▁of ▁either ▁eight ▁or ▁twelve ▁massive ▁to wers ▁linked ▁by ▁anti - sub marine ▁bo oms ▁and ▁n ets . ▁Only ▁two ▁to wers ▁had ▁been ▁constructed ▁before ▁the ▁Arm ist ice ▁with ▁Germany ▁caused ▁the ▁can cellation ▁of ▁the ▁project . ▁ ▁Origin ▁ ▁On ▁ 3 1 ▁January ▁ 1 9 1 7 , ▁it ▁was ▁announced ▁to ▁the ▁German ▁Reich stag ▁that ▁un rest rict ed ▁sub marine ▁war fare ▁would ▁res ume ▁the ▁next ▁day , ▁ 1 ▁February . ▁The ▁renew ed ▁U - bo at ▁campaign ▁was ▁initially ▁a ▁great ▁success ; ▁nearly ▁ 5 0 0 , 0 0 0 ▁tons ▁of ▁sh ipping ▁being ▁s unk ▁in ▁both ▁February ▁and ▁March , ▁and ▁ 8 6 0 , 0 0 0 ▁tons ▁in ▁April , ▁when ▁Britain ' s ▁reserve ▁of ▁whe at ▁fell ▁to ▁ 6 ▁weeks ' ▁supply . ▁ ▁The ▁British ▁Admir alty ▁red ouble d ▁its ▁efforts ▁to ▁find ▁effective ▁rem edy . ▁Att ention ▁turned ▁to ▁the ▁Stra it ▁of ▁D over , ▁which ▁was ▁used ▁by ▁U - bo ats ▁based ▁in ▁Bru ges ▁in ▁occupied ▁Belg ium , ▁to ▁gain ▁fast ▁access ▁to ▁the ▁Atlantic ▁trade ▁routes ▁and ▁the ▁busy ▁Western ▁Appro aches . ▁If ▁it ▁could ▁be ▁successfully ▁closed ▁to ▁them , ▁they ▁would ▁have ▁to ▁take ▁the ▁much ▁longer ▁" N or thern ▁Route " ▁around ▁the ▁north ▁of ▁Scotland , ▁and ▁therefore ▁be ▁able ▁to ▁spend ▁less ▁time ▁on ▁pat rol . ▁Ex isting ▁counter