No one!
Selecting the weak spots is only available if you have "No automatic targeting" turned OFF in realism Settings and are in a specific range to a target. (Dont know which range exactly). The weakspot then appear automatically
For seeing them in the manual without being able to select them (you then have to place the torps manually via periscope and have a solution plotted by the Weapons officer) there is the "merchant vulnerability mod" available from
http://u-boot.realsimulation.com