Batteries take a long time to recharge. Hence, when transitting, I try to be on the surface (or using schnorkel) as much as possible to preserve battery. When forced under by planes, I run at the slowest possible speed (ahead 1/3rd?) and I find that the battery is only about 1/5th used when I need to surface for air. Fresh air (i.e., resetting co2 levels) comes quickly, and hence is no big deal. Recharging a completely run-down battery can take a long time, so it's not good to burn it out while transitting - just stay under long enough for the planes/attackers to go away(like an hour or two), then surface and try to get recharged and out of the area. The logic is, when you encounter a TF or a fast-moving convoy, you will probably have to dive and run in as close as possible (i.e., running flank on battery) to get a good shot, since you can't overhaul them. You will need a full battery to pull this off along with the evasion that follows.
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