Q. regarding propellers and noise - updates?

[Catfish]

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

[Onkel Neal]

Nothing I have heard about. I always assume there is new tech on vessels today, but I won't get a hint of it for 10 years. I still remember laughing when I first heard about the 200mph "super-cavitating" torpedo!

[Aktungbby]

At seventy, the only cavitation I ponder is my teeth! https://wonderfulengineering.com/her...kept-a-secret/ & https://nationalinterest.org/blog/re...minance-164733 https://www.popsci.com/resizer/etaCx...GP5QWOYVC4.jpg

A rim-driven pumpjet has a ring-shaped electrical motor inside the pumpjet shroud, which turns the vane rotor (a vane rotor has the fan blades attached to a rotating band built on a cylinder interior, as opposed to a propeller shaft) inside the pumpjet cavity to create thrust. Previous submarine pumpjets are "shrouded propellers," which consist of a tubular nozzle covering the propeller. By removing the shaft of the propeller, the reduction in the number of moving parts decreases the noise made by the pumpjet, as well as saving hull space. Civilian manufacturers also claim that rim driven pumpjets are easier to maintain, and have less cavitation (bubbles that form during propeller movement), making them even more quiet.

[Catfish]

@Neal yes we of course only learn years later of the newest inventions, no navy wants to give away its advantgages, or hints on how their system works.

@Aktungbby: Interesting links, thanks!

I thought the rim-driven impeller maybe with a stator and a rotor, where the inner moving part (screw) does not even need a coil but just has to be made of a metal 'core' looks promising. Less maintenance, well, as long as the coil in the outer rim is ok, otherwise changing the whole thing might be a nightmare, maintenance-wise.
There are numerous advantages though because there does not have to be any rotary shaft seals, indeed no shaft leaving the hull altogether.

The screw should be much longer though, and have no tips at all, just like the churro example, something we discussed back then. We thought of a "flat" Archimedes-like screw where revolutions probably could not have been counted, moving within a hollow cylinder or tube. Or something looking like those Colani-aircraft propellers, adapted for underwater dynamics.

Interesting that they now think of a "churro" extending over the whole submarine, i wonder how this design would look like. Maybe with a tube running through the whole of the sub, with water intake and outlet, i doubt they would put the moving part on the outer hull