 No armour plate could be spared to protect the ships' bridges and gun positions so the Admiralty recommended that ship owners fit concrete paving stones in layers up to six inches thick to protect the vulnerable crew.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.As the fighting in the English Channel intensified in August 1940, casualties rose and the prospect of a collapse in morale threatened.