No vulnerabilities shown in targeting manual?

[Sailor Steve]

Originally Posted by Zorrin

Does the Blue area mean if I get a torpedo there using a magnetic pistol I'll kill it in one go? Or is it more that this is a good place to have your torpedo hit if you want to inflict as much damage as possible?

That's the main part of the keel. You might break the ship's back if you aim a magnetic-fused torpedo below the keel.

Bear in mind that in real life they had photos of the ships, not convenient directories on best hit locations. Of course any submarine captain had years of experience and would know that instinctively, so it's not really a cheat.