Evolution of the Big Mortars: 13-inch Mortars Model 1861, Part 1

Yesterday I discussed the classifications of mortars used in the Civil War period.  Today I’ll focus on the largest of the “heavy” mortars – the 13-inch Model 1861.  Due to employment, this mortar fits into two of those classifications – seacoast and sea.  So let me provide a brief history of the type and some of the particulars (saving details about the ammunition, operations, and service history for later posts).

The Americans used 13-inch caliber mortars in the Revolutionary War.  Among the siege weapons used at Yorktown in 1781 were 13-inch bronze mortars. Along with many other colonial vintage weapons, these soldiered on through the War of 1812.  However in 1818, the caliber failed the cut when an Ordnance Board established the first true system of American artillery.1 On the other hand the Navy retained, at least for contingencies, the 13-inch mortar.  However after the Barbary Wars, the Navy had little need for mortar armed “bomb” vessels.2

During a revision of the artillery system in 1839, the Army reconsidered the 13-inch caliber for seacoast mortars.  The Ordnance Manual of 1850 included specifications for a 13-inch heavy mortar as part of the patterns of 1841.  The pattern called for a weapon 53 inches long, with a 39 inch deep bore (including chamber), that weighed 11,500 pounds.  Range tables credited this mortar with a maximum of 4325 yards firing a 200 pound shell propelled by a 20 pound charge.3  However only one of these were ever produced.  The Army had the second of two on contract bored out to 12-inch caliber instead (credited with a 4625 yard range).4   The Army did not contract for series production of either caliber, leaving these weapons only as experimental types.

Because the Army had standardized the 13-inch caliber, when the Ordnance Board revised the artillery system for 1861, a new pattern of the type emerged.  Indeed all the Army’s mortars received a pattern update.  The chart below provides the particulars of those weapons from 8-inch to 13-inch.5,6

Just as with other “Model 1861” weapons, the 13-inch mortar offered a more refined design.  Internally, matching the evolutions of Rodman’s heavy guns, an elongated hemisphere replaced the traditional sub-caliber chamber used on previous mortars.  The exterior had only functional moldings which help explain how the weapon was served by the crew.

To start with, unlike larger guns, the mortars didn’t have a cascabel or knob.  So on the top of an otherwise clean barrel, the 13-inch mortars had a lifting lug to aid handling.

McNair 10 Apr 10 184
Lifting Lug on 13-inch Mortar at Fort McNair

At the breech end, where other artillery types had a knob, the Model 1861 series mortars had elevating lugs.

McNair 10 Apr 10 185
Elevating lugs on 13-inch Mortar at Fort McNair

Although design diagrams show lugs inside a raised strip, all surviving examples I’ve seen are exposed as those seen above. In operation, an elevating bar, fed through an “eye” of an elevating post, engaged the lugs.  Much like the system used on the larger Rodman guns.

Fort Moultrie 3 May 2010 603
Elevating post with retaining bolt (Fort Moultrie)

Each 13-inch mortar had two vents.  Why?

McNair 10 Apr 10 180
Vents on 13-inch Mortar at Fort McNair

The dual vent was due to the lack of bouching on these weapons. Per Army specifications, the foundry drilled the left side vent through to the chamber.  But they left the right side vent incomplete stopping one inch from the chamber.  When the left side eroded with use, it was filled with zinc.  At that time the ordnance crews drilled the right side complete.7

To handle the 200 plus pound shells, the carriages included a gibbet (somewhat a double entendre there) or post, as seen on the examples at Charleston’s White Point Gardens.

Charleston 4 May 10 018
13-inch Mortar at White Point Gardens

The markings on 13-inch mortars are at times frustrating.  As per regulation, the registry number, foundry, year of manufacture, weight, and inspector stamps appear on the muzzle.

Fort Moultrie 3 May 2010 605
Weight Stamp on 13-inch Mortar at Fort Moultrie

As you see in the photo above, those were not scaled out to take advantage of the ample muzzle face.  Often these were only lightly stamped and easily eroded away.  Thus even with several quality photos of mortars like the “Dictator,” historians have trouble identifying specific mortars by registry numbers.

Fort Pitt, the sole manufacturer of the 13-inch mortar, placed the foundry tracking number in raised numbers above the right trunnion.  On many survivors, this is the only easy way to identify a specific weapon.

Fort Moultrie 3 May 2010 602
Foundry Number of Fort Moultrie's mortar

Then again, even that raised number disappears from some mortars.

