GF19? That's way down there...not too far from the southern tip of Africa. I know a Type IX could make the trip. As for a Type VII.....? I agree, even with maximum fuel conservation, you'd be pushing the envelope. The re-supply ship at the Canary Islands might still be there in Sept 43'. If it is, you should be OK...if not...?? If you are using GWX...you will not get a penalty and lose renown for patrolling your assigned area. I rarely "follow" the patrol orders. I go where I please....
