Twenty-five seconds

During the Second World War the aircraft of Coastal Command were responsible for the aerial war against the submarine. That war began badly when one of its aircraft attacked a surfaced Royal Navy submarine HMS Snapper. The attack was extremely accurate and an ‘anti-submarine bomb’ hit Snapper square on the base of the conning tower. The submarine suffered no damage beyond the loss of four electric light bulbs. Fortunately a more effective weapon was developed in the air-dropped depth charge.

A typical encounter between an aircraft and a submarine was a race with its timing determined by the ability of a U-boat to submerge very rapidly. (In training exercises submarines submerged fully in 25 seconds from the sounding of the warning gong.) The moment the aircraft spotted the submarine, it headed straight towards it and dropped depth charges over the point where the submarine was last seen. The ‘stick’ of depth charges fell in a more or less straight line along the line of flight of the aircraft. The depth charges then sank to predetermined depth and exploded. If the U-boat was within a certain distance of the centre of the explosion (the lethal radius; this was about six metres) it would almost certainly be sunk. Within a somewhat larger distance the U-boat might be damaged but, if none of the depth charges exploded close enough, the attack had failed completely.