Development
The machine had a simple task to perform. Essentially, the requirement was to cut a trench with a section of about six feet (2 m) square and for this some sort of cutter was required. Correctly estimating the power required to perform this feat was essential to the success of the project, but the nearest comparable machine the designers could take figures from were the giant bucket and chain excavators used in Germany for open cast mining of lignite. The key feature of such machines was that the cutting was a continuous process that required a fairly constant power and from this comparison the trenching machine power requirement was estimated at about 1,000 hp half of which was for cutting and half for driving the machine forward.
Initial designs envisaged a large circular cutter the diameter of the trench to be dug and operating in the manner of modern tunnel boring machines. However, the design evolved to a more efficient arrangement with a huge plough removing the top 2 feet 6 inches (0.76 m) of soil and a cutting cylinder rotating perpendicular to the line of the trench for digging out the lower 2 feet 6 inches (0.76 m). The trench profile was then squared off by a number of blades. The spoil was deposited on either side of the trench, wings on the plough blade pushing the spoil away from the edge of the trench to prevent it from falling back in. Hopkins presented this conception together with a static model to Churchill via Sir Stanley Goodall. Churchill approved the scheme and gave permission for development of a prototype to begin with an initial grant of £1,000,000.
The Navy turned to Ruston-Bucyrus Ltd, an engineering company specialising in excavating equipment. Ruston-Bucyrus had been established in 1930 and was jointly owned by Ruston and Hornsby based in Lincoln, England and Bucyrus-Erie based in Bucyrus, Ohio, the latter of which had operational control. On 6 December 1939, Churchill was told that Ruston-Bucyrus would be able to build 200 trench cutting machines by March 1941 and they proposed a wider version that would produce a trench in which tanks could drive. Churchill gave the go-ahead for the production of a prototype but deferred a decision as to the final quantities for the time being.
A scale model about four feet long was prepared by the firm of Bassett-Lowke, they worked secretly in the cellars of a hotel in Bath – Bath being the temporary home of the Naval Construction department at the time. As soon as it was complete, Churchill ordered that it be taken to London. The model together with its accessories was packed into a mahogany box resembling a coffin; as it was carried to the station in Bath, many bystanders respectfully bowed their heads.
The working model was demonstrated to Churchill on 12 December 1939. For this a simulated soil had been developed from a mixture of sawdust and Plasticine. The demonstration went so well that Churchill's smile of pleasure "almost dislodged his cigar" and he ordered that a further demonstration should be arranged for that evening, to which Churchill was accompanied by Prime Minister Neville Chamberlain, Chancellor of the Exchequer Sir John Simon and Chief of the Imperial General Staff Sir Edmund Ironside. There is an apparent disagreement in the sources as to exactly what time this meeting took place. Newman (1956) gives 11 pm whereas Ironside (1962) specifies 7 pm. It is possible that Ironside was treated to a preview. Ironside later recalled:
| “ | At 7 p.m. I went over to see Winston Churchill at the Admiralty. He told me that he wanted to show me his "Cultivator". I found that he had invented and reduced to a model a machine that would go through the earth at a good pace... I thought that we could make a great deal of these machines and they present the first of any possible offensive idea. | ” |
Churchill used the model (or possibly another static model) to persuade the French to support the project to which they somewhat reluctantly agreed. An official order was placed with Ruston-Bucyrus on 22 January 1940. On 7 February 1940 the government gave approval for the construction of 200 narrow "infantry" and 40 wider "officer" machines, the latter creating a trench wide enough for tanks.
In the following weeks, the Germans noticed intense patrol activity in front of the Siegfried Line as the French collected soil samples so that technicians could determine the most suitable places for the Cultivators to advance. However, the production of Cultivator almost immediately faced a problem as the Air Ministry reserved for RAF use all the Rolls-Royce Merlin engines being produced. Ruston-Bucyrus had intended to use the Merlin, but now had to change tack. They called in Sir Henry Ricardo who suggested using a pair of 600 hp lightweight diesel engines built by Davey, Paxman and Co. The change meant a great deal of redesign work, but the new arrangement had some advantages. Now one engine would be used for the cutter and one for moving the machine which simplified some aspects of the design and diesel fuel was safer than the petrol required by the Merlin engine.
Read more about this topic: Cultivator No. 6
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