The Three Inch Stokes Mortar
Excerpted from U.S. Infantry Weapons of the First World War by Bruce N. Canfield

.

Stokes Trench Mortar Deployed by American Troops

It quickly became clear that some type of weapon was needed to provide artillery-like fire support to the infantry. While rifle grenades were useful for small-scale work, they possessed insufficient range and power to serve such purposes. A weapon that was fully man-transportable yet could fire reasonably powerful shells at targets beyond the range of rifle grenades was badly needed. The Stokes trench mortar was developed for precisely this purpose.

The Inventor

The Stokes trench mortar was developed by an Englishman, Sir Wilfred Scott-Stokes (1860—1927) who was the managing director of a mechanical engineering firm, Ransome & Rapier of Ipswich, England. Although Stokes did not have a military background, he quickly grasped the need for a "portable gun" soon after war erupted in Europe. Stokes correctly reasoned that such a weapon would be valuable for reducing the deadly machine gun nests that were beginning to wreak much havoc on the Western Front. Stokes had a working prototype of his trench mortar ready for testing in December of 1914, barely four months after the war started.

Like many notable inventions, Stokes' mortar was ingenious in its simplicity. Stokes' mechanical engineering background enabled him to design a weapon and ammunition that was easy to manufacture and use. The weapon consisted of a smooth bore tube (barrel) 3 inches in diameter with a metal end cap that fitted into a flat base plate. The 51-inch barrel was supported by a bipod that could be adjusted for range. The weapon was broken down into three sections for easy transport — the barrel (tube) which weighed 43 pounds, the base plate (28 pounds) and bipod (37 pounds) for a total of 108 pounds.

The mortar fired a high-explosive shell weighing about 11 pounds. The maximum range was approximately 800 yards. Due to exploding fragments, the minimum safe range was about 100 yards. The range was adjusted by varying the amount of explosive propellant attached to each shell (or "bomb") and adjusting the tube's angle of elevation. The mortar was sighted by the simple expedient of lining it up with the target by means of a white line painted on the tube.

The Stokes mortar shell was loaded by dropping a shell down the tube barrel. The shell fired by means of a shotgun-like blank cartridge that exploded when it hit a fixed firing pin attached to the base cap. The exploding cartridge ignited propellant rings attached to the shell. The ammunition used with the weapon was variously termed "shell," "round" or "bomb" in contemporary military manuals.

The Stokes mortar was soon tested by the British Army and was deemed "...a brilliant concept." However, it was determined that the war currently raging in Europe would soon be over and there would not be time to produce the new weapon before the fighting ended! However, these pundits were soon proven dreadfully wrong and in the spring of 1915 the "3-inch Stokes Trench Mortar, Mark I" was adopted by the British Army. However, the production of other types of munitions was perceived as having higher priority, and manufacture of the Stokes mortars and shells was delayed. Eventually, sufficient numbers of these weapons were made and they began to see front-line service by late 1916. The first recorded use of Stokes mortars by the British was near Loos, France, in September where they proved to be very valuable weapons despite the limited numbers available. By this time, however, the British Army had begun to reduce its emphasis on rapidly moving infantry advances in favor of holding static trench positions. This strategy reduced the effectiveness of the highly mobile Stokes mortar and spurred the development of larger and more powerful, but relatively immobile, mortars. Even though the front-line "Tommies" liked the Stokes mortar, the British Army considered it a low-priority weapon and production remained limited.

Such was the situation when the United States entered the war. As we have discussed, the Americans were determined not to become mired in protracted static trench warfare and desired to utilize aggressive, rapidly moving infantry tactics. It was soon realized that the Stokes mortar would be ideal for the Doughboys and the weapon was soon approved for adoption by the U.S. Army. The weapon was known in British service as the "3-inch Stokes mortar," Mark I, and the U.S. Army designated it as the "3-inch trench mortar, Mark I."

There were not sufficient numbers of British Stokes mortars and ammunition to equip the U.S. infantry units, but our allies had agreed to produce the weapon under contract for the American government. The production rate of the British firms involved in Stokes mortar manufacture was not adequate to meet the high demand of the rapidly growing AEF. In order to boost the production rate, approval was granted for the Stokes mortar to be manufactured in the United States. . .

. Most of the 3" Stokes mortars used by U.S. troops during the war were British made. According to a War Department document, the United States procured 914 Stokes mortars from the British for issue to the AEF. Although 843 American-made 3" Stokes mortars were delivered to France before November 11, 1918, few actually made it into front-line combat before the Armistice. The remaining American-made 3" Stokes mortars were used for training purposes stateside. . .

There were two standard types of 3" Stokes trench mortar shells authorized for use with the weapon during World War I. The primary type used was the "Mark I" high-explosive shell. It weighed about 11 pounds, 11 ounces, and was filled with approximately 2 1/4 pounds of "Trojan" nitrostarch explosive compound. TNT was also approved but seldom used. The shell was fitted with the "Mark VI" fuse. The Mark VI was an "all ways" impact fuse designed to detonate regardless of the velocity or angle with which the shell hit the ground. The Mark I explosive shells were painted gray or khaki with a three-inch yellow or red band painted around the body. The shell was equipped with a "safety pin" that was removed prior to dropping the shell into the mortar tube. The "Mark III" practice shell was produced and issued for training purposes. This shell was filled with about 2.15 pounds of sand and a black powder "booster" that produced a puff of white smoke upon impact. The weight and ballistic performance of the Mark III practice round simulated that of the Mark I high-explosive shell. The practice shell was painted dark blue and stenciled in white letters "Practice" on the body.

Three-inch Stokes mortar smoke shells for screening purposes and some white phosphorus shells were tested in limited numbers. There was also some experimentation done with toxic gas shells for the weapon. The 3" Stokes mortar gas shells were usually painted with a pink or green band on the outside of the body for identification purposes. The various types of chemical/gas shells could hold up to seven pounds of chemical compound. This was deemed insufficient, and the 3" Stokes trench mortar was seldom employed as a smoke or chemical agent projector. Therefore, the only type of 3" Stokes trench mortar shell to see any significant combat use was the "Mark I" high-explosive shell. The 3" Stokes trench mortar propelling cartridges used by the U.S. resembled shotgun shells and were color-coded red, green or blue depending on the respective power of the charges. Tables were contained in field manuals that listed the range and "time of flight" according to the type of cartridge and the number of "ballistite" propellant rings that were attached to the shell. Up to four of the donut-shaped propellant rings could be attached to a 3" Stokes mortar shell.

Training in France

The 3" Stokes Trench Mortar

Preparing for Action