There is no evidence of submarines ever successfully escorting transports by using defensive fire. Submarines were successful because its enemy did not know where it was. Tie them to a fleet of transports...

The Germans did not possess nearly enough Type VII or better boats to provide anywhere near adequate escort, time on station and return and to provide escorts for later transports bringing basic supplies like POL, rations and ammo. For Operation Drumbeat, the Germans could only commit only 5 (or was it 6) boats for it's operation off the coast of the US in early 1942.

A large enough fleet transport to even carry enough German divsions to make a dent would not have gone unnoticed by the British as they assembled and would have been harrased by air, surface ships and their own subs well before they left German-controlled waters and all the way across the North Atlantic, not unlike PQ-17.

The Norwegian Campaign of 1940 showed the effectiveness of German sea-borne invasions. While it was ultimately successful, it wasn't without horrendous cost to the DKM.

I see no way this scenerio could get past the "what do y'all think about this?" phase.

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Best Regards,
JW