The big difference between 39 and 43 is the detection capabilities of the escorts. Early U-boat aces liked to slip through the screen on the surface at night and ride between the columns selecting their targets and firing point blank. Later in the war it became very difficult to even come within contact of a convoy because of the cracked Enigma code and then because of Huff Duff, Radar, better tactical doctrine, and a much larger pool of available escorts a successful attack was a feat both of genius and luck.
The early war is where the noobs love to play and the late war is where the old salts spend their entire career. Night and day is a very apt expression for this.
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