I couldn't resist adding my two cents.

'normal' sent my antennae up a bit.

Indirect drive was the standard form of propulsion for US boats in WWII, though the older, pre-fleet boats used direct, mechanical drive (the S-boats, for example). I believe that the US was unique in this regard. German U-boats generally used direct, mechanical drive systems: I believe that the other major navies (Italian, British, Japanese, etc) did as well, though I'd love to hear from anyone with specific info.

The US adopted indirect drive after attempts to make direct drive work fell victim to continuing mechanical vibration issues when the larger, fleet boat designs of the US Navy necessitated four instead of two engines. hooking two engines up to one screw with clutches etc just never seemed to work. Indirect drive also allowed greater packaging flexibility and better fuel economy (the driving engine could run at optimum fuel economy speed more often - the same reason that hybrid cars get better mileage). during the war it turned out to be much more damage resistant and tolerant.

direct drive was preferred in the two engine German boats, partially out of simplicity, but also because the indirect system required more weight and, especially, copper - which the US had in great supply but was critically short in Germany.

Which was normal is a matter of perspective.

Regards,

Kurt