[vdr1981]

Engine room on most SH5 ships is generally located slightly behind superstructure, that's where you should aim...

The best solution is to use magnetic detonators so that torpedo can detonate below keel and slightly behind superstructure. That way keel, two flotability zones(port/stb) and engine room damage zones will be destroyed from single torpedo and there is no way that ship can survive...However, this can be tricky with unreliable magnetic detonator at the beginning of the war...