Plastic armour

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Plastic armour is a type of vehicle armour developed for merchant ships by the British Admiralty in 1940. The original composition was described as 50% clean granite of half-inch size, 43% of limestone mineral, and 7% of bitumen. It typically applied in a layer two inches thick and backed by half an inch of steel. Plastic armour was highly effective at stopping armour piercing bullets because the hard granite particles would turn the bullet which would then lodge between plastic armour and the steel backing plate. Plastic armour could applied by pouring it into a cavity formed by the steel backing plate and a temporary wooden frame.

At the start of World War II, British production of steel plate was in inadequate to equip the merchant navy with armour that would stop the armour piercing bullets from German war planes. The main requirement was to protect the personnel within the bridges and gun emplacements. At the outbreak of war, a number of ship bridges had been encased in concrete. However, it was soon found that this shattered on impact, and created multiple secondary fragments. In many cases having concrete armour was worse than having no armour.

The interesting story of the development of plastic armour is described by the principle inventor, Edward Terrell, in his book Admiralty Brief. It was developed by a small team in the Admirality in August 1940, and first fitted to merchant ships in October of the same year. This was done despite resistance from the Director of Naval Construction who felt that the Admiralty should not be developing armour. The armour was cheap and easy to install on ships, and the skills and equipment for installation came from the under-utilized road building industry.

Once installed on ships, plastic armour proved highly effective. By some measures, it was as good as plate steel, and was widely adopted by allied ships^{[citation needed]}. In the United States, some 3,000 merchant ships and 1,000 other ships were equipped with it, and in Britain and the Commonwealth some 7,000 ships were fitted^{[citation needed]}.