Famous Monitor's XI-inch Dahlgren Shell Guns

When John Ericsson designed the Monitor, he knew that a 15-inch Rodman existed, but that was an Army gun. He hoped that that a gun like that could go in his design. Unfortunately, the largest gun adopted by the Navy at that moment was the XI-inch Dahlgren Shell Guns, which is still really big, but not what he hoped for. Dahlgren wasn't convinced that guns larger than 11 inches were safe, and in the confines of an armored turret, well, he had even more reservations about such big guns. So at his direction, Ericsson submitted his experimental plans for the Monitor tailored to fit two XI-inch Dahlgren Shell Guns in the turret.

  • Model: XI-inch Dahlgren Shell Guns
  • Type: Muzzleloading Smoothbores
  • In Service With: U.S. Navy, Aboard the U.S.S. Dacotah, transferred to the U.S.S. Monitor
  • Under the Command of:
    • Lieutenant John Lorimer Worden, in command of U.S.S. Monitor, Feb. 25, 1862 - Early Sept. 1862
      • Lieutenant Samuel Greene, Executive Officer, supervised loading and firing of one Dahlgren
      • Acting Master, Louis N. Stodder, supervised loading and firing of one Dahlgren
    • Commander John P. Bankhead, in command of U.S.S. Monitor, Early Sept. 1862 - Dec. 30, 1862
  • Purpose: All Purpose Naval Armament on Turret Ironclad
  • Gun Placement:
    • Gun 27: U.S.S. Monitor Turret, Port Side
    • Gun 28: U.S.S. Monitor Turret, Starboard Side
  • Used in Battle: March 9, 1862, Battle of Hampton Roads, Virginia, against ironclad C.S.S. Virginia
  • Invented By: John A. Dahlgren, USN
  • Lost at Sea: On-board the sinking U.S.S. Monitor, Atlantic Ocean, southeast off Cape Hatteras, on December 31, 1862
  • US Casting Foundry: West Point Foundry, Cold Springs, New York
  • Year of Manufacture: 1859
  • Tube Composition: Cast Iron
  • Registry Numbers: 27 & 28
  • Trunnion Markings: Not Available
  • Foundry Numbers: Not Available
  • Inspectors Mark: Not Available
  • Additional Engraving: added during a maintenance period in October of 1862...
  • Purchase Price in 1859: $1,391.00 ea. (US)
  • Bore Diameter: 11 inches
  • Bore Length: 131.2 inches
  • Tube Length: 161 inches
  • Tube Weights:
    • Gun 27: 15,720 lbs.
    • Gun 28: 15,617 lbs.
  • Carriage Type: Turret Carriages
  • No. of Crew to Serve: 7 men per gun
  • Rate of Fire: One round, every 7 to 8 minutes each
  • Rifling Type: None, Smoothbores
  • Standard Powder Charge: Up to 15 lbs. Cannon Grade Black Powder
    • Later, charges safely increased to 30 lbs., too late for Hampton Roads
  • Muzzle Velocity: 1,120 ft/sec.
  • Effective Range (at 5°): 1,712 yards (0.97 miles)
  • Projectile Flight Time (at 5°): 5.81 seconds
  • Maximum Range (at 15°): 3,650 yards (2.07 miles)
  • Projectiles: Round Balls, 166 lb. Solid Shot or 133.5 lb. Shells

