Holt SPGs
by Charlie Clelland
This article is a vehicle by vehicle discussion of the self-propelled guns produced by the Holt Tractor Company or by the Rock Island Arsenal under the supervision of Pliny E. Holt from 1917 to 1922.
The Holt tractor company was the largest manufacturer of tracked vehicles at the outbreak of WW1. Both Britain and France bought Holt tractors to tow artillery pieces in the early years of the war. These proved to be very successful since they permitted the movement of heavy guns in conditions which would have been impossible for the traditional horse transport. The U.S. Army bought Holt artillery tractors in 2½, 5, 10, 15 and 20-ton towing capacities. The 15 and 20-ton tractors were Holt commercial vehicles minimally modified for military service. However, the smaller tractors were specially designed for the U.S. Army by Holt. At the entry of the U.S. into WW1 some 24,791 tractors of all types were ordered. By the time of the Armistice in Nov 1918 2,500 had been delivered and more than half of these were shipped to France. The obvious requirement for tracked support vehicles was not ignored and the design of tracked munitions carriers and petrol tankers was well advanced in Nov 1918.
| Type |
Ordered |
Accepted 11 Nov 1918 |
Accepted 31 Jan 1919 |
| 21⁄2-ton |
5,586 |
10 |
25 |
| 5-ton |
11,150 |
1543 |
3480 |
| 10-ton |
6623 |
1421 |
2014 |
| 15-ton |
267 |
267 |
267 |
| 20-ton |
1165 |
126 |
154 |
The artillery tractors offered Holt vehicles on which to base self-propelled gun mounts. The concept of a self-propelled gun as a minimal tractor vehicle with a gun mount was a defining characteristic of U.S. self-propelled guns until WW2 and differed from British and French designs which were based (mostly) on tank chassis. It is clear from contemporary accounts which referred to these vehicles as “motorised gun carriages”, “self-propelled caterpillar mounts” that the SPGs were thought of as artillery pieces with superior mobility rather than weapon systems with their own unique characteristics.
Holt 55-1
The Ordnance Dept had been investigating mechanisation of the field artillery since 1915. As part of these investigations options for mobility of the (then) new 3inch M1917 anti-aircraft gun were sought from interested manufacturers. The Holt Company approached the U.S. Army Ordnance Department with a proposal for a self-propelled tracked anti-aircraft vehicle mounting the new 3inch M1917 anti-aircraft gun. The Ordnance Dept authorised Holt to construct a prototype, this was delivered in 1917. The Holt 55-1 was found on testing to have many faults but the concept of a gun mounted on a self-propelled carriage was worth pursuing. The Holt 55-1 was refitted with an 8inch Vickers howitzer on a temporary mount constructed of oak planks. It was found on firing tests that the caterpillar vehicle was able to withstand the howitzer’s recoil forces. At some stage during the testing the vehicle was evaluated as an unarmed tracked transporter similar in concept to the Renault FB.
Mark I
Pliny E. Holt, the chief designer of the Holt 55-1, was persuaded to work for the Rock Island Arsenal, Illinois, on SPG projects. He supervised the design and construction of the next series of SPGs which used components of the 10-ton Holt artillery tractor. Unlike the French St Chamond SPG design these vehicles were tailored for each type of gun rather than designing a single vehicle which could be fitted with a variety of guns. The Mark I SPG was designed to mount the Vickers Mark VIII 8inch howitzer which fired a 90kg projectile to a max. range of 11,000 yards. A small crane was fitted to the left side of the vehicle deck to assist with loading the howitzer. Three of these vehicles were built at the Rock Island Arsenal and tested in 1918. The chassis was box structure with an opening in the centre for the gun recoil. The gun was mounted on the front part of the chassis with the transverse mounted engine and transmission at the rear of the chassis. The engine was the same as the Holt 10-ton tractor, producing 80hp at 850 rpm. The transmission was a combination of selector and planetary gearboxes giving 4 forward and 2 reverse gears with a top speed of 4 mph.
