 The <unk> class were the only Japanese battleships to place the powder magazine above the shell magazine as the <unk> wished to put as much space as possible between the highly flammable propellant and mine and torpedo <unk> . The danger from <unk> shells at long distances was not appreciated until the fatal magazine explosions of three British battlecruisers during the 1916 Battle of Jutland <unk> demonstrated the point . To further protect the magazines the depth of the double bottom was increased to a total of 3 @.@ 58 metres ( 11 ft 9 in ) underneath the barbettes and magazines . Additionally , the vessels contained 660 <unk> compartments to preserve <unk> in the event of battle damage . In addition to the torpedo <unk> added when the ships were modernized , the deck armour over the machinery and magazines was increased to a total thickness of 140 mm . Inside the original skin of the ships , two torpedo bulkheads were also added and the turret roofs were increased to a total of 152 millimetres ( 6 in ) of armour . 
