Good greif.
Look, its late and im tired so im going to put this in undersimplified terms.
In SH3 you basically have this:
ENG_Power effects, how large a unit of fuel/energy the game will use. It also effects top speed. Increase this number, you engines will suck a largerfuel/energy unit.
Eng_RP are used in AI detection routines (IE, how noisy are you?) and are i beleive how many units of energy are being used exponentially at a given point in time, depending on what settign you define. slow, 1/3rd, etc.
Orginaly the submerged endurance was MUCH larger. So large infact, that you rarely ran low on batteries. With upgraded battery units, you probably never ran out of electricity. I dont recall of ever having run a battery dry in stock SH3.
What was the battery fix? It reduced the maximum range of your undwater endurance. So your power pool to draw from isnt so vast, and thats ALL it does. Everything else remains the same.
Energy expenditure does not fluxuate excpet by what you tell it to do via throttle settings. Your surface range varies by your uboats mass and displacement and its relationship with weather. You see this every time you play. at your crusiing speed you might be doing 9 kts on a clear day and 7 kts in a heavy sea.
Now, lets talk about Hydrodynamics, or rather, LACK THEREOF. Drag, and such is not modled. This means that your not subject to any restriction of forward momentum while submerged. By cruising at a low RPM setting to achieve say 2 kts or so you get the maximum mileage out of your batteries. Your batteries will run dry around the same time you run out of oxygen, maybe a little before that. Anyway because the maximum capacity of the batteries isnt very large, it only takes about 3 hours to recharge them on the surface. Thats only 3 hours of being subjected to drag and running your diesals such to top off a smaller battery that will see you through an entire day and then some if you manage it wisely.