I believe they still shroud them. And I vaguely recall reading about a museum that received a retired prop design; they had to vertically mount it for display partially underground, as the Navy wouldn't let them cut the bottom blades since there was something still secret in the construction of the casting. My guess is it was hollow like jet fan blades

Even if the design principals are known the acoustics of a particular sub are not and can be inferred from the design.

That's funny, because I remember watching a documentary about some form of caterpillar drive.

That was the one that was "narrated" by Sean Connery?

the hishtorically accurate shientific account of a rushian boat being reshcued?

The order ish, engage the shilent drive.

One ping only though

I would have liked to see Montana.

And all the while, the captain had a mouthful of shtew.

IIRC submarine screws use prime-numbered blades (5, 7, 9) to cut down on harmonics. Perhaps the same would be useful here, though I don't see many prime-numbered turboprops, which you'd expect to benefit from similar noise reduction. Related, I highly recommend HI Sutton's youtube channel if you're into submarines. https://www.youtube.com/watch?v=ugSEIiTZ1Pg

A magnus effect drone may have a decent chance at being quiet. IANAAeroEng but I'd guess that the moving part's interaction with the fluid can be made laminar or at least non-turbulent, and also unlike some propellers, the moving part shouldn't be approaching or interacting with the sound barrier. https://youtu.be/hlmvHfIAszo?t=16

However my money's on ionic propulsion drones since they have no moving parts to make noise. I wonder if the air accelerated through the grids produces an audible hiss. https://youtu.be/UGM4JXVB5FM?t=126

Ionic propulsion requires massive power. It's cool but you'll probably never see it in real life applications.

Clarke's first law applies here. Maybe beamed power could help around a delivery hub.

I love when primality is useful. Like cicadas. But 9 is not prime.

Whoops I meant to write 11, not 9. I haven't actually heard of an 11-bladed screw, though here's a picture of an 11-bladed pumpjet. https://youtu.be/ugSEIiTZ1Pg?t=386

In wartime it is, and also equals to pi squared.

>IIRC submarine screws use prime-numbered blades (5, 7, 9) to cut down on harmonics. Perhaps the same would be useful here, though I don't see many prime-numbered turboprops,

Might be just odd-numbered ones, not neccesarily prime. But other interesting case is how car tyres have variable feature size on the tyre so the produced sound is more spread over the spectrum and appear quieter

> However my money's on ionic propulsion drones since they have no moving parts to make noise. I wonder if the air accelerated through the grids produces an audible hiss. https://youtu.be/UGM4JXVB5FM?t=126

That would be literally shocking (and changes properties heavily based on humidity) so I'd think use would be limited, it's not exactly great way to lift anything heavy

That ionic propulsion drone/startup you linked to is a scam. Their drone has a ducted fan in the center that's providing the thrust.

Re: closely guarded submarine secrets, I always found this whole incident and the surrounding events just fascinating [1]; It involves giant machine tools, computer control systems, and a lot of international cold war politics.

[1] https://en.wikipedia.org/wiki/Toshiba%E2%80%93Kongsberg_scan...

From page 2 of that link: "Once they reach a certain speed, the blades begin to create a partial vacuum, which results in air bubbles. This is a state known as cavitation."

How does a vacuum result in bubbles? Wouldn't the water just clap back and close the vacuum?

Those bubbles do collapse, but the problem is that as they collapse they generate tiny little shockwaves. Apart from being noisy, these implosions can be quite destructive—the constant bubble collapses are fairly energetic, and cause cyclic stresses on materials. Over time, these can actually eat away at the surface of the propeller [0].

[0]: https://en.wikipedia.org/wiki/Cavitation

Think of what happens to a fluid when exposed to a vacuum.

The water surrounding the vacuum instantly vaporizes and creates bubbles of water vapor. Then these small vacuum bubbles collapse and create small shockwaves that create noise and damage the propeller/screw.