XV-inch Dahlgrens: Heavy guns for the monitors

On April 7, 1863, the Navy brought the heaviest guns in service to the entrance to Charleston, South Carolina. The action was not a combat debut for the XV-inch Dahlgren gun.  But the appearance of seven monitors, each sporting one of these massive cannons, was a ominous turn for Confederate defenders.  Not only could the Federals arm their forts with 15-inch Rodman guns, but now their most formidable ships carried guns of the same caliber.  And… the Yankees were mass producing these massive weapons.

Readers will note my persistent use of Roman numerals for the designation of the Navy guns.  Such was the naming convention used by that service for shell guns, and was continued well into the post war period.  So, I’ll use it here for historical accuracy.

I’ve briefly discussed the development of Admiral John Dahlgren’s shell guns.  But these big XV-inch guns were not simply “big brothers” of the VIII-, IX-, X-, and XI-inch shell guns.  Some of the concepts carried over, to be sure.  But as with the Army’s heavy seacoast guns, the large caliber Navy guns used new construction techniques. For sake of brevity, I won’t offer a detailed design history here (I lean on and recommend “The Big Guns” for those seeking detailed references).  The short story is that in 1862 the Navy realized a need for heavier weapons than the XI-inch turret guns of the USS Monitor.

Dahlgren, at the time the Chief of the Bureau of Ordnance, initially offered reservations about casting such large weapons. Recall that Dahlgren’s method for producing shell guns called for a whole casting that included extra metal around the chase.  After controlled cooling, the foundry then turned off the extra metal and reamed out the bore of the shell gun.  Dahlgren also specified a smooth, unadorned exterior, which he believed was an important contribution to the strength of the gun.  But that was a bone of contention.  Dahlgren felt the Army’s 15-inch gun, designed by Thomas J. Rodman, had “stolen” from his design.  Dahlgren argued the “hollow core” casting technique used with the Rodman guns offered little to strengthen the guns.  Instead, the exterior form, which the Army had “borrowed” without attribution, was the true source of strength.  Of course with 150 years of hindsight, we can say “all of the above” and then some contributed to the strength of the guns.  But in 1862, Dahlgren faced a dilemma – stick to his talking points and insist on a XV-inch gun produced strictly to Dahlgren’s method; or admit Rodman’s method had merit and produce a new gun cast hollow and water cooled.

Some have cast Dahlgren as a vain, egocentric type.  But here is a case of the Admiral accepting, and even outright adopting, the points of his opponent.  In other words, it took a big man to make the decision as to how this big gun would be made.  So Dahlgren offered the design of a XV-inch monitor gun that called for Rodman’s method of casting, but also including the extra metal around the chase keeping with Dahlgren’s practice.

Dahlgren also specified a “teet” chamber – a hemispherical bore bottom with a small, sub-caliber extension where the vent connected. And the vent was likewise unconventional, with an “elbow” arrangement.  The path of the vent went straight down from the base of the breech, then turned 90° before entering the “teet” of the chamber from the rear.  After the first few castings, the chamber-vent arrangement came into disfavor.  All surviving weapons and subsequent production reverted to either elongated-hemispherical or ogival chamber profile and conventionally drilled vents.

Two other features of the XV-inch guns were products of the confined monitor turrets.  Instead of a long knob with breeching loop, the big guns used a half-cylinder molding with a piercing for the elevating screw.  On the XV-inch gun this molding was just nine inches long.  In order to work with the Ericsson turrets designed for XI-inch guns, the XV-inch gun muzzles were turned down to an exterior diameter of 21 inches.  And even at that, the gun could only slip into the smokebox and not through the port itself.

Monitor with XV-inch Gun, on the left, unable to pass through gun port *

As completed these guns had a 130 inch bore (from base of chamber to muzzle) and weighed 42,000 pounds.  The 12-inch long trunnions sat on flared rimbases.  Breech moldings included two blocks for lockpieces and a center block for rear sight.  Another block over the trunnions supported the front sight.

USS Catskill with XV-inch (left) and XI-inch Dahlgrens, along with two boat howitzers

The XV-inch Dahlgren also underwent some evolution during the war.  After the first 35 deliveries of original version, production changed to a “long gun” version.  The 42,800 pound (rounded to 43,000 in the model designation) new model featured a sharper taper to the chase.  These guns could extend muzzles through the monitor turret. A redesigned smoke box for the later Tecumseh class allowed for this “long gun” profile.  The oft-reproduced diagram below shows the differences between the fittings and form of the original and the long gun:

Short gun on the left, long gun on the right

All told, another sixty-five of the “long gun” model were produced by 1866.

The Navy ordered twenty more XV-inch guns in the 1870s.  These differed having a hemispherical chamber and a thicker, 25-inch external diameter, muzzle.  This ended production of the XV-inch guns.  Fort Pitt Foundry delivered 100, including the last twenty.  Cyrus Alger and the Seyfert, McManus and Company each delivered ten weapons.  Of this tally of 120 guns produced, there are a handful of survivors.  Several presumably remain underwater within  remains of sunken monitors.  But there are three outside of those wrecks.

Two of those survivors are, appropriately, in Filipstad, Sweden near the tomb of John Ericsson.  Perhaps some of the European readers are more familiar with the location and could provide a better description. But I believe these were secured specifically for use in a memorial there.

The third survivor has a more round-about story.  Several of the XV-inch guns were sold to Peru as armament on ironclads purchased after the war – USS Catawba and USS Oneonta.  Neither ship survived the War of the Pacific, being scuttled.  At some point, someone salvaged part of the Catawba.  And somehow, one of the XV-inch Dahlgrens ended up in… Hong Kong:

The gun is part of the Hong Kong Museum of Coastal Defense.  Certainly far from it’s Civil War origins.


Sources and notes:

Olmstead, Edwin, Wayne E. Stark, and Spencer C. Tucker. The Big Guns: Civil War Siege, Seacoast and Naval Cannon. Alexandria Bay, NY: Museum Restoration Service, 1997.

* In the often reproduced photo of the monitor turret, there is no doubt the gun on the left is a XV-inch Dahlgren.  However, the gun on the right appears to have a muzzle face of a Parrott Rifle. The USS Patapsco was the only monitor engaged on April 7, 1863 with a Parrott.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

12 thoughts on “XV-inch Dahlgrens: Heavy guns for the monitors

  1. Can you share with me any information of the XV shells fired by the Dahlgren? I am particularly interested in the fusing system used. I know they were all fused with Borman fuses, but it is my understanding that the shells were double or triple fused. I am in search of pictures that are close up and I have been unsuccessful so far. Thanks, Harry Uffalussy elk11060@yahoo.com

    1. I don’t have any photos of the XV-inch shells at hand. These usually used Navy water-cap fuses, for obvious reasons. The multiple fuse arrangements were simply separate fuse holes made in the shell at casting.

    2. So they had several fuse holes. Did they fuse the shell with only one fuse? What was the purpose of multiple fuse holes? I saw one of these at the Ft Fisher State Park museum.

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