Can I Mod Using Just The UPC Files?
 

I've been using TT's tweak utilty to alter the sim and zon files, but I'm wondering if I can just do the same thing with the upc files?

Basically, I'm just trying to increase the speed, hitpoints, and attack damage of my vessels. I'm also trying to increase the amount of shell ammo for all the deck guns. Is this possible or do I need to stick with what I'm doing? Thanks!

Sub speed: sub's .sim file, found in Data\Submarine\USS_Subname folder
Sub hitpoints: sub's .zon file, found as above, AND some of the .upc files in the UPCData directory (I STILL don't remember which subfolder...damn my memory sucks!) for the individual compartments/subsystems.
Attack damage: US_Torpedo.zon file, in Data\Library, and Shells.zon in the same dir.
Deck Gun Ammo Capacity and Rate Of Fire: Each gun's .sim file, in Data\Library\USSubParts.

UPC Files
 

UPC files are good to change the available subs by flotilla, change availability of sub equipment, conning towers, weapons, ammunitons and upgrades. To tweak the values for speed, power, etc, you need to use the SubSim Tweak.