How a modern heavyweight torpedo threatens a battlecruiser

What damage would a modern heavyweight torpedo do to a Second World War battlecruiser?

It would detonate beneath the keel and try to break the ship's back, though keel damage is not invariably an immediate sinking.

A modern heavyweight torpedo relies on an under-keel explosion rather than a simple hole through side protection. The weapon attacks longitudinal structure in an effort to break the ship's back. A later answer distinguishes a focused charge from an unfocused explosive mass and notes that some Russian heavyweight torpedoes use charge shaping. HMS Belfast survived severe keel damage, so the eventual loss still depends on construction, damage location, and damage control.