Experimental 20 inch Rodman Gun

ARTILLERY PROFILE
  • In Service With: U.S. Army Experimental
  • Type: 20 inch Rodman Gun, Model of 1861
  • Purpose: Coastal Defense
  • Invented By: Major Thomas J. Rodman in 1861
  • Patent: Rodman's Casting Process, Patent Granted August 14, 1847
  • Years of Manufacture: 1864 and 1869
  • Tube Composition: Rodman Process Cast Iron
  • Bore Diameter: 20 inches
  • Rarity: Very rare
PERFORMANCE
  • Rate of Fire: 1 rounds every 7 to 10 minutes estimated.
  • Rifling Type: None
  • Standard Powder Charge: 200 lbs. Unknown Grade of Black Powder
  • Max Range (at 25°): up to 8,000 yards (4.5 miles)
  • Projectiles: 1,080 pound round ball, solid shot
WEIGHTS & MEASURES
  • Tube Length: 243.5 inches
  • Tube Weight: 116,497 lbs, or 58.2 tons
  • Carriage Type: Watertown Arsenal iron frontpintle barbette carriage
  • Carriage Weight: 32,000 lbs. or 18 tons
  • Total Weight (Gun & Carriage): about 148,500 lbs. or 74.25 tons
  • No. of Crew to Serve: 7 men + Gunner
  • No. in North America from 1861 to 1865: 1
    • No. of Original Pieces You Can See in the Field Today: 2
  • Cost in 1864 Dollars: $32,781.37 USD Each in 1864
20 INCH RODMAN NO.1
  • Muzzle Markings: NO. 1, FORT PITT, PA., S.C.L., 1864, 116497 lbs.
    • Casting Foundry: Fort Pitt Foundry, Pittsburgh PA
    • Casting Date: February 11, 1864
    • Foundry Number: 2053
    • Inspected by: Stephen Carr Lyford, Ordnance Officer Inspecting, 1863-65.
    • First Test Shot:October 25, 1864
      • Test-fired 8 times total; 4 times in 1864, and 4 additional shots in 1867
    • Service History: None. Stored awaiting further testing. Declared surplus property by the federal government in 1903, and given to the Borough of Brooklyn. On display at the Fort Hamilton military complex.
    • Current Disposition: On public display at Fort Hamilton, Brooklyn, NY​


20 INCH RODMAN NO.2
  • Muzzle Markings: NO. 2, KFPF PA., J.A.K, 1869, 115100 lbs.
    • Casting Foundry: Knap’s Fort Pitt Foundry, Pittsburgh PA
    • Casting Date: 1869
    • Foundry Number: 3387
    • Inspected by: John Alexander Kress, Ordnance Officer Inspecting, 1867-69
    • Service History: Sent to the Coast Artillery facilities at Fort Monroe, Hampton Roads, Virginia for testing, gun is never fired.
      • In 1876 gun is nearly lost in transport en route to Philadelphia International Exhibition when the 100 ton ship nearly capsized because the gun wasn't properly centered on the deck before setting sail.
      • After the exhibition, the gun was sent to the U.S. Ordnance Proving Grounds at Sandy Hook, NJ for further testing.
      • In 1903, the 20-inch Rodman is transferred to grounds of Fort Hancock for Preservation as a monument “to the old class of guns.”
    • Current Disposition: On public display at Fort Hancock, Sandy Hook, NJ


  • Records Broken: 2​
    • Largest muzzle loading guns ever made​
    • Largest iron guns cast in one piece
  • Special Notes: 1 additional 20-inch piece was cast, or perhaps it was a spare, and was sold to Peru in the Late 1870's, Unknown current disposition.​
HISTORY
Like the famed Gun Club of Jules Verne's "Journey from the Earth to the Moon and Around It," Rodman wanted an even bigger gun to test, and proposed building one as soon as the first 15-inch Rodman had been accepted by the War Department. In his report of April 17, 1861, he expressed no doubt that a reliable gun of almost any size could be made with complete success using his casting process.​
320px-Building-a-20-inch-rodman.jpg

