[PhasmaFelis]

Which explains the shapes of submarines in the two eras: they're optimized for their preferred mode of operation. Diesel submarines were shaped approximately like a regular ship, with a angled and tapered prow to cut the waves. Nuclear subs are shaped like a torpedo, roughly cylindrical with a rounded prow, to move through deep water.

[brandmeyer]

The shift in thinking about submarine hulls occurred at about the same time as changes in propulsion, but they were separate developments. USS Albacore was the first to prototype the hull shape. A short-run class of diesel-electrics were designed to that shape before being completely overtaken by nuclear propulsion in the US.

All modern submarines, including AIP, diesel-electric, and nuclear, use hull forms optimized for underwater operations.

[rtkwe]

There are some neat ones employed by Sweden that use Sterling engines powered by liquid oxygen and diesel which is pretty neat and makes them really quiet.