Care and Feeding of the Coehorn Mortar

Following up on the last post regarding Coehorn mortars, let’s turn next to the servicing of that diminutive piece of artillery.

As mentioned before, a two man crew could handle a Coehorn, but a four man crew was more practical for movement.  However the Instruction for Heavy Artillery published during the war indicated the nominal crew was three – a gunner and two cannoneers.  Implements used with the Coehorn reflected the pieces’ small size and projectile trajectory. A short staff, just 18 inches long, with a sponge doubled as a rammer.   A primer-pouch contained a priming-wire, primers, and lanyard. A gunners-pouch with level and pincers. For aiming, the kit included a quadrant and plummet.  A mallet was provided to drive the fuse plugs.  A small wedge was also included to allow small adjustments to the elevation.  And of course the crew would have the appropriate tools to cut fuses and measure powder for the charge.  The Coehorn didn’t need other implements such as scrapers or worms, since the bore was accessible to all but the shortest arms.

Seven Days 26 May 12 171

Coehorns fired, with a couple of exceptions, almost exclusively shells.  The manuals indicate these were standard 24-pdr shells as used on the larger 24-pdr howitzers or siege guns.  The shell was 5.68 inches in diameter (recall the bore of the mortar was 5.82 inches wide, giving some feeling for the windage at play).  By regulation specifications the empty shell weighed 16.8 pounds.  It took a full pound of powder, but only two ounces was required to blow out the fuse.

Since the standard 24-pdr shell had no “ears” to aid in handling, as see on the larger 8-, 10-, and 13-inch shells, the manuals specified that the 24-pdr shells should be “strapped with tin” to allow fixing of a rope handling loop.

I’ve found two exceptions regarding projectiles used by the Coehorns in American service.  For proofing, the ordnance officers fired a 24 pound solid shot with a 12 ounce charge.  Another exception had more tactical and operational justification.  Coehorns would fire grapeshot (yes, real grapeshot) in order to repel an enemy attacking through a breech in fortification walls.   But that application crosses over to the role of another mortar – the stone mortar – which I’ll save for another post.

Because the Coehorn had a fixed elevation, ranging the projectile required adjustment of the powder charge.  The Ordnance Manual published in 1861 rated the ranges using a small half-ounce up to eight ounces:

The columns, from left to right, reference the charge, shell weight, elevation, and range in yards.  One only needed an ounce of powder to put a 24-pdr shell over the top of a trench into an opponent’s trenches some 68 yards distant.

Tidball’s Manual of Heavy Artillery, published in 1884, offered some different ranges for consideration:

This post-war reduction in performance runs counter to the assumption that gunpowder performance improved during and after the war.  Take each with a caveat.  Personally I’m inclined to accept Tidball’s.  When preparing to fire, the crew consulted the firing tables to determine the appropriate charge.  As mentioned in the previous post, the crew drew the powder from cans using measures.  With open powder laying about, mortar crews were not in the safest place on the battlefield.

To burst the shell, the Coehorn could use the ancient wooden fuse, but for a little better precision, the paper fuse was more widely used. Although technically the Boreman fuse might have worked with mortars, the limited propellant charge and short bores reduced the chance the fuse would catch fire.  So the older methods were better for the mortars.

The paper fuse required a wooden fuse plug.  This was a hollowed out conical wooden plug.  In transit, a cap covered the opening of the fuse.  Before firing, the cap was removed and  the paper fuse pushed into place.

Fuse Plug

The paper wrapper of the fuse sealed a trail of composition.  The burn rate of the composition was known with relative accuracy.  So simply snipping the paper fuse at a given length would provide the desired burn time of the fuse.

Paper Fuse

Gibbon’s Artillerist Manual rated the burn time of the paper fuse in color coded grades:

  • Black – 2 seconds to the inch.
  • Red – 3 seconds to the inch.
  • Green – 4 seconds to the inch.
  • Yellow – 5 seconds to the inch.

Paper fuses were normally two inches long and were cut to a length that would provide the desired time of flight before bursting.  This system, while somewhat crude compared to modern standards, gave a reasonable chance that if properly fired the fuse would induce an explosion of the shell over the target.  It also meant that Coehorn mortar gunners had to at least be able to multiply and divide!

Aiming and pointing of the Coehorn did not differ greatly from its larger cousins.  But with the fixed elevation, the gunner had to put more emphasis on the selection of powder charges.  When firing in repetition against a single target, the Coehorn’s rate of fire was around twenty rounds per hour.  This allowed for adjustments called out from observing the fall of the last round.  If pressed, the rate could pick up.

A four man crew hefting about a 300 pound weapon that required precise measurement of powder and relied upon a paper fuse, with one round every three minutes, doesn’t sound attractive – even for the Civil War.  But the mortar’s value lay in its ability to place explosive projectiles on the other side of obstacles, be they natural or man-made, using horizontal trajectories.  As the war progressed, horizontal fire became more important.  Sort of a response to changes on the battlefield.  Decades to come, horizontal fire would become far more important.

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