Usually if you cant see the target because of bad weather its sometimes best to pass the target(s) up. Convoys it may result in you getting sunk but it also means you should be able to escape on the surface
If the torpedo does not have the range to arm itself it is just a 1+ ton paper weight. Occasionally you can get away with a 100 meter shot when the torp is set on impact but you will sustain light to moderate damage, severe if unlucky.