time ▁U - bo at ▁commander . ▁U B - 4 ▁soon ▁joined ▁the ▁other ▁U B ▁I ▁boats ▁then ▁compr ising ▁the ▁Fland ers ▁Fl ot illa ▁( ), ▁which ▁had ▁been ▁organized ▁on ▁ 2 9 ▁March . ▁When ▁U B - 4 ▁joined ▁the ▁fl ot illa , ▁Germany ▁was ▁in ▁the ▁midst ▁of ▁its ▁first ▁sub marine ▁off ensive , ▁begun ▁in ▁February . ▁During ▁this ▁campaign , ▁enemy ▁vessels ▁in ▁the ▁German - defined ▁war ▁zone ▁( ), ▁which ▁en comp ass ed ▁all ▁waters ▁around ▁the ▁United ▁Kingdom ▁( including ▁the ▁English ▁Channel ), ▁were ▁to ▁be ▁s unk . ▁V ess els ▁of ▁neutral ▁countries ▁were ▁not ▁to ▁be ▁attacked ▁unless ▁they ▁definit ively ▁could ▁be ▁identified ▁as ▁enemy ▁vessels ▁operating ▁under ▁a ▁false ▁flag . ▁ ▁U B - 4 ▁k icked ▁off ▁operations ▁for ▁the ▁new ▁fl ot illa ▁when ▁she ▁depart ed ▁on ▁her ▁first ▁pat rol ▁on ▁ 9 ▁April . ▁The ▁following ▁day , ▁she ▁s ank ▁the ▁first ▁ship ▁cred ited ▁to ▁the ▁Fland ers ▁Fl ot illa . ▁The ▁ 5 , 9 4 0 - ton ▁British - flag ged ▁, ▁which ▁had ▁been ▁char tered ▁by ▁the ▁American ▁Commission ▁for ▁Rel ief ▁in ▁Belg ium , ▁was ▁head ed ▁for ▁Nor folk , ▁Virginia , ▁United ▁States , ▁in ▁ball ast ▁after ▁deliver ing ▁relief ▁supplies ▁to ▁Rot ter dam . ▁U B - 4 ▁came ▁upon ▁the ▁ste amer ▁between ▁Har wich ▁and ▁the ▁Hook ▁of