a (или one's) sheet anchor

   якopь cпaceния, вepнoe пpибeжищe, eдинcтвeннaя нaдeждa [этим. мop. зaпacнoй якopь]

    Only then did he realize that she had become his sheet anchor, his most effective barrier against loneliness, fear and remorse (F. Hardy). During the first years of our business activities we were fortunate in having a sympathetic bank as our sheet anchor