Charleston 4 May 10 013
Missing Foundry Number on a Charleston Mortar

Thankfully, the number appears frequently enough to aid identification and interpretation.

Charleston 4 May 10 008
Foundry number 680 at Charleston

Foundry number 680, today at Charleston’s White Point Gardens, was among the first batches of 13-inch mortars from Fort Pitt.  It received registry number 22 and was credited in January 1862.

Charleston 4 May 10 007
13-inch Seacoast Mortar, Fort Pitt #22

Given the early reception of this particular mortar, there’s a high likelihood it saw some action in the spring of 1862 – Island No. 10, Fort Pillow, New Orleans, Fort Pulaski, or Yorktown.

If only the mortar could speak….



  1. Birkhimer, William, Historical Sketch of the Organization, Administration, Material and Tactics of the Artillery, United States Army  (Washington:  James J. Chapman, 1884), Pages 275-279.
  2. Tucker, Spencer, Arming the Fleet: U.S. Navy Ordnance in the Muzzle-loading Era (Annapolis: Naval Institute Press, 1989), pages 107-109, 143.
  3. U.S. War Department, The ordnance manual for the use of officers of the United States Army (Washington: Gideon & Company, 1850), pages 7 and 364.
  4. 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), Appendix C163, page 261.
  5. Data for the chart assembled from:
    1. U.S. War Department, The ordnance manual for the use of officers of the United States Army (Philadelphia: J.B. Lippincott & Company, 1862), page 18.
    2. Tidball, John C., Manual of Heavy Artillery Service (Washington: J.J. Chapman, 1884), pages 145-166.
    3. Olmstead, et. al., Appendices C118, C146, C257, and C164; Pages 239,  255, 257, and 261.
  6. There is a discrepancy seen between the length of trunnions specified in the Ordnance Manual and those on surviving examples.  Surviving 8- and 10-inch mortars have 3.25 inch lengths, while the 13-inch have 4.5 inch trunnions.
  7. Tidball, page 36.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

16 thoughts on “Evolution of the Big Mortars: 13-inch Mortars Model 1861, Part 1

  1. Thanks for helping to understand this aspect of operations at Island No. 10. I don’t think I read it stated explicitly, but would it be correct to infer that these were the guns put aboard boats as part of Foote’s flotilla?

    I’ve been reading (and just posted) the letters of a fuse-cutter aboard one of those boats, and he described mortars that I believe match the description you gave:

  2. How many of the 13-inch mortars are at Fort McNair? I’m working on a project to document the location of all of the existing 13-inch mortars.

    1. At one time there were three. I know one was relocated to New York. And I have not been back at Fort McNair for many years.

    2. Do you know where it is in New York? Supposedly there are 27 in existence today. I think I have found 26.

  3. There was a 13″ tube at the Watervliet Arsenal Museum but they closed years ago. It was No. 992, 17200 lbs JMB by my notes of the time.
    A complete piece sat in front of the Saratoga Military Museum, No. 978,17196 lbs. I have photos somewhere. Hopefully it’s safe from the left wing vandals.
    I’ve been building a 3/16 scale model in steel. The tube, trunnions, and lifting lug are done and need welding. I have the carriage drawings and need to go steel shopping when this shutdown breaks
    There were two 10″ model 1871’s in a park in Cohoes! NY as well.

    1. Sorry for the typo. Should read two 10″ model 1861’s in Cohoes park. I’ll have to take a drive and check if they are still there. Maybe collect some details. Can photos be attached or sent somewhere?

  4. What is the length of the gimlet for the 13 in mortar and who made them?
    I have one I found at Petersburg Va. at a yard sale many years ago. Thought it was a woodworking tool.
    The wooden handle is marked H. H. M. CO on one side and the other side the letter S in a elongated diamond and the number 8 to the right of the diamond. 23 1/4 inches long. Shaft is 10mm Dia.
    Any information would be helpful.
    Thanks W.K. H.

  5. How was the 13″ motor ignited? The touch hole is about 15″ deep. My drawing is not clear but appears the diameter is 0.2″ surrounded by a semi cylindrical hollow pan. It seems the length of the hole would be a challenge to a friction primer. The hole diameter seems small to fill with a powder trail touched off with a match and the time delay excessive if ignited with ordinary cannon fuze. I’ve seen the video of the reproduction mortars being fired but it wasn’t completely clear how it was done. If the gimlet was 10mm diameter I guess that would open other possibilities.

  6. Thank you for your informative site. I’m researching how shells are specifically loaded into the mortar cannon using the gibbet. How does the gibbet work?

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