Type II makes it
kiel-canal-AN14 @ 1/3rd speed surfaced
patrol AN14 mostly creeping submerged for 4-6 days
and back to kiel at 1/3rd

(I tried, and even entered Scapa Flow that way)
If you want refueling operations, then you are better of with SH3commander and after entering cuxhaven you make sure, that the patrol grid is set back to the original setting of your patrol via SH3commander and that you are only spending 1 day in harbor
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SH3 battle capability upgrades: GWX2.0, JSGME2.0, SH3Commander 2.7, SH3Gen 0.8.2, TorpDamageMod 2.0, OLC GUI 1.1.5
Awaiting combat readiness for GWX2.0: SH4 effetcs for SH3, SH3Weather 1.5
Following development of:
www.subwolves.com
Realism: 90%