I sank a T3 tanker in rough seas yesterday. No fog though, which is good. I fired two electrics, one for the bow and one for the fuel tank. The first shot hit the bow, didn't appear to sink much into the water though since the sea was rough. I couldn't really tell. I didn't want to wait for it to sink itself, so I sent a second electric towards the fuel tank. At this point, I thought that if this torp doesn't make a clean hit I'd have to fire a third. So, impact was made on the fuel tank. Some small explosions went off. It didn't break in half but it did go down in one piece. A shot to the bow and then a shot to the engine or fuel tank worked out nicely in this case.
I'll have to see if it'll work on other large merchants.