[KaleunMarco]

Originally Posted by Mios 4Me

Yeah, Balao + 4 more torps is a better solution instead of upgrading. Can't remember if it were you or sniper who showed me how to do that.

could have been me.
you need to perform this only once per boat (Gato, Balao, etc) and while you are in port between missions.
browse to the Savegame folder.
make a copy of the ActiveUserPlayerUnits.upc file.
Open the original ActiveUserPlayerUnits with Notepad.
find:
[UserPlayerUnit 1.Compartment 1.Bunker 1]
ID=SternTorpedoRoom
and change the Capacity to whatever you want.
then find:
[UserPlayerUnit 1.Compartment 5.Bunker 1]
ID=BowTorpedoRoom
and change the Capacity to whatever you want.

you can preload the additional slots with warshots by following the torpedo pattern in the line following the Capacity line:
IDLinkAmmunitionLoaded=

or

wait until you reload that Savegame and add the fish via the weapons screen.

voila!

good luck!