Dahlgren Boat Howitzers: Service Charges

Let me start by sending thanks to John Morris, fellow member of the Company of Military Historians, for clarifying a point about the Dahlgren Boat Howitzers.   In the comments on Monday’s piece on the small howitzer, John pointed out that even though the pieces had the same sized chamber, the howitzers had different service charges.

Back last May when discussing the ammunition used by the howitzers, I wrote:

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.

I’m still searching for a specific statement from a source such as the Ordnance Instructions that would detail the differences in sabot sizes.  But John offered a couple of pages he found in the John A. Dahlgren papers in the Library of Congress.  (The pages, as well as a link to pictures of other papers John referenced, are posted on the Company of Military Historians forum.)  The first page appears to be a roll-up of test results.  I’ve attempted to recreate the table from the page in this spreadsheet:

(Click to Enlarge)

I’ve used strike through to indicate where Dahlgren crossed out figures in the original, and given my best guess as to the underlying numbers.  Please don’t accept my guess as-is, go to the photo or original and look it over. Dahlgren tested the howitzer system with at least six different sizes of the howitzer ranging in weight from 276 to 768 pounds for the 12-pdr caliber (as noted by the 9 1/2 pound projectile), and one 1300 pound 24-pdr model.

As the title over the table indicated, he used the charge of the 12-pdr Heavy Howitzer as a basis for testing.  That matches the fifth row down, showing a weight of 768 pounds for the howitzer, using a 20 ounce powder charge (1-1/4 pounds).  The second line offers a weight (324 pounds) close to the standard Small Howitzer, for which Dahlgren noted an 8.4 ounce (or half pound) charge.  The third line matches the Light Howitzer (443 pounds) with an 11.4 ounces (just under 3/4ths a pound) of powder.

The second page offers a bit more complex table titled “Proportional Charges.”  Columns offered figures comparing weights of cannon to projectile and projectile to powder charge.  Here’s my interpretation of that table:

This table referenced two separate experiments using different baselines.  Notes at the bottom of the table stated the lightest howitzer (piece A) could not handle an 8 ounce charge.  Perhaps the upper set of results was found unsatisfactory.  The second remark indicates the 443 pound howitzer, used as the baseline for those tests, performed satisfactorily with 10 ounces of powder.

The 443 pound howitzer matched the production Light Howitzer in weight and caliber.  The 10 ounce charge translated to 0.624 pounds, which was very close to the charge specified later in the Ordnance Instructions. Also for those experiments, the 324 pound 8.6 caliber howitzer (closest to the production Small Howitzer) used a 7.4 ounce or 0.46 pound charge.  The 768 pound  12 caliber howitzer, which matched up to the production Heavy Howitzer, used a 17.6 ounce charge (just over 1 pound).

With this in mind, considering the charges specified in the Ordnance Instructions more authoritative, I’ve updated the Boat Howitzer charge provided earlier:

Comparison of Navy and Army Howitzers

As specified in the Ordnance Instructions the Light Howitzer used a 0.625 pound charge and the Heavy Howitzer used a full pound.  Dahlgren’s notes the Small Howitzer used nearly a half pound charge.  However, Dahlgren’s experiments noted a 30 ounce charge for the 24-pdr Howitzer, some two ounces short of the two pounds indicated in the Ordnance Instructions.  A small, but notable, discrepancy.

So while clarifying somewhat the powder charge weight, the figures leave questions about the sabot sizes open.  One would expect handling instructions to indicate some means of separating the different ammunition sets, as light and heavy howitzers served side-by-side in some situations.  Who knows, maybe there is a table for “howitzer sabot sizes” in Dahlgren’s papers too!

Certainly all fine points which need clarification.  Again, let me thank John Morris for pointing out the error and offering up the fruits of his research.  This is exactly the collaborative scenario I had in mind when starting this blog.  Further I must echo John’s observation of the need to digitize the Dahlgren papers.

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

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