Stokes mortar

This article is about the World War I Stokes 3-inch mortar. For the World War II mortar, see Ordnance ML 3 inch mortar.

3 inch Stokes mortar
Sir Wilfred Stokes with example of his mortar and bombs. Typical 3-inch bombs used are 2nd and 6th from left
Type	Light mortar
Place of origin	United Kingdom
Service history
Used by	British Empire French Third Republic Kingdom of Greece Kingdom of Italy Paraguay Second Polish Republic Portugal United States
Wars	World War I World War II Banana Wars Chaco War
Production history
Designer	Sir Wilfred Stokes KBE
Designed	1915
Specifications
Weight	104 lbs (47.17 kg) total
Crew	2
Shell	HE 10 lb 11 oz (4.84 kg)
Calibre	3.2 in (81 mm)
Action	Trip
Elevation	45°-75°
Rate of fire	25 rpm (maximum) 6-8 rpm (sustained)
Effective firing range	750 yards (686 m)
Maximum firing range	800 yards (731 m)
Filling	amatol
Filling weight	2lb 4 oz (1 kg)

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 later 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

Portuguese Expeditionary Corps soldiers loading a Stokes Mortar, in the Western Front

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 that 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 four additional "rings" of propellant were used for incrementally greater ranges. See range tables below. The four 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 modern mortar, Stokes was given several forms of monetary reward by the Ministry of Munitions for his invention.

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 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 2,500 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 extensive use of the Stokes mortar during the Chaco War, especially as a siege weapon in the Battle of Boquerón in September 1932.

Image gallery

• 

  High Explosive bomb
• 

  No. 145 percussion fuze
• 

  Men of the KOYLI fusing Stokes shells near Wieltjie, October 1, 1917

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

See also

• Mortar (weapon)
• List of infantry mortars

Bibliography

• (Provisional) Range Table For 3-Inch Stokes Mortar, September 1917. United Kingdom War Office.
• "Stokes' trench howitzer, 3", mark I". US Army War College, January 1918. Made available online by Combined Arms Research Library
• Field Artillery Notes No. 7. US Army War College August 1917. Provided online by Combined Arms Research Library
• Bruce N. Canfield, The Three Inch Stokes Mortar. Excerpted from U.S. Infantry Weapons of the First World War
• General Sir Martin Farndale, History of the Royal Regiment of Artillery. Western Front 1914–18. London: Royal Artillery Institution, 1986
• W L Ruffell, The Stokes Mortar

External links

• "Handbook of the M.L. Stokes 3-Inch Trench Mortar Equipments. 1919." Published by His Majesty's Stationery Office, London, 1920.
• "Basic Field Manual. Volume III, Basic Weapons. Part Four, Howitzer Company. 3-inch Trench Mortar". United States War Department, 1932. Made available online by Combined Arms Research Library