I've heard the Mk. 48 torpedo is wire-guided, why not make it wire powered? The range would only be limited by the length of the wire. I'm wondering why this is not used and what problems it would have.
The main reason is that the power requirement for running the torpedo is vastly larger than that for merely sending control signals to it. This means much thicker wire would be needed to actually power the torpedo over the wire. Even if cost is not a consideration, a usefully long spool of such thick wire would be very large.
You can reduce the size of the wire by increasing the voltage, which thereby reduces the current at the same power. However, that then increases the demands on insulation, which takes significant space. Thin insulation, like enamel coated, can't stand off much voltage. To make things even worse, this all has to work in the a conductive medium (seawater).
I don't know how much power a torpedo requires, but probably a few kW at least. Let's say 5 kW just to pick something (if anyone knows the real number, please tell us). That could be 10 A at 500 V, 50 A at 100 V, etc. Even considering the heat sinking provided by the seawater, 50 A still requires a substantial wire, and 100 V insulation is going to double the thickness, which quadruples the volume.
I'm going to stop here since figuring out the volume of wire, volume and weight of copper, power loss, and voltage drop requires knowing too many specifics that I can only guess at. However, it should still be obvious that this is going to be a lot worse than the torpedo spooling out double-stranded #30 wire with thin insulation behind it.