Reloading the torpdo tubes inside the boat was one thing and then you also could reload the external reserves which rendered your boat unable to dive for quite some time.
1. Raise External reserve storage
2. Remove torpedo from external torpedo storage
2a. Lower external reserve storage again.
3. Bring it down into the boat via front torpdo loading hatch
5. Now the torpedo is in the front compartment and can be reloaded into the tube via the usual method.