Little Mortars with a Big Job: The American Coehorn Mortar

Several weeks back I briefly discussed the evolution of the very light Coehorn mortars.  The original concept called for a light mortar for use in the narrow siege line trenches. But the small size of the type allowed gunners to operate with the maneuvering field armies, yet provide the high angle fire against fortifications.  Even though their caliber was too small for serious siege work against well fortified positions, the Coehorn was an attractive addition to a commander who expected to face only light, open fortifications.

Again, it is my opinion that the American Army entered the 19th century with a quantity of Coehorns (be they named “Royals” or “Coehorn” at that time).  As with other mortar designs, on hand quantities must have argued against all but limited production of the type. For a point of reference, here’s a reproduction (?) Coehorn mortar of the type used by the British during the colonial era, and the type inherited by the Americans:

Mortar (Photo credit: Jeff Kubina)

Furthermore, the Coehorn mortar was most needed for an army on the offensive in the traditional sense. The Americans didn’t see that as an important role (although at the operational level at least, the Army had operated offensively during the War of 1812). Operations on the frontier rarely called for artillery, much less mortars.

At least one surviving American 24-pdr iron Coehorn from the first quarter of the 19th century exists today.  Reported among the Petersburg National Battlefield collection, I have not examined it up close so I’ll save that for another day.  But I mention it here because of the stampings and manufacture date.  The register of surviving pieces indicates its registry number is 13, produced in 1827, and is marked “U.S. Arsenal, Washington.”  If correctly reported, such indicates limited, and non-standardized, production during the years following the War of 1812.

By the late 1830s, the Ordnance Department sought to standardize nearly everything.  Among those designs was the Model 1838 Coehorn Mortar.

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Coehorn Model 1838 at Cold Harbor

The mortar pictured above is on display at the Cold Harbor visitor center, outside Richmond.  Fitting, as the mortars began playing an important role during the Overland Campaign of 1864.

In profile, the Model 1838 resembled British models, minus the handles of course.  The proportions are not far off those described in Muller’s manual from the 1760s.  The Ordnance Manual of 1841 specified the 24-pdr Coehorn weighed 160 pounds and was 15.32 inches overall.  As you can see from the overhead view, the mortar’s muzzle had a flat lip, reinforced by a generous ring (nearly an inch thick and one inch wide).  Over the mouth of the chamber, a four inch wide reinforcing band provided additional strength.

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Profile view of the Coehorn

Looking at the muzzle face, most of the stampings appear in compliance with the Ordnance Instructions of 1861 – “1864 // A.M.Co.  // R.M.H. // 160 // No. 154”.  Translation – this mortar was produced in 1864 by Ames Manufacturing Company (Springfield, Massachusetts), inspected by Richard M. Hill, weighing 160 pounds, and is registry number 154.

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Muzzle markings on mortar

The depth of the bore, not including the chamber, was 8.82 inches. A further look down the bore, taking advantage of the mortar’s shallow throat, shows the powder chamber.

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Powder chamber of Coehorn

The chamber formed a conical “cup” at the bottom of the bore.  At the top it measured 3 inches in diameter.  At the bottom, according to regulations, the diameter was 2 inches, although some sources say 1.75 inches.  The depth of the chamber was 4.25 inches.

Looking outside to the rimbases, there is one other marking that often appears on Coehorns – the foundry number.  For this piece that is 263.

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Rimbase number on Coehorn

The low mounted trunnions attached to the breech of the piece.  The trunnion molding actually formed a solid cylinder across the breech, by regulation 2.75 inches in diameter.  The overall length of that cylinder was 12.5 inches, standing out 2.5 inches on either side of the mortar.

Looking back over the reinforce band, this particular mortar exhibits the traces of painted markings.  These appear to be post-war arsenal stenciling, and not from wartime service.

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Stenciling on Coehorn

I didn’t capture a close up view, but at the base of the breech was a single vent hole.

Production of these mortars began in 1839 with a lone pattern weapon ordered from Ames.  After its delivery in 1840, the Army ordered no more Coehorns until 1855, when Ames produced a batch of 30.  That was the entire pre-war production run for Army orders.  Of course that does not rule out state, militia, or private orders, but no evidence of such has surfaced.

Only in 1862 did the Army resume requests for Coehorn mortars.  Ames delivered about 200 more between April 1862 and the end of the war.  Cyrus Alger contributed 47 more in a production run during 1862-63.  Some have mentioned a spike in production to field experiences at Vicksburg.  I would simply point out that the Army had already received 36 Coehorns on wartime orders before the first spadeful of siege lines was turned at Vicksburg.  Another fifty were ordered before the first field reports were filed.  Ames later received orders for three batches of fifty each starting the spring of 1864 running through January 1865.  Given those production statistics, I’d argue this was more so a gradual procurement strategy as opposed to a reaction to the immediate wartime situation.  (My Artillery History revisionist moment of the week I guess…)

Having “walked around” the little Coehorn mortar, I’ll turn next to the operational at tactical particulars – how it was mounted, transported, and operated in the field.

6 thoughts on “Little Mortars with a Big Job: The American Coehorn Mortar

  1. Craig, great stuff! You may cover this in the operational post, but what constitutes “light, open fortifications”? I envision quick breastworks constructed from trees and rocks, like those at Gettysburg.

    • Perhaps I’m using a non-authentic term, but yes I’d consider the field breastworks as light, open fortifications. But when writing this, I had in mind the prepared field fortifications such as used at the Wilderness and beyond.

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