Germany started the war with 57 u-boats (30 type 2, 22 type 7A and 7B, and 5 type IXA). Let's assume all 57 were type 21 instead.
Let's also assume that every ship that was sighted by the original 57 was also sighted by the 57 type 21's.
They would have one tactical advantage in that instead of only having 27 boats capable of operating beyond the North Sea, the whole fleet could have done so. That still would not have been a lot of help. Why not? Because the u-boats had far bigger problems:
1) Faulty torpedoes - finding and attacking a target does not equate to having sunk it. Early u-boats fired hundreds of eels that hit and failed to detonate. Our theoretical 1939 types XXI's would have the same problem.
2) No radar to find targets in the vast ocean - what good is a superior weapon if it doesn't encounter something to shoot at? The convoy system would not come into existence until 1940. Ships were sailing solo, so there was no real advantage to having 6 or 60 torpedo tubes that could reload in 15 minutes.
Even after the convoy system began and escorts appeared, there was no real advantage to the type 21 until the torpedo problems were overcome.
Only after that would there be a significant advantage to the 21's with their high underwater speed and fast reloads......but only when they found a convoy.
A type 21 in 1939 or no, you might as well ask the question "what if Germany had possessed nuclear weapons in 1939?". Answer - the whole world would be speaking German today. They didn't, and we don't.
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Raptor
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