A long time ago i was loosely involved in submarine "acoustic isolation" measures/rafting. But apart from just transmitting noise/vibration/harmonic pressure waves to/through the hull (which easily becomes a huge bell), there are problems with all kinds of machinery that is directly or indirectly connected to the surrounding seawater, like with pipes and pumps (indirectly and active/passive) or the "active" propeller/screw or a "passive" towed array, both in direct contact to the water column.
Looking for noise reduction in the propulsion screws you could usually hear more or less how much blades the prop had, and even without cavitation this could be quite loud, depending on form and design. Using noise cones or pumpjet systems have done a lot against noise, and also for efficiency, but often have other disadvantages as speed reduction or more complicated maintenance and underlying machinery.
So have their been new ideas about the design of screws/props let's say conventional without using pump jets or noise cones?
This is now three decades ago, but already then we spoke about "soft" filled screws adapting to the pressure, isolating the shaft from the prop, and screws using no blades.
Even if new tech will certainly not be spread publicly, wherever i look now from China, Russia, to the US all still seem to use the good old designs from the 1990ies
