15-inch Rodman Prototype: Trials of the New Gun

Discussing the prototype 15-inch Rodman gun, I left off with the 25-ton iron form arriving at Fort Monroe for trials.  Now it’s time to talk about shootin’ that big gun.

Thus far in the discussion of Rodman guns, my articles focused on “experiments” within the scientific sense.  But prototyping takes on a different focus.  To pass trials, a prototype undergoes tests to ensure it meets requirements and expectations.  Beyond that, prototype testing ensures the system is a practical fit in the desired role.  The focus extends more to “will it work the way we intend to use it?”

Instead of placing the gun on very restrained test mount, Captain Alexander B. Dyer had a wrought iron carriage built at Fort Monroe Arsenal for the prototype.  Generally, the carriage matched that of the Columbiad barbette center pintle design – just bigger all around to fit the bigger gun.  Recall too the prototype 15-inch Rodman retained a 1200 pound preponderance at the breech along with the ratchet elevating system.

For trials in 1860, the crew at Fort Monroe placed the gun on a concrete and cut stone platform along the beach.  As with the standard service mounting in a fort, the trials mount had concentric rings to allow the carriage to traverse.  The mount allowed just over 28° elevation.

Normally any new gun undergoes proofing to verify it could withstand service charges.  However, since the 15-inch was an entirely new design proofing involved testing to see just how much load the gun could reasonably withstand.  Proofing with 0.6-inch grained powder started with 25 pound charges propelling a 330 pound shell.  After five shots this increased to 30 pounds with a 310 pound shell, which the gun fired five times.  At subsequent intervals, the powder charge increased by five pounds until reaching 40 pounds.  Rodman also fabricated several “perforated cake” charges for these tests (and those deserve treatment in a separate post). During these proofs, the greatest range was with a 40 pound charge at 28° elevation reaching 5730 yards.

After forty fires during the proof, a board of officers, representing the engineers, ordnance corps, and artillery, determined the gun was ready for evaluation.  Just to drop names, the board included General Joseph Totten, Major John Barnard, Captain Horatio Wright, Major John Symington, Captain Dyer, Captain Josiah Gorgas,  Colonel Justin Dimick, Major Robert Anderson, Captain J. H. Carlisle, and Lieutenant G. Tallmadge.   For the board, the gun fired forty-nine more times at an elevation of 6°, with a 35 pound charge and a 317 pound shell.  Ranges varied between 1873 and 2017 yards.  During these fires, the upper carriage recoiled between 68 and 77 inches.

Rodman attempted to measure muzzle velocity with a Navez device, but results were erratic.  Instead, the crew fired a few rounds at a short range target (885 feet, because “short range” is relative) and estimated velocity from the time of flight. With 35 pounds of powder, observers figured velocity at 1328 feet/second.  Using 50 pounds of perforated cake, velocity dropped to 1282 feet/second.  Interestingly, these tests used rope grommets instead of sabots.

After a few more ranged shots, the trials got down to some fun trials – ricochets on the water!  The board wanted to know the behavior of the large projectile when used in one of the favored anti-ship techniques of the time.  With elevations ranging between horizontal and 5°, using 40 pound charges and 318 pound shells, five shots fired over the open waters around Fort Monroe.  However due to rough waters, these tests were far from conclusive.  Yet the conclusion was higher elevations yielded better ricochet patterns.

At the end of all these fires, the board examined the bore of the gun.  Using a star gauge, the officers found no enlargements in the bore.  For all practical purposes, the gun stood up well to use.  However, the carriage had sustained some damage.  The original traversing wheels cracked under the strain of firing, and were replaced by a stronger set.

The trials also considered how the crew operated the gun.  Obviously with a larger projectile than any weapon then in service, handling was a consideration.  Rodman’s report noted that three men could load the gun – two carrying the projectile and one pushing it into the muzzle with the rammer.  However, a team of five was preferred.  Running the gun into battery required seven men, and an eighth used to ease the work.  Although two men could traverse the gun, the preferred number was four.

For a shot at horizontal, time required for servicing and loading the piece fell to just over a minute after practice.  Running the piece out at maximum elevation required between three and four minutes.  The crew could traverse the gun at a rate of roughly 45° per minute.  Not bad for the “first run” of the gun.  Even in the 1890s with crews intimately familiar with the carriage and gun, officers planned for one shot every four or five minutes in combat.

At the end of these trials the board gave the gun positive marks.  The gun survived the trials and performed well.  Despite the size, the gun was just as well handled as the contemporary Columbiads.  The officers believed, “… the introduction of guns of much larger caliber than any now in the service, is desirable and practical.”  They went on to say, “the efficiency of our present armament for harbor defense would be improved by the addition of a judicious portion of guns of this class.”  However, the board did express caution about the endurance of the gun, asking for additional fires.

Starting in December 1860, Dyer supervised additional tests on the 15-inch prototype.  All told, he exceeded 500 fires.  At the end of which, he reported no measurable wear in the bore.  Vent erosion was less than normal for a gun at that endurance.

While Dyer fired this big cannon, the country was falling apart.  I find interesting how members of the board were soon swept up in events.  Anderson, of course, would move from this trial to his next post at Fort Sumter.  Within a few months, Gorgas resigned his commission and became the Confederate chief of ordnance.  But Rodman left the trials and returned to Pittsburgh.  There he worked on a 12-inch rifled gun constructed in the same manner as the 15-inch gun.

One final note, perhaps bringing this story from one about “cannons” to one about people – describing the gun crew at one point, Rodman indicated the composition to be “one sergeant and six negroes.”  Such raises several questions.  Why wasn’t a detail of artillerymen used?  Were these freedmen or slaves?  Were these men trained in artillery handling before these trials?  Regardless, I find an interesting thread here.  On the eve of the Civil War the Army used African-American labor to test its most important new weapon.  And that very gun later acquired the nickname “The Lincoln Gun.”


Thomas J. Rodman’s report of the 1860 prototype trials appears in Reports of Experiments on the Properties of Metals for Cannon, and the Qualities of Cannon Powder; with an Account of the Fabrication and Trial of a 15-inch Gun (Boston: Charles H. Crosby, 1861), pages  281-293.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

4 thoughts on “15-inch Rodman Prototype: Trials of the New Gun

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