on fuel usage, remember the U-297 in late 44 would have travelled almost the whole way underwater, so the surface fuel consumption is not really applicable.
second, from what I read in Blair, U-Boats only used snorkels to recharge batteries, say around 4 hours out of every 24 and travelled the other 20 hours on batteries, so the fuel usage would be even less.
a similar boat, the U-299, left Trondheim 24/01/45 and came back 82 days later on 15/04/45. U-297 was only out about 12 days, so it may not have used up much fuel.
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