Mark II
The Mark II was designed to carry the 155mm GPF gun which was built under licence as the 155mm M1918. The GPF gun fired a 43kg projectile with a max. range of 15,700m. The gun was mounted on a short pedestal mount bolted to the chassis. The chassis was slightly shorter and lower than the Mark I. The engine and transmission was positioned in the front of the chassis and the rear of the chassis was opened out to allow for the gun recoil. A total of 10 Mark II SPGs were constructed, the first two with a 145hp Duesenberg engine and the remainder with a 130hp Sterling engine. A transmission similar to the Mark I was fitted. Unlike the Mark I the driver’s position was in the centre on the right side of the gun rather than at the rear.
Mark III
The Mark III was designed to carry the 240mm Schneider howitzer. This was a smaller calibre, longer barreled version of the French 280mm Schneider TR M1914 howitzer. To fit this heavy howitzer on a tracked chassis a special mounting for the howitzer was developed which slid up a pair of inclined rails to absorb most of the recoil. In addition a pair of hydraulic recoil cylinders were added as a secondary recoil absorber. There were two Mark III vehicles built with a cast steel frame with a 210hp 8-cylinder Cottrell-Holmes engine and a single M1 version with a rivetted steel frame and a 225hp 6-cylinder Duesenberg engine. The Mark III was very heavy at about 48 tons and exceeded the weight limits set by the U.S. Engineers for highway bridges. The max. speed was very low at 4.2 mph.
Ordnance Dept 75mm Experimental Gun Mounts
The Ordnance Dept designed SPGs based on the Holt 2½ and 5-ton artillery tractors as gun carriers at the same time as the Rock Island Arsenal was building SPGs based on the 10-ton tractor. Very little information has been published on these vehicles but some observations may be made from surviving images of these vehicles. The gun used was the 3inch M1916 gun - an advanced US-designed light field gun intended to become the standard field gun of the U.S. Army. However, the French 75mm Mle 1897 built in the U.S. as the M1897 was chosen as the standard field gun and the M1916 was discarded. The 2½ ton tractor was fitted with an M1916 gun by extending the side plates of the tractor chassis to support trunnions in which the gun axle fitted. The excess axle length was cut off. There were a pair of outriggers fitted at the rear to stabilise the vehicle during firing since the short length and high, rearwards position of the gun would have tended to topple the vehicle. The other vehicle was based on the 5-ton tractor and mounted the gun on frames bolted to the chassis. The gun was mounted fairly centrally so outriggers were not required. There was sufficient space on the 5-ton tractor to fit a deck behind the gun.
75mm SPG based on 2½-ton tractor
75mm SPG based on 5-ton tractor
Mark IV and Mark IVA
In 1918 the Rock Island Arsenal built a tractor (Mark IVA) and gun vehicle (Mark IV) based on St Chamond plans for the Mortier 280mm TR de Schneider sur affût-chenilles St Chamond. This SPG mounted the 240mm Schneider howitzer rather than the 280mm of the St Chamond SPG. There were some differences between the American and French vehicles: the engine was a more powerful 150 hp 6-cylinder Van Blerck petrol engine driving a General Electric 70 kW 400V generator. Both the tractor and gun vehicle were driven by a pair of 70 hp 400V electric motors. The charging trolley and rails on the St Chamond gun vehicle were replaced by a crane which could reach the ammunition stored on the tractor and deliver shells to the loading tray of the howitzer. The small crane on the tractor of the St Chamond vehicle was deleted. The tractor could carry 42 161kg projectiles and the howitzer’s max. range was 15,000 m. The Mark IV permitted the carriage of the 240mm howitzer without the excessive weight of the Mark III although the max. speed was quite modest, about 8 mph.
