▁fired ▁and ▁four ▁U - bo ats ▁s unk . ▁These ▁problems ▁came ▁at ▁a ▁time ▁when ▁the ▁war ▁in ▁the ▁Atlantic ▁was ▁still ▁going ▁exception ally ▁well ▁against ▁Britain . ▁The ▁deployment ▁of ▁U - bo ats ▁during ▁Operation ▁Wes er üb ung ▁is ▁occasionally ▁critic ized ▁as ▁a ▁waste ▁of ▁man power ▁and ▁mater iel ▁for ▁this ▁reason . ▁The ▁use ▁of ▁magnetic ▁p ist ols ▁and ▁operations ▁in ▁cold ▁weather ▁rendered ▁the ▁G 7 e ▁tor ped o ▁less ▁effective . ▁Coast al ▁operations ▁may ▁have ▁also ▁ham per ed ▁command ers ▁used ▁to ▁the ▁open ▁sea . ▁Spe cul ation ▁pers ists ▁that ▁massive ▁tor ped o ▁failure ▁during ▁this ▁campaign ▁prov oked ▁German ▁naval ▁high ▁command ▁into ▁impro ving ▁the ▁weapon ' s ▁performance . ▁U - bo at ▁command ers ▁like ▁Pri en , ▁Sch ult ze ▁and ▁Sch üt ze ▁had ▁the ▁only ▁pat rol s ▁of ▁their ▁care ers ▁without ▁any ▁t onn age ▁s unk . ▁ ▁Hart mut ' s ▁execution ▁First ▁hints ▁of ▁an ▁imp ending ▁invasion ▁of ▁Norway ▁and ▁Den mark ▁came ▁in ▁March ▁ 1 9 4 0 . ▁By ▁early ▁April , ▁most ▁of ▁the ▁U - bo ats ▁to ▁be ▁used ▁for ▁Operation ▁Hart mut ▁as ▁part ▁of ▁Operation ▁Wes er üb ung ▁were ▁ready . ▁Bd U ▁Karl ▁D ön itz ▁inter mitt ently ▁sent ▁ships ▁to ▁recon no it re ▁areas ▁of ▁the ▁Norwegian ▁coast ▁and ▁to ▁try ▁to ▁locate ▁port ions ▁of ▁the ▁British ▁fleet