Question: Stock SH4 Ship weak spot?

[Mday]

Is it a matter of chance that a torpedo hit at a ship's weak spot can kill the ship in 1 hit? or is it a guarantee that the ship will sank as long as the hit has indeed landed on the weak spot? So for example, if a torpedo hit a battleship under it's b turret but didn't trigger a magazine explosion, does that mean the torpedo didn't hit the right spot? Or is it just by chance the magazine explosion don't get triggered and there is nothing wrong with the captain's aim?

Also would like to know the week spot of Hiryu.
I know that a hit right under its funnel can disable the ship.
I have also notice that sometime a hit under those aircraft can trigger some kind of secondary explosion and sank the ship. However since it doesn't happen everytime and I have never sank a hiryu with just 1 torpedo, I am wondering where is the exact weak spot located. So far the best result I have is one hit at the funnel and another hit at the aircraft for a 2-hit kill.