A rare mortar, with a familiar form: 10-inch Seacoast Mortar, Model 1861

While the early patterns of 10-inch seacoast mortars pose questions, even down to definitively establishing the year pattern, the model set in the 1861 Ordnance Instructions offers no such ambiguities. The “new” pattern issued at the eve of war conformed to similar designs set at that time. The table below compares the particulars of the mortar designs issued that year.

(See footnote 5 from the post on 13-inch mortars for the sources for this table.)

The 10-inch Model 1861 Seacoast Mortar was, for all intents, a little brother of the 13-inch model. About seven inches shorter and thirteen inches smaller in girth, the 10-inch mortar was five tons lighter. Like other mortars of this series, the 10-inch variety featured a simple exterior, lacking any non functional moldings. The an elongated hemispherical powder chamber measured seven and a half inches in depth. This held a service charge of 10 pounds, although the 12 pound heavy charge is listed as standard by post-war manuals. The seacoast mortar fired a 98 pound shell.

Production of the 10-inch Model 1861 began very late in the first year of the war. Cyrus Alger received a contract for 10-inch seacoast mortars that year. The first five became the last batch of Model 1840 mortars (two of which are seen in that photo from Dutch Gap). After that batch, Alger shifted work to the “new” model, with the requirement for hollow-core, water-cooled casting. But the run of 10-inch mortars came to an abrupt end in July 1862 with only eight accepted. One of those sits in downtown St. Augustine, Florida.

St. Augustine 1 Aug 11 571
10-inch Seacoast Mortar Model 1861, #8

The markings on the muzzle provide the administrative details of this piece.

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Muzzle face and markings on 10-inch Mortar

The markings read, clockwise from the top, “C.A. & Co // Boston // 7640 // 1862 // T.J.R. // No. 8.” Translated this indicates the mortar was the eighth of the type produced by Cyrus Alger in 1862, weighing 7,640 pounds. Thomas J. Rodman inspected the mortar, as well as the other seven produced in the batch. This particular mortar was the last of its kind produced.

The base of the breech has a strip with rectangular pockets to allow the elevating bar to engage. Note the “eye” on the carriage behind the breech.

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Elevating Fixtures on the 10-inch Mortar

However, this carriage also has a post-war elevating mechanism to augment, if not supplant, the original.

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Elevating ratchet on 10-inch Mortar

One other mark appears on the mortar, over the top between the trunnions. The “U.S.” indicates acceptance by the ordnance officer.

St. Augustine 1 Aug 11 577
Acceptance mark on 10-inch Seacoast Mortar

One feature not visible on the mortar at St. Augustine is the lifting loop seen on the top of all model 1861 mortars. There is a flattened spot just above the acceptance mark. My guess is the loop was removed, with the metal milled down, at some point after manufacture. Perhaps to allow use of the updated elevating mechanism.

Another feature not seen here are the vents.  And there’s no evidence the vents were ever drilled out!  Perhaps No.8 was used only for mock-up or handling tests.

Of the eight produced, only the St. Augustine mortar and another in Peoria, Illinois survive. The Peoria mortar retains the lifting loop. These survivors, of that very small production run, are the rarest of the model 1861 mortars.

So why were only eight of these mortars produced? Once again the records are silent in this regard. However, consider the 10-inch mortar fired a projectile less than half the weight of the 13-inch mortar, to a slightly shorter range. The only tactical advantage the 10-inch mortar might offer was lighter weight. Considering the intended employment of these weapons, in seacoast forts, the extra five tons was not a major concern. Instead, planners would desire the heavier projectile to defeat enemy ships.

What few 10-inch Model 1861 Seacoast Mortars were produced remained in the inventory for a short time after the war. By the 1880s, however, the type was not listed in manuals and thus likely removed from the inventory.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

5 thoughts on “A rare mortar, with a familiar form: 10-inch Seacoast Mortar, Model 1861

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