In earlier posts I discussed the design history, carriages, and variants of the boat howitzer family. Now I turn to discuss the ammunition used by the boat howitzers. While generally similar to projectiles used in land service, the naval ordnance offered a few notable differences. In A System of Boat Armament (1852) , John A. Dahlgren recommended shell, shrapnel (case shot), and canister. Grape shot, while mentioned, was not considered standard.
Dimensions for the Navy’s 12-pdr shells matched almost perfectly to Army shells of the same caliber. The Navy’s 24-pdr shell, with a 5.72 inch diameter, had less windage compared to the Army’s 5.68 inch diameter shell. But for reasons not noted, the Navy’s projectiles were listed as slightly heavier than the Army’s. Otherwise the specifications were the same. (See page 34-35 of the Army Ordnance Manual of 1862 for details.)
Similarly, the Navy’s specifications for shrapnel case appeared similar to the Army’s comparable projectile, called spherical case shot. As with the shells, the Navy’s 24-pdr retained the slightly larger diameter. While in 1852, the Army’s specifications closely matched the Navy’s, by 1862 the Army opted for Bormann fuses requiring different diameters for the fuse-hole. As result, empty Army case shot was slightly heavier. Both Navy and Army case were filled with a mix of lead balls and sulphur. The Army specified 82 lead 0.79 caliber musket balls for a 12-pdr case shot, and 175 for a corresponding 24-pdr. Dahlgren specified 80 lead 0.65 caliber musket balls for his 12-pdr shrapnel and 175 for the 24-pdr.
Dahlgren, a strong proponent for shells, also championed the use of shrapnel. In addition to a detailed discussion of the history of shrapnel, Dahlgren offered the results of tests performed to determine the dispersion of fragments. Test indicated the optimum bursting height for 12-pdr case was 15 to 20 feet above and 50 to 75 yards in front of the target.
Navy canister differed in construction with the Army’s. Navy 12-pdr canister consisted of 39 1-inch diameter cast iron balls stacked inside a tin case. The 24-pdr used the same number of 1.3-inch balls. The Army opted for 48 1.17-inch balls for the 12-pdr and the same number of 1.35-inch balls in the 24-pdr howitzer’s canister. The Navy’s 12-pdr canister weighed 7.75 pounds complete. The 24-pdr was 14.55 pounds when dressed out. With more and larger sub-projectiles the Army’s weighed 10.8 pounds and 21.25 pounds respectively. Of interest, the Army’s 12-pdr Mountain Howitzer fired a 11.2 pound canister with 148 0.69-inch led musket balls. (See the Navy Ordnance Instructions of 1866 and The Army Ordnance Manual of 1862 for more details) Tactically, the differences between Navy and Army canister was negligible, but the lesser weight of each round was a mobility factor.
The Navy’s ordnance manuals of the Civil War era did not identify grape-shot for the boat howitzer calibers. I would assume from the omission that Dahlgren only mentioned grape-shot to cover all options. For arguments’ sake, the Army’s 1862 Ordnance Manual listed particulars for both 12- and 24-pdr calibers. Grape-shot always took the form of a stack of nine cast iron balls – three rows of three each. 12-pdrs used 1.14 pound, 2.06-inch diameter cast iron balls. The 24-pdr grape-shot used 2.4 pound, 2.64-inch diameter. Overall grape fell into disfavor as canister became practical and popular. While offering some range advantage over canister, grape was best used against rigging. And of course with the advent of steam power, rigging was less critical to a ship’s operations.
For the 12-pdr and 20-pdr rifled howitzers, the Navy issued Hotchkiss, Schenkel, and Dahlgren’s own projectiles. The listed weight of a 12-pdr shell was 11 pounds, while that of a 20-pdr was 18 pounds. The Navy Ordnance Instructions do not mention bolt or canister projectiles for these cannon.
One discrepancy arises when comparing service charges listed in the Ordnance Instructions against Dahlgren’s notes and physical dimensions of the boat howitzers. The instructions note two pound charges for the 24-pdr smoothbore and 20-pdr rifle. The 12-pdr heavy and 12-pdr rifle used one pound service charges. But, although the 12-pdr medium and the 12-pdr small featured the same chamber dimensions, the Ordnance Instructions list 0.625 pound service charges. Such loading would either require a special sabot, which is not noted in any instructions, or leave an unacceptable air gap between the charge and projectile. (UPDATE: Dahlgren Boat Howitzer service charges)
The Navy transported the boat howitzer projectiles in boxes with either nine projectiles. The 24-pdr boxes measured roughly 22 x 21 x 14 inches, weighing 270 pounds loaded with shells. And the 12-pdr, measuring 19 x 18 x 12 inches, weighed 140 pounds. Often the boxes were issued in sets. As time progressed, the field carriages were modified with braces, with cleats, to mount these boxes beside the cannon. This of course added 300 to 500 pounds to the weight in action.
In summary, the Navy used projectiles similar to the Army’s in most regards, but with slight variations in weight and composition. Perhaps, if I were carrying about canister rounds in a haversack, I would appreciate the two-and-a-half pound difference between 12-pdr rounds!