In 1913 Krupp produced a 42cm howitzer on a Radgürtel wheeled chassis under the code name ‘M-Gerät’. It had an 800kg shell, with 150kg of explosive filler, and a 9,300m range.
It was road-transportable in five sections, although each of the five pieces weighed between 16–21 metric tons (16,000–21,000kg). Each prime mover was 5m long, the entire vehicle 12m. In total the mortar weighed 70 metric tons (70,000kg). The mortar could be reassembled using a crane in four hours, but six to eight hours of hard work was necessary to prepare the entire mortar position. It is easy to see the mobility advantage possessed by the 21cm howitzer, which weighed about an eighth as much.
The availability of high-capacity mechanical prime movers was the prerequisite for the towed 42cm mortar. Even heavy draft horses were limited to loads of 6,000kg, and only for a limited time. In the absence of mechanical prime movers, heavy guns and mortars could only be moved by rail and fired from on or next to the tracks. The first experiments with mechanical prime movers were conducted in exercises at Metz in 1908.
Short Naval Cannon Battery 3 was pulled by requisitioned agricultural steam ploughs and locomotives. Although they weighed 2,000kg, they had never been designed as artillery prime movers and therefore were of limited tactical usefulness, as they could not pull such colossal loads for long distances. They also varied considerably in size and manufacturer. They required large quantities of coal and water and gave off clouds of black smoke. The crews were their usual farm drivers. Nevertheless, this field-expedient mobility for the 42cm was a resounding success. Due to Belgian demolitions the rail-mobile guns would never have got near Liège in time to affect the outcome of the battle.
All this made the choice of a firing position difficult. In addition, once the mortar was emplaced, the barrel could move left and right only as much as necessary to adjust fire. Shifting targets required a prime mover to haul the trail of the mortar around, necessitating an even larger gun position.
Ten to fifteen 900kg shells were brought forward on each rail wagon to be offloaded at a rail siding about 15km from the battery position, onto lorries, three to a vehicle. Unloaded near the gun position, each shell and powder charge were put onto a shell carrier and pushed by three men along a plank road to the gun position. They were hauled up to the loading tray by the ammunition crane on the mortar. The range was regulated in part by the size of the powder charge, in part by the elevation of the mortar
The first test-firing of two M-Gerät mortars took place at Krupp’s private firing range at Meppen in February 1914, followed by more test-firing at the artillery MTA at Kummersdorf. These required ‘certain improvements’ on the mortars. Mobility exercises were conducted using the steam agricultural tractors near the Krupp works at Essen. Large farms were given subsidies to have the tractors on hand when needed.
A total of four batteries with seven 42cm mortars were on hand in 1914: three rail batteries with five mortars and one battery with two wheel-mobile mortars.
The development of the mobile, fast-firing, highly accurate 21cm mortar, directed by FOs with telephones, led the German siege artillery to develop a new form of siege warfare, the verkürztes Verfahren, a hasty attack. Where normal siege doctrine required a laborious and methodical deployment and advance, in the verkürztes Verfahren the 21cm would rapidly occupy a defilade position, followed by an immediate, intense bombardment and, if necessary, quick assault by infantry and combat engineers. At Liège the Germans employed the verkürztes Verfahren, and not the conventional siege doctrine, with devastating effectiveness, and which caught the Belgians and French completely by surprise.
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