The newest US subs use shroud protected pump jet propulsor systems, which already minimize acoustic signatures. Especially when it comes to the cavitation noise from propeller tip vortices, which is what Toroidal props are fixing for "open" propeller designs.

https://twitter.com/toughsf/status/1024083748407713792?lang=...

https://www.researchgate.net/publication/302920496_Numerical...

https://www.thedrive.com/the-war-zone/31708/veteran-sonarman...

https://www.globecomposite.com/blog/x-factor-columbia-class-...

https://www.sciencedirect.com/science/article/pii/S209267821...