He felt, or at least said, however, since he seems to have limited his ambitions rather reluctantly, that a 20-inch gun firing a half-ton shot would be quite big enough. Anything larger would require massive machinery for loading, and "it is not deemed probable that any naval structure, proof against that caliber, will soon if ever be built...."​
Rodman's newest monster--one of the largest iron castings to say nothing of the largest gun ever attempted--was three years in the making. Expected to weigh over 100,000 pounds finished, the gun was much heavier than the 40-ton capacity of Knapp, Rudd's largest furnace. The foundry, however, had a total pouring capacity of 185 tons, and expected to cast the new gun from six furnaces at once. New plans had to be drawn, molds had to be made, new casting procedures were essential, and new finishing machinery had to be designed and built.​
The great day finally came on February 11, 1864. With Major Rodman, then superintendent of Watertown Arsenal, Mass., supervising the operation, the huge gun was poured. Filled in sequence from different furnaces, the 4-piece mold took 160,000 pounds of molten iron. Cooling, by both running water and streams of air, took nearly a week, after which the gun was finished on a specially built lathe. The finished barrel weighed 116,497 pounds, and the muzzle of the gun was inscribed: "20 inch, No. 1, Fort Pitt, 116,497 lbs."​
Destined for Fort Hamilton in New York harbor, the gun was placed on a double railway truck, also specially built, at the foundry to await shipment. As the Pittsburgh Gazettte reported on July 23, 1864, "Juveniles, aged from ten to fifteen years, were amusing themselves today in crawling into the bore on their hands and knees. A good sized family including ma and pa, could find shelter in the gun and it would be a capital place to hide in case of a bombardment....​
Rodman supervised the building of a special carriage for the 20-inch gun at Watertown Arsenal, for the cannon was far too big for any standard mount. The finished product, an iron frontpintle barbette carriage weighing 36,000 pounds was shipped off to New York and assembled at Fort Hamilton.​
The 20-inch gun was a sizable piece of artillery. Total length was 20 feet, 3 inches, with the bore length 17 feet, 6 inches; thus the bore length-diameter ratio of 10.5 was even lower than that for the 15-inch Rodman gun. Both the shot and the shell for the 20-incher were more than twice the weight of the same projectiles for the 15-inch model, the solid shot weighing 1,080 pounds, slightly over half a ton, and the explosive shell 725 pounds empty of the bursting charge.​
The First Test
The first test, not for range but simply to see if and how the gun would shoot, was held on October 25th, almost as soon as the gun was mounted. A huge crowd turned out, including Rodman, of course, and even Secretary of War Edwin Stanton. A 100-pound blank charge loaded for the first shot wouldn't fire with a standard friction primer, and at first it appeared that the gun had a blocked vent. After the charge was pulled, a man was sent down the bore, which Harper's Weekly reports he did very easily, to check for obstructions from the inside.​
The trouble was finally found. The 20-inch gun, over 5 feet in diameter, had a small vent hole almost 23 inches long, and a standard friction primer simply hadn't the power to carry its flame that far to the charge. When the vent was filled up with fine powder before the primer was inserted, the blank charge fired perfectly.​
The next shot was fired with a 50-pound powder charge and the 1,080-pound solid shot, at zero elevation. The Scientific American's on-the-spot correspondent wrote that "the shot struck the water throwing up showers of spray as large as a ship." The third and final shot of the day used 100 pounds of powder behind a solid shot, with the gun at an elevation of 25 degrees.​
"At the report the ponderous globe rushed up through the air with a hoarse roar, and sweeping its long ellipse, fell a great distance, estimation 3 1/2 miles, away into the sea...." The shot's clearly visible flight was timed at 24 seconds.​
The tests were continued on October 27th, again with a huge crowd present. Only two shots were fired, both with round shot and the gun at zero elevation. On the first shot, with a 100-pound powder charge, the ball hit and richocheted 8 times on the water. Recoil drove the gun and carriage back 6 feet, 10 inches, on the base. The second shot, with a 125-pound charge, drove the gun back 7 feet, 5 inches, but the ball, hitting rough water, skipped only 5 times.​
While the Ordnance Department announced that another test would be held as soon as a hulk or ship could be found for a target, the gun was never fired again during the Civil War. The huge cannon was simply included with a battery of fifteen-inch guns as a part of the permanent defenses of New York. Another test, held in March 1867, included four shots fired with 125-, 150-, 175- and 200-pound charges, all with the gun at an elevation of 25 degrees. The maximum range attained was 8,000 yards, or a little under 5 miles.​

A second and slightly lighter 20-inch gun also may have been cast for the Navy in February 1864, and another was later cast in 1866. For obvious reasons, however, the guns were never much more than experimental pieces.​
Rodman's heaviest cannons were fantastic weapons for their time, but from a practical point of view their usefulness was extremely limited.​
Aiming time depended on the extent of adjustment, but it took an additional 2 minutes and 20 seconds to traverse the gun and barbette carriage 90 degrees. The 20-inch gun certainly would have required twice the loading and aiming time of the 15-incher.​
Hitting a fast-moving ship at any reasonable range with the one shot that could be gotten off in time would have depended largely on luck.​
Rodman's guns proved his theories, but the 20-inch gun was still too big to be a really effective weapon. These large guns still exist. Old "No. 1" still sits at Fort Hamilton in Brooklyn, New York, now a public park located near the Verranzano Narrows Bridge. The second 20 inch Rodman looks out over New York Harbor from Fort Hancock at Sandy Hook, in New Jersey.​

FOR FURTHER READING
  • 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.
  • Ripley, Warren, Artillery and Ammunition of the Civil War, Charleston, S.C.: The Battery Press, 1984.
ASSOCIATED LINKS
 
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Norm53

First Sergeant
Joined
Feb 13, 2019
Location
Cape May, NJ
"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….."

