How many torp this Hog Island took (exploding)?
They are tough, and many times they need 2 to go down...
In fact all SH4 stock ships are stronger than others with letters names (tankers are strong too!)
Sometimes only 1 do the job, but you have to hit the hull right under the funnel (machine room) and just at the limit of draft (magnetic pistol).
Furthermore they never sink right away. We have to wait for the flooding doing its job. But again sometimes the ship crew can manage to repair the damages.
As it was in RL...numerous ships were only badly hurt, and kept sailing.
Keep the good job on! Sink them all!