FWIW, I found an interesting book while poking around for more information on the subject. It claims that until Rodman came up with cooling the bore during casting, it was impossible to make an effective solid-cast gun over 10" bore. Anyone know how the big mortars were cast?
The heavy, 1861 13" Seacoast Mortar was made using the same "Water Core" Rodman process as many other seacoast guns. I saw a photo of 14 or 15 of them on the foundry dock in South Boston once. Wish I had saved that one. Approx 1/2 of the heavy Parrott rifles were also made with this same process. The one we have in Denver, Colo. at City Park is one of those cast per Rodman's process. It has WATER CORE stamped on the face of the muzzle in capital letters as does our re-creation of that piece.
Presumably the max mortar charges were similar to those used in direct-fire guns of the time. Do, say 10" guns of the period have roughly the same thickness of wall for the first few calibers as a 10" sea-coast mortar?
That's an excellent question. Offhand we could not answer that with certainty, so we looked it up in the Big Red Book, The Big Guns
by Olmstead, Stark and Tucker. We found that the 10.0" Mortar that you mentioned has a dia. of 27.5", bore of 10.0" and walls of 8.75". Don't get all twitterpated, folks. Remember that mortars have chambers and that the largest chamber dia. of 7.5" is LESS than this mortar's wall thk. of 8.75". The big seacoast guns and howitzers do even better. All of these have 10.0" bore dias.: Seacoast Howitzer, Pattern 1839, wall thk. of 11.5". How about the Columbiad, Pattern 1844, wall thk. 10.5", or the New Columbiad of 1857, wall thk. of 10.5". As you would expect, the 10" Rodman Gun, Pattern 1861 has a wall thickness greater than the bore dia., it's 11.0" as is the wall thickness of the XI Dahlgren Shell Gun. Even this big Navy gun satisfies the 1:1:1 Rule if you consider the 7.0" chamber on the the 10" Navy Shell Gun of 86 cwt. It's wall thk. is only 7.75. Big guns=Thick Cast Iron.
Tracy and Mike Yes, we occasionally work on Saturdays.