Plastic armour
Plastic armour replaced the use of concrete slabs, which although expected to provide protection, were prone to cracking and breaking up when struck by armour-piercing bullets.
The Admiralty had done no testing with armour-piercing bullets, and when the fighting started in earnest, it became evident that concrete armour was almost useless against German machine-gun fire.
[1] Edward Terrell was a successful barrister and magistrate[2] with a flair for invention; by 1940, he had registered a number of patents relating to pens, ink bottles, and peeling knives.
[4] Terrell was brought into the Admiralty to run an information-gathering section, concerned with the manner in which small ships were attacked by aircraft.
When visiting damaged ships, he found bridges and chartrooms riddled with bullet holes and sections of concrete in shattered pieces.
[6]Terrell found that the worn-out ship had been heavily caulked with Insulphate, a slightly elastic compound of asphalt filled with small bits of cork.
Insulphate was a popular solution to the problem of coping with the flexing of a ship that was old or that was now being used in waters rougher than those for which she was originally designed.
In the resulting discussions, Terrell suggested replacing the cork with rock to deflect the hard core of an armour-piercing bullet so that it hit the backing plate at an angle, dispersing its energy so that it would not penetrate.
[9] On 27 August, Terrell, Glanville and a Lee drafted a report detailing their efforts and giving a recipe for plastic armour; the entire development cycle had taken just 10 days.
Since this was from what vital parts of a ship's superstructure were made, it was possible to cast plastic armour in situ between existing plates and temporary wooden shuttering, usually to a thickness of 4 inches (100 mm).
[11][12] The war had brought road building to a virtual halt, so plenty of suitably qualified workers and machines were available for the task of armouring Britain's ships.
Terrell and the chief of the Directorate of Miscellaneous Weapons Development (DMWD), Commander Charles Goodeve, met with representatives of DNC, who rejected plastic armour out of hand.
DMWD performed a series of independent tests at the Royal Navy shore establishment HMS Excellent, resulting in a positive report: There is no doubt that Plastic Armour is very greatly superior to any other non-magnetic material, excluding non-magnetic bullet-proof steel, so far tried ... it is most strongly recommended that the fitting of concrete protection should be discontinued and Plastic Armour fitted in its place.
The Trade Division insisted that the term "armour" was important for morale and that at this stage higher authorities decided that the DNC would be bypassed and production would start without their formal approval.
[24] The court hearing lasted for a full week, with the Crown protesting that plastic armour had been developed in the normal course of the men's work.
The court found in favour of Terrell as sole inventor, granting an award of £9,500 in recognition of the usefulness of the invention and the initiative with which it was developed.
The original plan for tank protection with plastic armour was to produce HCR2-filled steel panels, small in size to reduce the area damaged by a single projectile, which could be fastened to an M4 Sherman in an emergency.
