me asures ▁were ▁the ▁D over ▁Pat rol ▁of ▁cru is ers ▁and ▁destroy ers , ▁and ▁the ▁D over ▁Bar rage , ▁a ▁line ▁of ▁steel ▁anti - sub marine ▁n ets ▁supp lement ed ▁by ▁mine fields ▁stretch ing ▁across ▁the ▁Stra it , ▁pat rolled ▁by ▁destroy ers , ▁armed ▁tra w lers ▁and ▁dr if ters ▁equ ipped ▁with ▁powerful ▁search light s , ▁work ▁on ▁which ▁had ▁started ▁in ▁ 1 9 1 5 . ▁Although ▁this ▁had ▁been ▁improved ▁by ▁use ▁of ▁the ▁new ▁Mark ▁H 2 ▁mine ▁from ▁mid - 1 9 1 7 , ▁it ▁was ▁not ▁initially ▁very ▁successful ▁as ▁there ▁was ▁no ▁agreement ▁on ▁the ▁depth ▁at ▁which ▁the ▁mines ▁should ▁be ▁t ether ed . ▁Until ▁December ▁ 1 9 1 7 , ▁only ▁two ▁U - bo ats ▁had ▁been ▁destroyed ▁by ▁the ▁bar rage . ▁Furthermore , ▁the ▁pat rolling ▁vessels ▁were ▁vulner able ▁to ▁attack ▁by ▁German ▁tor ped o ▁boats , ▁as ▁in ▁the ▁action ▁of ▁ 2 6 – 2 7 ▁October ▁ 1 9 1 6 , ▁and ▁the ▁action ▁of ▁ 2 0 ▁April ▁ 1 9 1 7 . ▁ ▁The ▁Engine er - in - Ch ief ▁to ▁the ▁Admir alty , ▁Sir ▁Alexander ▁Gib b , ▁de vised ▁a ▁scheme ▁of ▁eight ▁or ▁twelve ▁large ▁concrete ▁and ▁steel ▁to wers , ▁placed ▁at ▁intervals ▁on ▁the ▁sea ▁bed ▁across ▁the ▁D over ▁Stra it ▁from ▁Fol k estone ▁to ▁Cap ▁Gr is