1. Someone will correct me if I'm wrong but I'm fairly certain that by the time you see a ship's hull in your periscope from underwater you are WAY too close to even use your torpedoes. Even if they did work, you are almost guaranteed to waste at least two torpedoes out of a shot of four unless you converge them which, again, makes no sense and is a waste of your ordnance.
2. Why would you want to set up an attack like this anyway? In the same time it would take you to do this you could manoeuvre left or right and have a beautiful 90-degree shot from your aft tubes. You would just have to sit and wait for the ship to cross your path. In other words why set yourself up to hit a small target (a skinny bow) when you can set up for a large target (a nice fat side of hull)?
3. If you are so close that you have to "crash-dive" then;
a. Your watch crew is doing an extremely lousy job at spotting
targets, and
b. The merchant will NOT be continuing straight towards you
anyway but will be zig-zagging rendering a nose-on shot both
impractical and that much harder.
4. In other words it would appear to be an extremely poor tactical choice for an attack.
