which is the easiest way to sinking ships in GWX3?

[JT1981]

if i use torps to attack enemies,the magnetic or the contact fuse,which is the best that i should choose?

bty,i heard that torpedos exploded below the ship's keel will be more powerful and make more damage,is it true in GWX3.0?because i usually used contact fuse and found it is not powerful enough,even though a 4k tons ship needed 2 torps