I don't understand all this irrational romanticism about a failed weapon. Germans used deck guns because they couldn't carry enough torpedoes to get the job done. The deck gun was an admission that the war was already lost without a better U-boat, one that wasn't coming. Had the Type XXI ever seen battle it would have done so without a useless deck gun.
Also, very early in the war, armed merchies were non-existent and U-Boats were going to black holes where warships wouldn't oppose them. That didn't last long as, the British reasoned the equation out. Hundreds of hits from the submarine to sink a merchie. One hit from just about anything bigger than a popgun to the sub's pressure hull and they're an artificial reef with fish food.
German submarines, being unable to carry enough torpedoes to make a difference, were forced to use deck guns in really dangerous situations. The much better designed American submarines came loaded for bear with torpedoes and did the math to see that deck guns were more of a danger to their user than their target.
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