▁help ▁mit ig ate ▁damage ▁and ▁flo oding ▁from ▁damage ▁inf lic ted ▁by ▁a ▁weapon ▁and ▁to ▁preserve ▁a ▁ship ' s ▁mob ility ▁by ▁phys ically ▁separ ating ▁the ▁engines ▁and ▁bo ilers ▁into ▁at ▁least ▁two ▁groups ▁so ▁that ▁a ▁single ▁tor ped o ▁hit , ▁for ▁example , ▁could ▁not ▁flo od ▁all ▁of ▁the ▁bo iler ▁or ▁engine ▁rooms , ▁dis abling ▁all ▁of ▁the ▁ship ' s ▁prop ulsion ▁mach in ery . ▁A ▁single ▁World ▁War ▁II ▁tor ped o ▁hit ▁would ▁typically ▁blow ▁a ▁ ▁hole ▁in ▁the ▁h ull ▁and ▁would ▁comprom ise ▁the ▁integrity ▁of ▁adjacent ▁wat ert ight ▁bulk head s ▁over ▁twice ▁that ▁length ▁and ▁further ▁if ▁the ▁ship ▁was ▁of ▁riv et ed ▁construction ▁rather ▁than ▁w eld ed . ▁This ▁would ▁usually ▁flo od ▁two ▁compart ments ▁and ▁possibly ▁three . ▁ ▁The ▁unit ▁system ▁invari ably ▁added ▁length ▁to ▁accommod ate ▁the ▁additional ▁pip ing ▁to ▁cross - connect ▁the ▁engines ▁and ▁bo ilers ▁and ▁the ▁widely ▁separated ▁bo ilers ▁required ▁two ▁fun n els ▁which ▁reduced ▁the ▁fields ▁of ▁fire ▁of ▁the ▁ship ' s ▁anti - air craft ▁guns ▁and ▁added ▁top weight . ▁There ▁could ▁be ▁significant ▁knock - on ▁costs ▁as ▁well . ▁For ▁example , ▁the ▁second ▁batch ▁of ▁the ▁British ▁Le ander - class ▁light ▁cru is ers ▁of ▁the ▁ 1 9 3 0 s ▁( which ▁were ▁all ▁eventually ▁sold ▁to ▁Australia ) ▁were ▁modified ▁to ▁use ▁the ▁unit ▁system