I think 8 knots will give you the best range in a VIIc and will probably get you there and back without refuelling (subject to weather conditions), including spending some time on actual patrol. I believe Doenitz himself was surprised that a type VII boat could undertake patrols on the US eastern seaboard without the need to refuel. However, the crew did make some sacrifices in that every space was crammed with supplies and even tanks normally used to store fresh water were used to hold fuel oil.
Nemo
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"I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic".
Sir John Slessor GCB, DSO, MC, DL
AOC-in-C Coastal Command RAF
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