My impression is there is some WWII US realism modeling going on with diesels in that setting the lower speeds takes engines offline and directs them to battery charging making that process a bit faster.
In my opionion, no modding is necessary if the captain just uses smarts and watches the fuel usage. At lower speed settings (slow and 1/3) you have a great amount of range (much more than the SH3 boats.) Although I can see it as attractive to be able to switch to batt on the surface (and I have read real historical accounts where this was done to make the boat quieter while passsing near the enemy and near coasts) in a more practical sense, anything that would make less than full battery available when you get jumped or forced under for extended periods is not a good practice. WWII commanders protected the battery charge like a glass of water in the desert.
On efficiency...
Careful observation of both SH3 and 4 behavior indicates there is a slight fuel extension to running on the battery even if submerged so don't worry you can't do it on the surface, just go ahead and do some battery-range extension submerged anyway. In a tough situation, it might mean the diff between getting home. I did that once in SH3. But again, if you are smart and reserve the amount of fuel you need to get home and don't let youself get distracted, the crew will appreciate getting their mail and you will have a better next mission with a rested crew, skilled replacements, and upgrades to the boat.
-Pv-
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