The Nellie Project
The first call for the 12 TP was as the propulsion engine for a large tank needing 600-650 hp where an absolute minimum of space was available. A view is held by some that the letters TP stood for "tank propellant", and not "three pieces" as suggested elsewhere. Two TPs were supplied for the prototypes of the tank, the story of which is told on our page Paxman's Tank Propellant. The next call for the TP was to power a very large excavator, the brainchild of Winston Churchill. According to one source the first bulk order for TPs was one of 550 engines for this machine and the tanks.
The construction of the French Maginot and German Siegfried Lines during the inter-war years led to an assumption that any future conflict would again involve trench warfare. Mindful of the appalling loss of life in the trenches of 1914-18, Churchill wanted to find a way of allowing troops and supplies to advance in relative safety and quickly break through the German front line. He came up with the idea of machines which would dig large trenches through No Man's Land under cover of darkness and the noise of an artillery barrage. Troops and tanks would follow in these trenches, coming to the surface at or behind the enemy front line. The mechanical 'mole' designated NLE, standing for Naval Land Equipment, came to be known as 'Nellie' or in Paxman's Drawing Office as 'No man's Land Excavator' - a very apt description.
On 7th February 1940 Cabinet and Treasury approval was given for the construction of 200 narrow 'infantry' machines and 40 wide 'officer' machines. The original planned production rate was 20 machines (requiring 40 engines) per week. It her final form Nellie was 77' long, 6' 6" wide, 8' high and made in two sections. The main section, driven on caterpillar tracks, looked like a greatly elongated tank and weighed 100 tons. The front section, weighing another 30 tons, was capable of digging a trench 5' deep and 7' 6" wide. It comprised a plough which cut the top 2' 6" of the trench, and 'pick and shovel' cutting cylinders which excavated the bottom 2' 6". The spoil was carried away by conveyors to the top of each side of the trench to create 3' parapets. Nellie could move at just over half a mile an hour, removing some 8,000 tons of spoil in the process. When she reached the enemy's front line she would stop and act as a ramp for following tracked vehicles to climb up out of the trench onto open ground. Originally she was to be powered by a single 1,000 bhp marine version of the Rolls-Royce Merlin engine. Apart from the fire risks inherent in a petrol engine, it was soon pointed out this engine could only be expected to produce 800 bhp under continuous load, less than was required for the task. Shortly after, all Merlin engines were urgently needed by the RAF and Sir Harry Ricardo's advice was sought on a suitable alternative. He recommended using two Paxman-Ricardo engines of the type already in service with the Navy and of a proven design. The decision was made to use two 600 bhp Paxman 12TPs, necessitating a complete redesign of Nellie. One engine was to drive the cutter and conveyors at the front and the other used to propel the machine itself.
The war very quickly took a totally unexpected course. After Dunkirk and the fall of France the Nellie project collapsed. Large scale production of TPs for Nellie was abandoned and the engine capacity was turned over to the Admiralty. (Field trials of the pilot machine commenced in June 1941 and were not completed until about January 1942.)
The most detailed history of Churchill's trench digger is 'Nellie' - The History of Churchill's Lincoln-Built Trenching Machine by John T Turner, published in 1988 by The Society for Lincolnshire History & Archaeology (ISBN No 0 904680 68 1). It is a very well researched and written publication. According to this history only five of the smaller 'infantry' version of Nellie were actually completed. Four were scrapped at the end of the war and the fifth, believed to have been the pilot machine, was scrapped in the early 1950s. There is a suggestion that work was commenced on four of the wider 'officer' machines but that these were scrapped when Winston Churchill finally agreed to cancellation of the project in May 1943.
