▁operations ▁between ▁England ▁and ▁Norway . ▁On ▁ 6 ▁April , ▁the ▁code word ▁" H art mut " ▁was ▁transm itted ▁and ▁German ▁sub mar ines ▁began ▁their ▁designated ▁operations . ▁ ▁Att acks ▁on ▁al lied ▁vessels ▁German ▁sub mar ines ▁operating ▁near ▁the ▁Norwegian ▁coast ▁made ▁numerous ▁attacks ▁on ▁British ▁ships , ▁although ▁many ▁proved ▁un success ful ▁due ▁to ▁fault y ▁tor ped o es . ▁ ▁U - 4 ▁U - 4 ▁s ank ▁the ▁British ▁T - class ▁sub marine ▁HMS ▁Th ist le . ▁ ▁U - 1 3 ▁U - 1 3 ▁s ank ▁the ▁British ▁steam ▁merchant ▁Sw ain by ▁off ▁the ▁Sh et land ▁islands ▁with ▁one ▁tor ped o ▁on ▁ 1 7 ▁April . ▁ ▁After ▁dock ing ▁in ▁Ber gen ▁for ▁a ▁short ▁time , ▁U - 1 3 ▁then ▁s ank ▁the ▁Dan ish ▁vessel ▁L ily ▁that ▁had ▁been ▁taken ▁as ▁a ▁prize ▁by ▁the ▁British ▁after ▁the ▁German ▁occupation ▁of ▁Den mark . ▁The ▁first ▁tor ped o ▁failed ▁to ▁arm , ▁but ▁the ▁second ▁broke ▁the ▁ship ▁in ▁half . ▁A ▁few ▁days ▁later ▁U - 1 3 ▁dam aged ▁another ▁British ▁ship ▁before ▁returning ▁to ▁port . ▁ ▁U - 3 7 ▁U - 3 7 ▁s ank ▁one ▁Swedish ▁tank er , ▁one ▁Norwegian ▁fre ighter ▁and ▁one ▁British ▁fre ighter ▁equ alling ▁ 1 8 , 7 1 5 ▁tons ▁of ▁sh ipping ▁between ▁ 1 0 ▁and ▁ 1 2 ▁April .