surface of the bore is absorbed in expanding the tube, and that which finally reaches the cast-iron being much reduced in amount, and also spread over a surface relatively much greater than that of the bore, is largely within the limits of safety for the comparatively brittle envelope. The ductile metal of the tube also cushions the cast-iron against the effects of severe vibration and shock. Guns thus constructed have great power of endurance, and when put to extreme test it has been found that the cast iron casing does not burst explosively but cracks and gives way without violence." (Pg. 34-35)
The cutaway illustration may be found on Page 701. The range table on page 133.
From the side and rear these Rodman Rifles (except those converted using the "breech insertion" method) are identical to the original 10-Inch smoothbore. The concentric rings of the muzzle give away the piece as a conversion.
Three 8-Inch Rifles are present at Fort McHenry. Quite a few others were donated to communities around the country.
Cutaway image of the 8-Inch Rifle Conversion. "Manual of Heavy Artillery Service" by John C. Tidball, pg 701
Range table for the 8-Inch Rifle Conversion. "Manual of Heavy Artillery Service" by John C. Tidball, pg 133.
8-Inch Rifle at Fort McHenry
