I don't think German boats were lower quality than those of the other belligerants, and their ability to survive depths that would have destroyed most Allied boats testifies to this. Most German designs were, however, more primitive than enemy designs. This was particularly so in the area of habitability. (U.S. boats had air conditioning, freezers, refrigeration, ice cream machines, and were even capable of producing enough fresh water to allow showers.)

The smaller German designs imposed limitations on the designers. American subs had nothing but trouble with H.O.R. engines (a license built MAN design), and some German boats using the same basic engine didn't do much better. Others worked fine. German boats were prodominantly twin engine types using motor/generator propulsion. (On the surface the diesels drove the shafts and the motors were used as generators.) The U.S. boats were diesel-electric. (The diesels drove generators and separate motors drove the shafts.) Fleet boats had four engines (five, including the auxiliary), and four motors (2 per shaft).

The Germans had better torpedoes (or, at least, when they discovered problems, they got them fixed quickly and didn't spend the first year and half of war shooting duds). The U.S. had a much more sophisticated Torpedo Data Computer. The best torpedoes, from the standpoint of effectiveness as ship killers, were probably Japanese (with the 24" destroyer torpedo the champion).

Each country played to their strong points. From the standpoint of using their boats to the most of their potential, Germany and the U.S. (after mid-1943) were the most effective. Japan was the most wasteful of submarine potential, though the blame for that seems to be well above the level of the actual commanders.

J.T. McDaniel