Wartime Production
The logic of SPGs offering superior battlefield mobility for artillery was accepted by War Munitions Board in 1918. Production contracts were signed for the Marks I through IV SPGs with delivery expected in early 1919. The production contracts of the SPGs were;
50 Mark I (8 inch howitzer) - Harrisburg Manufacturing & Boiler Co., Harrisburg, PA
50 Mark II (155mm GPF) - Morgan Engineering, Alliance, OH
250 Mark III and Mark IV (240mm howitzer) - The Standard Steel Car Co., Hammond, IN.
There is some confusion about the numbers of 240mm howitzer SPGs. Benedict Crowell, Director of Munitions 1917-20, notes in his report on munitions production in WW1 that a total of 270 SPGs were ordered which means that actual production contract was for 170 240mm howitzer SPGs. After the Armistice in Nov 1918 the contracts were cut back to a few vehicles for experimentation. It is unlikely any of the production 240mm howitzer SPGs were delivered since there had been delays in U.S production of the 240mm howitzer - the first example had exploded on test. The only production SPGs delivered appear to be 8 Mark IIs.
Mark VII
The Mark VII appears to have been based on the experience with the experimental 75mm SPGs. The 2½-ton tractor was selected as the base for a new 75mm SPG in 1919, the 5-ton based vehicle presumably was dropped due to excessive weight. The Mark VII reused the 3-speed transmission and track system of the 2½-ton tractor and was powered by a 70hp V8 Cadillac engine. The engine was waterproofed to allow for deep wading to a depth of 0.85m. The large outriggers were replaced by much smaller folding jacks. The gun, as in the earlier vehicles, was the 75mm M1916. Two vehicles were produced at Holt’s Stockton factory and tested at the Aberdeen Proving Ground (APG) in 1919-20.
Mark VI
Two Mark VI vehicles were built at Holt’s Stockton factory in 1920-21. These were designed to carry either a 75mm M1920 gun or (interchangeably) a 105mm M1920 howitzer. The use of tractor components was abandoned and these vehicles were designed from the ground up. They were powered by a Holt 75hp V8 engine with 3-speed transmission. The waterproofing investigation started on the Mark VII was improved on the Mark VI so it could run submerged provided the inlet and exhaust extension pipes remained clear. The tracks used rubber pads on the track shoes and rubber rims on the roadwheels, return rollers, idlers and drive sprockets to reduce vibration, reduce damage to roads and extend track life. It was noted that the Mark VI did not have the loud track noise of other tracked vehicles which would have been an obvious tactical advantage had they been deployed. There was a removable armoured enclosure available for the Mark VI which was designed to protect the driver and gunner(s) without interferring with the gun elevation and traverse. Holt claimed the Mark VI was capable an emergency max. speed of 30mph but the normal max. speed was 14mph - during a drive from Stockton to San Francisco (120 miles) a top speed of 27mph was recorded. There was a surviving Mark VI at Aberdeen, MD - it is believed the vehicle is in storage at Fort Lee, VA.
Mark IX
The Mark IX was built in 1921 by Holt at Stockton, CA and tested in 1922 at APG. It was designed to mount either the 155mm GPF gun or 8inch howitzer. It appears to have been a second generation of heavy SPG and used many of the design ideas from the Mark VI applied to a larger vehicle. The vehicle was waterproofed and it could ford to a depth of 2.13m. The Mark IX used rubber track blocks and rubber rims on the wheels to reduce track wear and vibration. The Mark IX was powered by a 6 cylinder 250 hp engine which gave a max. speed of 12 - 16 mph.
Mark X
The Mark X was built at Rock Island Arsenal in 1922. In design it appears to be intermediate between the Holt Mark VI and Mark IX. The gun mounted was a 4.7inch M1920 gun but this could be interchanged with a 155mm howitzer - probably the Schneider Mle 1917. The vehicle weighed about 10,900 kg, it was powered by a 4-cylinder 150hp Sterling engine and had 3-speed transmision which gave a max. speed of 16 mph. Although tests with this vehicle were successful, lack of funds and a hostility by sections of the Field Artillery Branch to SPGs meant that the Mark X was not produced. The Mark X is one of two surviving SPGs from this period, the survivor is at the Field Artillery Museum, Fort Sill, OK.
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