Funny math. U-85 missed with her stern torpedo and was eventually sunk. That is case 1 where defense didn't work. The O-21 used her stern torpedoes against the following, then pursuing U-95. That is case 1 where defense did work. The Tang successfully used her stern tubes against ships that attempted to ram her and also against a destroyer firing on her as she was making her escape. That is case 2 where defense did work. The Spadefish missed with her stern torpedoes but forced a destroyer to zig and attempt to depth charge her but she escaped. That is case 3 where defense did work. So what was the Norwegian sub going to do to defend themselves? Use harsh words to discourage them? Now you're just being silly. That torpedo tube is there for offense and defense. A pursuing ship on the stern of a sub would qualify for a down the throat shot. Or no different than the E-D firing her aft torpedoes to slow down or blind a pursuing enemy ship to affect an escape. You're oversimplifying it. For daytime and nighttime periscope attacks that would be true but not for daytime and nighttime surface attacks. Depending on visibility conditions the WW2 submarine could choose to fight on the surface and utilize her faster surface speed. I was speaking in reference to the sub conducting combat on the surface. This occurred during mostly nighttime when conditions in WW2 allowed for surface attacks. According to "Naval Weapons of WW2", p264 by John Campbell the T5 torpedo was in use in Sept 1943 and used 640 times with 58 hits (9% hit rate) against typically "difficult" targets. This is compared to the aggregate hit rate of over 20% for other German torpedoes. That doesn't sound experimental or used only a handful of times. You should read "US Submarine Operations in WW2" by Theodore Roscoe. It's a fascinating account of combat operations in WW2 and it's not all "daytime periscope" combat as you'd like to make it out as.