If I remeber correctly in SH3 8 knots is the optimal speed for IXs. In reality IX boats used to spend a couple of weeks patroling before heading home. If the weather is good (i.e. you travel at 8 knots) and you don't need bursts of speed or to recharge batteries you can make it back with fuel. If you encounter loads of targets and you have to avoid DDs then there is a chance you will need to stop by one of the U-Tankers.
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