British 13pdr AA Gun on Thornycraft J-Type Lorry
or on Daimler Mk.3 Lorry
with a plan by Ken Musgrave and photos by Knut Erik Hagen

The huge impact of aviation had on warfare, came as a surprise for many in 1914, leading not only to a massive expansion of the air arms, but also making the need for anti-aircraft artillery clear to all involved. Many more or less improvised solutions came into being on all sides of the front lines. And in many cases these guns were motorized, in order to make their tactical use as flexible as possible. One of these early improvisations can be seen here, in two guises: the British 13pdr AA Gun mounted on a Thornycraft J-Type Lorry, or on the Daimler Mk.3 Lorry.

The firm of John Thornycroft Ltd. of Basingstoke had a long history of producing vehicles to meet military requirements and it was not inappropriate that Thorny­croft's J Type lorry should come out best in the 1913 and 1914 War Office subsidy trials. Not only did it lead the field in most of the trials runs, but it was also the lightest vehicle competing in the 3-ton Class A. The prototype vehicle was designed in 1912 specifically to qualify for the newly announced subsidy scheme. It took part in the 1913 trials and was exhibited at the Commercial Motor Show, Olympia, the following July. The wheelbase of the J Type was 13 ft 72 in., and the width was 7 ft 2.5 in. The four-cylinder side-valve engine developed 40 h.p. In all respects the vehicle conformed to the War Office subsidy requirements; it weighed 3.25 tons less its body, and had a top speed of 14.5 m.p.h. More than 5.000 J Types were supplied to the British Army in the years 1914-18 and this vehicle won a great reputation on all fronts. The Thornycrofts were chosen for the more exacting special-purpose roles and those most associated with the J Type were the mobile workshops and the mobile anti-aircraft guns. The vehicle below shows the latter configuration with 3-inch A.A. gun, one of the best­known vehicle types of the First World War. The Germans had designed this sort of vehicle many years earlier, but in the British Army the mobile A.A. gun was very much a last-minute improvisation produced in 1915. 

Another variant was to put the 13-pdr AA gun on the Daimler Mk. 3 Lorry, as shown in the excellent plan below by Ken Musgrave. (Any commercial use of this plan must be cleared with mr Musgrave first.)

The gun in question was a 13-pdr. field gun, as used by the Royal Horse Artillery, removed from its normal field carriage and fitted to a rotating pedestal mount. However, soon the need for a more powerful AA gun arose. At first the 18pdr was used in the same role as the 13pdr, but this was a failure ballistically speaking, so instead it was decided to use the 18pdr, but to reline it to the calibre of 76mm, thus enabling it to take the grenades of the 13pdr. This proved a very successful solution, and this gun, the 13pdr 9cwt AA gun became the most common of all British Anti-Aircraft weapons during WW1. The muzzle velocity of this gun was 655m/sec, giving it an effective ceiling of some 5.790 meters.

The photo below show the 13pdr mounted on the Daimler Mk.3.

The gun was used in conjunction with a visual rangefinder carried in an accompanying vehicle and set up in a convenient position alongside the emplaced gun carrier. Jacks on the chassis steadied the vehicle when the gun was fired. The usual gun section was equipped with two Thornycroft gun-carrier lorries, each supported by two 3-ton lorries which carried the gun crew, the rangefinder, and ammunition. 

Regarding the Thornycroft Lorry: The Thornycroft mobile workshop was another very important type and had a van-like body with drop sides which opened out to form a working platform. Inside the body a lathe, anvil, work bench, drilling machine, grinding machine, and small auxiliary petrol engine driving a gener­ator were the usual fittings, though equipment was changed for special functions on some vehicles. The mobile workshop companies carried out all minor running repairs (and not a few major repairs) to vehicles and guns behind the front lines. Thornycrofts served on all fronts with the British Army, and the J Type remained in production as a commercial vehicle until 1926. Early J Types had wheels with cast spokes, but these were soon changed for pressed-steel disc types as shown in the photos.