How was the shell loaded?
 

CivilWarTalk

Lieutenant General
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Managing Member & Webmaster
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Was 25 degrees the maximum elevation of these guns?
The information on these is very limited, so I'm not sure.

I don't know the limitations of the carriage to take forces at higher angles, or if the barrel could take the pressures of the ball working against gravity at higher angles. Or if the physical girth of the thing made it impossible to elevate any higher than 25 degrees. It has so many things working against it.

Like if you are thinking of using this as an offensive weapon like the Swamp Angel, say on a Flatbed Rail Car, the height of the thing would be enormous, not to mention the center of gravity would be far too high... I would worry it would topple over in the slightest turn.
 
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CivilWarTalk

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"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….."

How was the shell loaded?
Bring the gun level, and they had a crane to lift and position the ball, it only took between 7 and 9 men to service the gun, but it was very slow to move the gun on the platform and point it.
 

Rhea Cole

1st Lieutenant
Joined
Nov 2, 2019
Location
Murfreesboro, Tennessee
Bring the gun level, and they had a crane to lift and position the ball, it only took between 7 and 9 men to service the gun, but it was very slow to move the gun on the platform and point it.
I am sure that everything involved with servicing that thing went slowly... except for the get back out of the way before you fire it part! It must have really been something to see.
 

wausaubob

Lt. Colonel
Joined
Apr 4, 2017
Location
Denver, CO
Its a good indication that by October 1864 a practical 16" gun, with up to 35 degrees of elevation was possible. Two of them mounted on swivels, on a vessel that had adequate support vessels, would have destroyed any port or fortification.
 
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Rhea Cole

1st Lieutenant
Joined
Nov 2, 2019
Location
Murfreesboro, Tennessee
Its a good indication that by October 1864 a practical 16" gun, with up to 35 degrees of elevation was possible. Two of them mounted on swivels, on a vessel that had adequate support vessels, would have destroyed any port or fortification.
You might want read what happened when the British fleet anchored in Alexandria Bay & shelled the Egyptian forts. Jacky Fisher commanded a vessel with rifled 16" guns. They fire 2,000 (+/-) rounds & scored twenty something hits. One captain, distressed at his ship's poor gunnery, had the hands run athwart ship, rocking it to simulate sea going conditions. (This from memory of reading Dreadnaught, the story of the battleship contest between Britain & Germany contained within is absolutely fascinating. )
 

Seduzal

Major
Retired Moderator
Joined
Jun 19, 2013
Location
Canton, North Carolina
Thanks for sharing this awesome article of a gun that was never fired. Looks like to me that if it took 9 or more men to load this gun they would be wore out before they ever fired the shell..
 

Lubliner

Captain
Forum Host
Joined
Nov 27, 2018
Location
Chattanooga, Tennessee
Yesterday I stumbled upon the 15 inch Brooke Rifle being fired at Columbus, Georgia. The weapon was taken from the CSS Jackson. My thought also wandered into and about the device for loading. Here they fire a charge. I hope it is okay to share.
Lubliner.
 

RoadDog

Corporal
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May 29, 2008
Location
The Great Midwest
I kind of have to feel sorry for the guy who went down the cannon's bore to find out why it wouldn't fire during the first test.

I need a volunteer...you!

Please, Mr. Custer, I don't want to go.

No amount of 'cue in the world would have gotten me to volunteer for that.
 

CivilWarTalk

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I kind of have to feel sorry for the guy who went down the cannon's bore to find out why it wouldn't fire during the first test.

I need a volunteer...you!

Please, Mr. Custer, I don't want to go.

No amount of 'cue in the world would have gotten me to volunteer for that.
On one hand, I understand where you are coming from, but knowing what I've read about this, it's probably not that guy's first trip head first down the bore of the gun.

My biggest question is, what was his light source to check the inside of the vent? Besides the vent hole itself of course! The cannon is a big pipe, surely you want to "find" the vent hole if it's plugged, so you'd want a light. It's not like they had lighters back then, certainly not flashlights. He wouldn't be lighting matches in the breech would he?! Unless they had a small lantern, a small candle seems like the only option!
 

wausaubob

Lt. Colonel
Joined
Apr 4, 2017
Location
Denver, CO
On one hand, I understand where you are coming from, but knowing what I've read about this, it's probably not that guy's first trip head first down the bore of the gun.

My biggest question is, what was his light source to check the inside of the vent? Besides the vent hole itself of course! The cannon is a big pipe, surely you want to "find" the vent hole if it's plugged, so you'd want a light. It's not like they had lighters back then, certainly not flashlights. He wouldn't be lighting matches in the breech would he?! Unless they had a small lantern, a small candle seems like the only option!
Someone holding a mirror at the muzzle might be a better solution.
 
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