The thing is damage percentage in DW is not very useful. The vessel suffering damage even to 90% are not affected in any way with regard to their efficiency and lethality.
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DW is not so great when simulating damage. it's just numbers without consequences aside now the vessel is more susceptible to being killed.
In RTS games the units come with life bar. In my honest opinion damage in DW with respect to AI controlled platforms is nothing but a life bar with a twist in name only.
I would imagine that even a small diesel electric sub suffering 40% damage would be totally incapacitated. The shock damage to its various war systems, mechanical, sensor and weaponry would deem the vessel basically to be inoperative.