That seems to be classified.
There's a "NSfuelbunker" and a "FuelBunkersB" and just plain "FuelBunkers" in the zones.cfg file, the zone "spheres" and "boxes" in each individual sub's zon file are unlabeled and nothing in the dat files gives any hint, so I dunno where the fuel tanks are.
"I would like to play around with a "harder to destroy" but not "indestructible" rudder and props."
Doubling the hit points might help there, main reason I hacked them to ridiculous values is because there's no recovery from destroyed propulsion/steering - even if the rest of the sub is repairable there's no point, if you can't move or steer you can't get back to base for repairs. Having all torpedo tubes and/or both periscopes destroyed means you have to cut the patrol short and return to base, but if you can't return to base the career is over.
Just took a look, stock depth charge is 170-230 hit points, stock rudder is 150 hit points, so a single charge within 30 meters behind the sub and your rudder is gone, no point in continuing. Try jacking it up to 400 points, see if that results in repairable damage.