The maximum Diesel speed of the XXI is a complex story. On the surface using full Diesel power it was 15.2 knots. The engines produced 2 x 2000 HP, but there were mechanical losses of 5% in the system plus the electric motors took away another 7%. There was no way to bypass the E-motors, so they were always turned through when the Diesel was running. This 7% loss could be compensated by using some electricity, so the surface speed becomes 15.7 knots. Actually, the superchargers ran fine when snorkeling, but the engine was very sensitive to pressure changes caused by momentary loss of air flow , so the valve timing on the engine was changed to make it less sensitive. Running on the snorkel valve timing, the maximum Diesel engine power was only 2 x 1400 HP. Since surface operation was rather academic by late 1944 and snorkel speeds over 6 knots were accompanied by a lot of problems, mainly vibration, 2 x 1400 HP were enough. The supercharger was not even installed in later XXI which led to a snorkel power of only 2 x 1200 HP and a surface power of 2 x 1400 HP. The theoretical snorkel speed at 2 x 1200 HP was 11 knots, but it could not be achieved in practice. All this suggests that it was probably common practice to run the XXI on the creep motors while the Diesels were used 100% to charge the batteries.

Considering that snorkel development did not start until 1943, it clear why they were so primitive that only 6 knots were possible. Developing good snorkels takes a lot of empirical work and hence time.

Regards,
SuperKraut