Stokes mortar

The Stokes mortar was a British trench mortar invented by Sir Wilfred Stokes KBE that was issued to the British, Commonwealth and U.S. armies, as well as the Portuguese Expeditionary Corps (CEP), during the latter half of the First World War. The 3 inch trench mortar is a smooth-bore, muzzle-loading weapon for high angles of fire. Although it is called a 3-inch mortar, its bore is actually 3.2 inches or 81 mm.



Design
Frederick Wilfred Scott Stokes – who later became Sir Wilfred Stokes KBE – designed the mortar in January 1915. The British Army was at the time trying to develop a weapon which would be a match for the Imperial German Army's Minenwerfer mortar which was in use on the Western Front.

Stokes's design was initially rejected in June 1915 because it was unable to use existing stocks of British mortar ammunition, and it took the intervention of David Lloyd George (at that time Minister of Munitions) and Lieutenant-Colonel J. C. Matheson of the Trench Warfare Supply Department (who reported to Lloyd George) to expedite manufacture of the Stokes mortar.

The Stokes mortar was a simple weapon, consisting of a smoothbore metal tube fixed to a base plate (to absorb recoil) with a lightweight bipod mount. When a mortar bomb was dropped into the tube, an impact sensitive primer in the base of the bomb would make contact with a firing pin at the base of the tube, and detonate, firing the bomb towards the target.

The barrel is a seamless drawn-steel tube necked down at the breech or base end. To the breech end is fitted a base cap, within which is secured a firing pin protruding into the barrel. The caps at each end of the bomb cylinder were 81 mm diameter. The bomb was fitted with a modified hand grenade fuze on the front, with a perforated tube containing a propellant charge and an impact-sensitive cap at the rear.

Range was determined by the amount of propellant charge used and the angle of the barrel. A basic propellant cartridge was used for all firing, and covered short ranges. Up to 4 additional "rings" of propellant were used for incrementally greater ranges. See range tables below. The 4 rings were supplied with the cartridge and gunners discarded the rings which were not needed.

One potential problem was the recoil which was "exceptionally severe, because the barrel is only about 3 times the weight of the projectile, instead of about one hundred times the weight as in artillery. Unless the legs are properly set up they are liable to injury".

A modified version of the mortar, which fired a modern fin-stabilised streamlined projectile and had a booster charge for longer range, was developed after World War I; this was in effect a new weapon.

History
The mortar was in no sense a new weapon, although it had fallen out of general usage since the Napoleonic era. In fact, while the British and French worked on developing new mortars, they resorted to issuing century-old mortars for use in action.

The Stokes mortar remained in service into the Second World War, when it was superseded by the Ordnance ML 3 inch mortar, and some remained in use by New Zealand forces until after the Second World War.

As well as receiving a knighthood for inventing the mortar, Stokes was given several forms of monetary reward by the Ministry of Munitions for his invention including royalties of £1 per Stokes mortar shell produced.

The French developed an improved version of the Stokes mortar as the Brandt Mle 27, further refined as the Brandt Mle 31; this design was widely copied with and without license. Despite their indigenous production, out of 8,000 81 mm mortars in service with the French in 1939, 2,000 were of the original Mk. I build purchased from Great Britain.

About 700 Stokes mortars were acquired by Poland between 1923 and 1926. In 1928 an unlicensed Polish copy was made as the Avia wz.28, but due to French pressure it was abandoned in 1931 because the French Brandt company held the patent for the ammunition. The Polish then produced a licensed copy as the wz.31 model (Polish: Moździerz piechoty 81 mm wz. 31) starting in 1935; 1,050 were made in Pruszków. By 1939, the Polish army was equipped with some 1,200 Stokes-Brandt mortars, most of them the newer 1931 model. Each Polish infantry battalion was intended to be equipped with four such mortars, but there were not enough mortars available to fulfill this disposition. The upgraded 1931 version was used by the Polish Army during, amongst others, the Battle of Westerplatte in 1939.

Combat use
British Empire units had 1,636 Stokes mortars in service on the Western Front at the Armistice.

In World War I the Stokes Mortar could fire as many as 25 bombs per minute and had a maximum range of 800 yards firing the original cylindrical unstabilised projectile. By World War II, it could fire as many as 30 bombs per minute, and had a range of over 2500 yards with some shell types.

A 4 inch version was used to fire smoke, poison gas and Thermite (incendiary) rounds but this should be considered a separate weapon to the standard 3 inch version firing high explosive rounds described in this article.

The Stokes mortar was used in the Banana Wars and helped American forces defeat Sandinista rebels during the Second Battle of Las Cruces on January 1, 1928.

The Paraguayan army made an extensive use of the Stokes mortar during the Chaco War, especially as a siege weapon in the Battle of Boquerón in September 1932.

Range tables
(Provisional) Range Table For 3-Inch Stokes Mortar, Printed in September 1917. Cartridge : 95 grains ballistite, reinforced with Charges : 5 grains, guncotton yarn Rings : 110 grains, .3 mm flake cordite Projectile : Bomb, 10 lb. 11 oz

Weapons of comparable role, performance and era

 * 7.58 cm Minenwerfer : approximate German equivalent

Surviving examples

 * Australian War Memorial, Canberra
 * An example with bombs is displayed at l'hotel de ville d'Arras, France. Bernard Plumier : Link to his web page which has details and photograph Direct link to photograph