▁time . ▁The ▁operation ▁subsequently ▁dis rupted ▁training ▁exer cis es ▁in ▁the ▁Balt ic ▁Sea ▁as ▁six ▁out dated ▁Type ▁II A ▁training ▁sub mar ines ▁were ▁brought ▁into ▁the ▁North ▁Sea . ▁ ▁At ▁this ▁point , ▁command ers ▁and ▁cre ws ▁were ▁simply ▁ordered ▁into ▁the ▁North ▁Sea . ▁A ▁se aled ▁en velope ▁ab o ard ▁each ▁of ▁the ▁U - bo ats ▁held ▁their ▁official ▁orders . ▁The ▁en velope ▁was ▁to ▁be ▁opened ▁upon ▁receiving ▁the ▁radio ▁code ▁word ▁" H art mut " ▁( h ence ▁the ▁attributed ▁name ▁Operation ▁Hart mut ). ▁ ▁D ön itz ▁predicted ▁spect ac ular ▁results ▁for ▁his ▁sub mar ines . ▁In ▁his ▁mem oir s ▁he ▁wrote , ▁" Und ou bt edly ▁the ▁enemy ▁would ▁react ▁sharp ly ▁to ▁the ▁landing ▁of ▁German ▁troops ▁in ▁Norway . ▁Their ▁operations ▁could ▁be ▁directed ▁at ▁our ▁occupied ▁ports ▁or ▁their ▁own ▁facilities ▁- ▁that ▁is , ▁strateg ically ▁important ▁sites ▁for ▁the ▁English ... ▁The ▁enemy ▁was ▁also ▁bound ▁to ▁the ▁narrow ▁water ways ▁of ▁the ▁f j ords ; ▁their ▁ships ▁would ▁have ▁to ▁pass ▁near ▁the ▁U - bo ats . ▁They ▁could ▁only ▁go ▁un seen ▁in ▁very ▁tur bul ent ▁weather . ▁The ▁deployment ▁of ▁multiple ▁u - bo ats ▁in ▁st agger ed ▁formation ▁would ▁likely ▁yield ▁more ▁shooting ▁opportun ities ." ▁ ▁By ▁the ▁beginning ▁of ▁April ▁ 1 9 4 0 , ▁ 3 1 ▁U - bo ats ▁were ▁ready ▁for