How to Raise a 25-Ton Cannon: Elevation System for Rodman Guns

Yesterday I covered the mechanical operation of the elevating system on pre-Civil War columbiad gun carriages.  This type of elevating gear worked for columbiads, seacoast howitzers, and even a few Navy shell guns that entered service from the 1840s to the close of the 1850s.  Almost all Confederate columbiads used the system.  Even the first batches of Rodman guns used the system.

However the weight of the larger Rodman guns exceeded the practical limits of the Columbiad’s system. The chief problem was the preponderance required to keep the weighted on top of the elevating gear (specifically the pawl).  While this ensured the elevation remained true during firing, during handling the preponderance made the gun crew’s work difficult if not outright dangerous.  The 15-inch Rodmans entering service in 1861 exceeded the limits of what a reasonable gun crew could manage – 1,200 pounds.   So an alternative system came into use.

To my knowledge, no documentation details the experiments and design of this modified elevation system.  The Rodman guns arrived on the scene about the same time as wrought iron carriages for seacoast guns.  However the early diagrams of Rodman guns on iron carriages seems to show the older style elevating system.

At some point that elevating gear gave way to this:

Fort Foote 1 Mar 08 138
Breech of 15-inch Rodman at Fort Foote

The post, sometimes referred to as a ratchet post, replaced the elevating screw, elevating box, and pawl of the earlier arrangement.  Instead, the crew simply inserted the elevating bar directly into the post and against the sockets on the breech face.  With that the crew could lever the gun’s breech up or down.  Here’s how the manual of service described the action:

No. 4 [crewman] mounts upon the chassis and, embarring through the ratchet-post with the elevating-bar, raises or lowers the breech as directed by the gunner.1

Very simple.  Angle the gun appropriately.  Just as seen in this posed photo from Battery Rodgers, Alexandria, Virginia.

The bar had a squared off head to fit into the sockets on the gun’s breech.

Fort Trip 26 Mar 11 107
Close up of Sockets on 15-inch Rodman

The post sat just a few inches from the breech face, but that was all the room needed.

Fort Trip 26 Mar 11 129
Breech Clearance

What did Archimedes say about just needing a place to stand?  And notice the post does not touch the breech.  And there is no elevating screw or other point supporting the breech.

Fort Trip 26 Mar 11 130
Inside of the Post

Inside the post are several slots with the open ends towards the breech.  Presumably these engaged with pins or other fittings on the elevating bar.  The orientation would allow the bar to remain firmly seated while the crew worked.

Now how exactly was this an improvement on the older ratchet-pawl system?  Well take note of the 15-inch gun in profile.  How many points does the gun itself touch the carriage?

Fort Trip 26 Mar 11 140
15-inch Rodman - Alger #1

Give yourself a gold star if you said two points.  Only the trunnions touch the carriage (recall the spacing of the post above).  So even after 150 years, this 25 ton gun balances on the trunnions.  After the 15-inch Rodman prototype, and small batches of 8- and 10-inch models, all Rodman guns were cast “without preponderance” by moving the trunnions just a few inches back.  So instead of working against 1,200 pounds of preponderance while elevating the piece, the No. 4 man needed only to press the lever up or down.

But with that careful balance, I’ve often wondered just loading the piece would serve to drop the gun off balance.  The addition of a 450 pound solid shot might create a giant, dangerous teeter-totter.  Although the manuals do not indicate such, retaining the elevating bar in place during firing might prevent the fall of the gun.  The orientation of the slots in the post would allow such, by not allowing the bar to fly loose when the gun fired.  And at least one photo shows the bar remaining in place with the gun in position, this one again credited to Battery Rodgers.

Another photo captioned as Battery Rodgers shows a gun brought back to a stowed position.  If you look close, laying behind the elevating post is the elevating bar.

So, for these big Rodmans, the crew had a much easier job with elevation as compared to the older Columbiads.  Still, ease of use must be taken in context.  The rate of fire for a well drilled crew on a 15-inch Rodman was one round every five minutes.2 Even without preponderance on the breech, crews moved slowly around these heavy iron monsters.

————————————-

Notes:

  1. John C. Tidball, Manual of Heavy Artillery Service: For the Use of the Army and Militia of the United States (Washington: James J. Chapman, 1891),   Page 114.
  2. Ibid, Page 127.
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Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

6 thoughts on “How to Raise a 25-Ton Cannon: Elevation System for Rodman Guns

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