Both have their advantages and disadvantages. The direct Diesel drive as used by the U-boats was first and foremost lighter and much less raw material intensive. There was an extreme copper shortage in Germany. Having both E-motors and generators required about twice as much copper per horsepower as having just the E-motor. Further, the installed electric power was only about ¼ of the power of the Diesel, so the direct drive wound up using only about 1/8 of the copper which would have been needed for a Diesel electric drive. Also, the overall mechanical efficiency of the direct drive was a bit better than Diesel electric.

In the case of the Type XXI, the copper usage advantage for the direct drive was less than ½ since the electric motor horsepower was a bit greater than the Diesel and it had two creep motors which were separate from the main drive system. The XXI was essentially a Diesel electric up to 6 knots.

The main advantage of the Diesel electric was operational simplicity and flexibility and quieter operation. The direct drive transmitted the vibrations of the Diesel engine directly to the screw. Electric motors are almost vibrationless.

The direct drive U-boats could run up to ¾ power (3,000 HP Diesel minus 750 HP generating capacity) while running at full battery charge simple by having the Diesels turning the electric motors (in generator mode) and the screws at the same time. Power management on an XXI got a lot more complicated above 6 knots.

Regards,
SuperKraut