John Ericsson had been assured that two XI-inch Dahlgren shell guns would be provided for the new Monitor project. When it was discovered that the intended guns had not shipped, and were not available, a search for available guns was made. The U.S.S. Dacotah which just happened to be docked nearby, had two slide-mounted pivot guns installed, these just happened to be lightly used XI-inch Dahlgren Shell Guns, Registry numbers 27 & 28. It was just what they needed.​
The Dahlgren guns were removed from Dacotah, and mounted aboard the Monitor, inside the new armored rotating turret.​
Back in 1860, before the Monitor was designed, during a test firing, a Dahlgren shell gun exploded. To prevent any catastrophic gun bursting within the confined turret on the Monitor, each of the XI-inch Dahlgren guns was restricted to using 15-lb gunpowder charges by the always cautious Commander John Dahlgren.​
When the Monitor entered it's first Battle at Hampton Roads, it fired it's Dahlgrens in anger against the C.S.S. Virginia, formerly the Merrimack. Forty-one shots were fired by the Monitor in that engagement, but with the restricted gunpowder charge of 15 lbs., even though the 165 lb. solid shot easily dented and scuffed the armor plate on the Virginia, it didn't do any serious damage to the iron-clad vessel.​
Tests conducted after the battle confirmed that using 30 lbs. of black powder in the 11-inch Dahlgren would have easily punctured the Virginia’s hull.​
After the Battle of Hampton Roads, the Monitor attempted to engage the Virginia when it came out on May 8th, firing a few shots at distance, but the Virginia didn't take the bait. The Confederates abandonded the City of Richmond a few days later, burning the Virginia in their wake.​
Free from patrolling the Virginia, the Monitor moved on to participate in the Battle of Drewry's Bluff, firing at a few targets with the Dahlgrens and scoring hits, but finding it difficult to elevate their guns effectively at short range.​
When the U.S.S. Monitor was ordered to move down to North Carolina in late December, it took a voyage that it wouldn't sail home from. In the evening of December 30th, a storm hit off the coast of Cape Hatteras, and waves caused the ship to take on water and begin sinking. Later that night the doomed ship took 16 men with it to the sea floor, and the two XI-inch Dahlgren Shell Guns.​
  • Wreck of USS Monitor Discovered: August, 27, 1973
  • Location of Wreck: 35°0′6″N 75°24′23″W, designated as Monitor National Marine Sanctuary
    • Atlantic Ocean, about 16 mile SSE of Cape Hatteras Lighthouse, North Carolina, about 230' below the surface.
  • Turret / Dahlgrens Recovery Date: August 5, 2002
  • Dahlgrens Current Disposition: Undergoing Conservation at the Mariners' Museum in Newport News, Virginia
After the turret was raised in 2002, conservators began the long process of excavating the fragile cannons from the turret and stabilizing them. The cannons were removed from the turret in 2004 and placed in conservation tanks. The guns underwent an extended soaking process to remove chlorides from the iron. This process took approximately five years. Additional work to remove concretions outside and inside the guns has been completed. Both guns are currently undergoing electrolytic reduction and desalination in the Batten Conservation Laboratory Complex.​


Photos, L to R: USS Monitor Turret Recovery 2002, Monitor's Dahlgrens going into Conservation Tanks,
and Excavating the Bore of one of the Monitor's Dahlgrens. Photos from NOAA.gov
Navy Official Reports, North Atlantic Blockading Squadron
Report of Lieutenant Jeffers, U. S. Navy
Regarding ammunition expended by the U. S. S. Monitor

Hampton Roads, March 16, 1862.

SIR: In answer to your enquiry I have to report that the Monitor expended forty-one solid cast-iron shot in her engagement with the Merrimack, equally divided between guns 27 and 28.

On inspection of the bore with a mirror no trace of injury can be observed. I have no means of examining the vent by taking an impression.

Unless absolutely necessary I shall fire no more cast-iron solid shot, as I am satisfied that shells are not more liable to fracture. The bronze coated shot I shall reserve for especial occasion. The wrought-iron shot I shall send on shore to remove the temptation to fire them. I am satisfied that the Merrimack can not seriously injure the Monitor, but an explosion of a gun might destroy the turret.

I have the honor to be, very respectfully, your obedient servant,

Lieutenant, Commanding.​
Flag-Officer L. M. GOLDSBOROUGH,
Commanding North Atlantic Blockading Squadron.
NAVY OR, Series I--Volume 7, From March 8 To September 4, 1862. pp. 1-81

  • The Story of the Monitor: The First Naval Conflict Between Ironclad Vessels - Archive.ORG (Free)
    by William S. Wells, Issued by the Cornelius S. Bushnell National Memorial Association, New Haven, CT; 1899.
  • Shells, and Shell-guns by John Dahlgren, King & Baird, Philadelphia, 1856. - Google (Free)
  • The Big Guns: Civil War Siege, Seacoast and Naval Cannon
    Olmstead, Edwin, Wayne E. Stark, and Spencer C. Tucker, Alexandria Bay, NY: Museum Restoration Service, 1997.
Last edited:


Feb 18, 2017
Back in 1860, before the Monitor was designed, during a test firing, a Dahlgren shell gun exploded. To prevent any catastrophic gun bursting within the confined turret on the Monitor, each of the XI-inch Dahlgren guns was restricted to using 15-lb gunpowder charges by the always cautious Commander John Dahlgren.
I'm not sure you can draw a causal link here. The 15 lb charge was the full normal charge of the Dahlgren gun in contemporary tables - they don't even mention a 20 lb charge - and it would seem odd to me for Dahlgren to make a positive announcement to the effect that the permitted charge is exactly the same as before.