The Crab ended up as the ultimate version of the flail tank and, thanks to its service on D-Day and throughout the campaign in North-West Europe, probably the most well-known. The design was inspired by the Scorpion but had the crucial difference of taking the power for the flail drum from the main tank engine, instead of an auxiliary one. It also kept the gun turret and as such could be used as a normal tank when not flailing.

Experimental prototypes were ready for trials by the end of September 1943. Not all recommendations from the Scorpion were implemented as these tests were mainly to see how the new power configuration would work, rather than as a final product. When flailing, the Crab could run at a speed of 1.66 mph (with this only dropping by 10% when on hills), a mine detonating efficiency of 91%, and flail damage similar to those results on the Scorpion. As such, it was deemed a success and work moved ahead on the production prototype, built on the Sherman V, ready by the end of October.

The Crab Mk I prototype on the Sherman I.

At the same time, a flail tank known initially as the Octopus and later the Marquis was in prototype. This was more closely based on the Scorpion as it still had an auxiliary engine but did address the issue of raising and lowering the rotor arms by hydraulics instead of requiring crew to get out of their armour to do it manually. The movement was intended to avoid obstacles and for better packing in landing craft, not to follow the contours of the ground when flailing as with the Baron. The Marquis project was abandoned in October after the success of the Crab but part of it lived on as the hydraulic arms fitted on the second Crab prototype were identical to those on the Marquis.

The Marquis with its arm raised.

In addition to hydraulics, the production prototype saw the arms strengthened as they were deemed 'somewhat flimsy' on the experimental one. Trials were satisfactory – mines laid in mud and barbed wire were successfully detonated but those in hedges were occasionally missed, the flails having a tendency to push the hedge down over the mine and protect it – but the rotor and arm were badly damaged after 27 tellermine detonations and so increased armour in these areas was recommended. Confidence was high in the Crab however and work on production of 300 began almost immediately with the aim to have them ready by the end of March 1944.

The second round of trials faced a new mine laying technique wherein one mine would be buried deeply under another one. The top mine was connected to the bottom one by a fuse and the idea was that a flail tank safely detonating the first would then set off the second one, by then under the belly of the tank. However, trials found that the flails would actually cut the connecting fuse unless it had been laid more than 2" below ground level. Users also found a much greater loss of flail links than previously, with one incident seeing 100 chain links and the rotor lug lost in a single detonation. This posed a major problem. While at first it was thought chains were being dashed and broken against the drum (and a series of trials set up to test alternatives), it was later proved to simply be a decline in chain quality.

Samples were therefore sent in from the 5 different manufacturers responsible for producing the Crab’s chains and these were tested to find which ones were not working to the required specification. Each set was subject to an hour’s flailing and the resultant damage varied from 16 to 470 links lost. The best chain tested was known as the Wiggins "K" type and production shifted to match this configuration and standard. However, even if manufacturers ignored all other chain requests (such as for the Navy’s anchors), it was soon realised that it would be impossible to produce the amount of chain required for all the Crabs to have a set of chains and two spares. Thankfully, during chain trials, an alternative was proposed which used straight/plate links instead. This was found to be able to withstand 2.5 hours of flailing without serious damage and so, in February 1944, efforts turned towards this type of flail instead and the chain manufacturers were able to continue their normal jobs. Nevertheless, some production of the "K" type did continue so that it would be available in special circumstances should the plate type fail.

Stages in flail development for the Crab. The sample on the far right was the one adopted for final production.

Unfortunately, the plate type did have an issue with knotting, though it would also quite often unknot itself as well. It was at a standard where users were happy to go to war with it, but there was time for improvements to be made and so much experimentation followed to find an improved configuration of links even while production of the original continued. The final design was not fully approved until April and became available in June – it therefore missed the landings themselves but served well in the first weeks of the battle for France with over 1400 mines destroyed. In fact they were so satisfied with the result of the final iteration of the plate type, production of the "K" type chain was finally fully stopped in December 1944.

There was much more development on the Crab than merely the flail however, with it steadily building on lessons learnt from the other flail tanks such as with visibility and wire cutting, and in May 1944, a Crab II was even approved. 

Information and pictures in this article are taken from E:05.0177.01 and E:07.0038.15.

Zoe Tolman, Assistant Archivist. Published in The Craftsman, August 2023.