▁ 2 5 ▁officers ▁and ▁other ▁ranks ▁and ▁were ▁the ▁first ▁to ▁adopt ▁s addle ▁t anks . ▁ ▁For ▁surface ▁running , ▁the ▁boats ▁were ▁power ed ▁by ▁two ▁ ▁dies els , ▁each ▁driving ▁one ▁prop eller ▁sh aft . ▁When ▁sub mer ged ▁each ▁prop eller ▁was ▁driven ▁by ▁a ▁ ▁electric ▁motor . ▁They ▁could ▁reach ▁ ▁on ▁the ▁surface ▁and ▁ ▁under water . ▁On ▁the ▁surface , ▁the ▁D ▁class ▁had ▁a ▁range ▁of ▁ ▁at ▁. ▁ ▁The ▁boats ▁were ▁armed ▁with ▁three ▁ 1 8 - inch ▁( 4 5   cm ) ▁tor ped o ▁tub es , ▁two ▁in ▁the ▁bow ▁and ▁one ▁in ▁the ▁stern . ▁They ▁carried ▁one ▁reload ▁for ▁each ▁t ube , ▁a ▁total ▁of ▁six ▁tor ped o es . ▁ ▁Const ruction ▁and ▁career ▁D 6 ▁was ▁laid ▁down ▁on ▁ 2 4 ▁February ▁ 1 9 1 0 ▁by ▁V ick ers ▁at ▁their ▁Bar row ▁ship yard , ▁launched ▁ 2 4 ▁October ▁ 1 9 1 1 ▁and ▁was ▁commission ed ▁on ▁ 1 9 ▁April ▁ 1 9 1 2 . ▁She ▁was ▁the ▁first ▁British ▁sub marine ▁to ▁be ▁equ ipped ▁with ▁a ▁deck ▁gun ▁when ▁built , ▁a ▁ 1 2 - p ound er ▁( 3 - inch / 7 6 mm ) ▁gun . D 6 ▁was ▁s unk ▁by ▁U B - 7 3 ▁ 7 3 ▁miles ▁north ▁of ▁In is ht rah ull ▁Island ▁off ▁the