The swedish Gotland-class submarines are know for their stealth. This is achieved by using stirling engines and other improvements. See wikipedia on them: https

The swedish Gotland-class submarines are know for their stealth. This is achieved by using stirling engines and other improvements. See wikipedia on them: https://en.wikipedia.org/wiki/Gotland-class_submarine https://en.wikipedia.org/wiki/Gotland-class_submarine I think the reason that nuclear subs are hard to make quiet is that the reactor needs quite a lot of supporting machinery, which tends to make noise (pumps etc. for cooling), and these can't be quickly shut down. Hence diesel-electric subs can be a lot quieter (but of course can't win in endurance). But a silent sub can still do a lot , especially if it's targets really can't quiet themselves...