What turtleback armour is trying to save

What is the purpose of turtleback armour?

It is intended to protect machinery and magazines from penetrating fire, preserving mobility and reducing the risk of catastrophic loss.

The scheme accepts that a ship may suffer damage above the waterline while protecting the spaces that keep it moving and prevent a catastrophic magazine loss. It therefore emphasizes survival and mobility rather than guaranteeing that every fighting function remains intact.