Light Trench Mortar Batteries AIF

About This Unit

This is a generic administrative grouping that facilitated the Enlistment and Embarkation of reinforcements destined to serve in the Light Trench Mortar Batteries organic to each Infantry Brigade.  They would have been assigned to one of 15, following completion of training in France.  The original units had been raised from their parent Brigades after the relocation of the ANZAC forces to the Western Front in early 1916.

The numeric designation indicates the Brigade to which the Battery belonged.  For example the 8th Light Trench Mortar Battery belonged to 8th Brigade comprising the 29th (NSW) 30th (VIC) 31st (QLD and VIC) and 32nd (SA / WA) Battalions.

Organisationally, Light Trench Mortar Batteries were under the direct command of the Brigade Headquarters but were generally deployed close to the front line so that the enemy was within range.

Light Trench Mortar Batteries went where their parent Brigade went and their involvement in battles / campaigns reflects that of the Brigade.

Trench Mortar Batteries and Machine Gun Companies gave the Brigades their own integral fire support. The trench mortars were the Brigade's own ‘artillery’ and the soldiers were generally drawn from units within the Brigade. The Battery would have, as in the accompanying photo, provided support exclusively to the Battalions of the Brigade.

Mortars fire high-explosive fragmentation bombs at a high angle, which detonate on impact or very soon thereafter.  In military parlance the mortar provides 'Indirect' fire.  In this context that means the weapon is not aimed along a line of sight directly at the target like a rifle or machine gun.  In the case of the mortar the projectile follows a high looping path to the target not unlike a tennis ball lobbed over the net to the opponent.  Its point of impact is determined by the power of the propelling charge, the elevation of the barrel or tube, and the direction (or bearing) it is pointing.

The light trench mortar was the Stokes 3 inch mortar.  It was effectively not much more than a simple tube for a barrel, with a bipod to support it and a steel base plate on which to mount the tube and bear its recoil.  It fired a cylindrical bomb to about 800 yards, although its range improved throughout the war.  Later, the bombs were fin stabilised which greatly improved accuracy and consistency.  These were issued to Light Trench Mortar Batteries, attached to each Brigade Headquarters.  They were relatively mobile and could be dismantled and packed up in minutes.

  • It comprised a barrel, a smooth bore steel tube which had a firing pin fitted in its base.
  • A bipod held the tube at the required elevation and azimuth (bearing) and was fitted with rudimentary sights.
  • A base plate absorbed the recoil and reduced the rate at which the weapon embedded itself the earth on which it was positioned, from the recoil of each round fired.
  • Cylindrical time-fused and bombs fitted with impact fuses were launched from the mortar by a simple percussion firing system.  Fin stabilised bombs did not appear in general use until after the War.  They and the weapon system evolved into the British 3 inch mortar used in WW 2 and up until the 1970s in the Army Reserve.

The Stokes Mortar generally had a crew of two or three men.  

The ‘Number One’ in the crew would lay or aim the mortar - although they were not in direct line of sight to the enemy.

  • He would use rudimentary 'sights' aligned with a known fixed point as the datum or ‘zero line’ and then a bearing could be set.
  • An elevation would be set using a clinometer and range tables. The mortar fired a characteristically high angle trajectory and maximum range was achieved at about 45 degrees.

The ‘Number Two’ loaded and fired the mortar.

  • He would load a bomb into the muzzle of the mortar by hand. As he released it, the bomb would slide down the barrel until it hit a firing pin in the mortar’s base.
  • This would initiate a primer in the base of the bomb, igniting the cordite propellant charge, the blast from which would then project the bomb out of the barrel.

The ‘Number Three’s’ task was to keep a supply of bombs up to the mortar.

  • This involved collecting ammunition from carrying parties and stowing it nearby, then unpacking the ‘ready’ bombs from their transportation packaging.
  • On receipt of an order to fire, the Number Three would adjust the charges if necessary to match the range and elevation necessary to engage the target by removing charge bags of propellant from the bomb.
  • When the fire mission commenced, he would remove the safety pin from the fuze before passing the bomb to the Number Two.

At high rates of fire it was possible to get 15-20 bombs in the air before the first one hit the ground; this was perhaps the mortar’ most valued attribute - a high volume of fire in a short space of time. The high angle trajectory meant that bombs descended steeply and could lob directly into a trench or dugout and then detonate to deadly effect. Hundreds of jagged steel splinters would be created by each bursting bomb.

The flash and blast of the mortar could give its position away so they were generally fired from cover - such as from the base of a trench - the deeper the better!

Trench Mortar Batteries would have been high on the list of priority targets for enemy counter battery fire - fired by heavy artillery. This meant they may not be the most welcome of companions in the front line! Many of the casualties sustained by mortar battery personnel would have been thus inflicted. The other vulnerability lay with their being relatively close to the Front Line and thus potentially vulnerable to enemy infantry raiding parties.