As I continue on the thread discussing Civil War mortars, the next in the queue are the lightweights – the Coehorns. This class of mortars bears the name of Dutch Baron Menno van Coehoorn, a contemporary and adversary to the French Sébastien Le Prestre de Vauban, from the 17th century.
Although better known in his times for refinement of fortifications, the Dutch baron also devised better siege techniques. One particular tactical problem which Coehoorn attempted to solve was the mobility of mortars. Often a besieging force needed high angle fire, from mortars, on targets only ranged from the advance parallel trenches. The heavy mortars often used in the siege were simply too heavy to man-handle into those forward positions. Coehoorn devised a light bronze mortar, of 12-pdr caliber, which a two- or four-man crew could handle with relative ease. Other European armies followed suit with similar weapons, calling them “pack mortars” or simply light mortars. Regardless of the name, the application was the same – a lightweight weapon, maneuvered to forward positions by a small team of men, and used to deliver shells at a high angle on enemy positions.
British gunmaker John Muller provided specifications for two calibers of light mortars in his Treatise of Artillery (various editions from the mid-1700s on). Muller described a 5.8 inch caliber, roughly 24-pdr size, mortar, measuring 16.5 inches long and weighing about 150 pounds. He also included a 12-pdr caliber mortar with a 4.6 inch bore, measuring 13.5 inches and weighing around 80 pounds. The British convention gave these two calibers the names “Royal” and “Coehorn” respectively. However the inventor’s name was most closely linked with the weapon class. Over time the exact measurements varied, some with different bore sizes than Muller’s specifications, but the general application remained the same. These two basic types remained in British service well past the Napoleonic era to the mid-19th century.
Of course these light mortars were no strangers to the Americans. General Oglethorpe used them against Spanish defenders at St. Augustine. General Braddock carried some on his ill-fated campaign. And there were plenty inherited by the Continental Army. General Knox reported having six 5 ½ mortars in the siege train taken to Yorktown. But direct references to the light mortars fall off after independence.
It is my opinion that the young American Army continued to maintain numbers of the light mortars in the inventory. However, given the defensive posture along the borders and the mortar’s limited value on the frontier, there was little need for new weapons of this class. The ordnance patterns established in 1819 lacked any light mortars. Not until the 1830s did the Army return to the Coehorns. Patterns established in 1838 included a 24-pdr Coehorn mortar.
Much like the siege mortars of the same vintage, the Coehorn pattern and its bed borrowed heavily from British designs.
I will discuss the Model 1838 in more detail in later posts. But, let me close this introduction by splitting hairs a bit on the naming convention. Many cannon historians note the change of naming convention – from “Royal” to “Coehorn” – as evidence of American disdain for the monarchies of Europe. This may have weight, but there is no firmly documented trail on the matter. Eventually, even the British changed the naming convention. No doubt except for the Ministry for Redundant Nomenclature, which would have preferred the “24-pdr Royal Mortar from the Royal Mortar Foundry.”