I thought you could seek refuge in any neutral/friendly harbour and start the next patrol from there. There is no flotilla based at Tobruk but that port is okay (you start in a nice berth at the jetty with the bow pointing seawards) - so is there another explanation?
Here I am in Tobruk...:

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Regards
Vermin
Now if I could get some rollers under my U-boat I might be able to get it to the water - unfortunately it just rolls from left to right until it expires!