I'm not a fluid mechanics guy, but all that cavitation from the screws seems like preventably wasted energy, not to mention a huge source of potential surface wear.

Yes you're right.

I'm surprised PBCF isn't more common. It essentially reduces swirl and cavitation in the central vortex, converting it into additional thrust while also reducing shaft torque. It's an inexpensive retrofit (payoff time <6 mo) with broad compatibility, and the swap can be done while unloading with no downtime. And yes, it reduces rudder surface erosion which is caused by cavitation.

Seems like a no-brainer. Maybe folks just aren't aware of the technology?

https://www.youtube.com/watch?v=a8PjWSeLUzY

Wow, that's awesome! Looks like a big win all-around. Save money, save fuel, save wear.

Fascinating, I'm really into boating and sailing and have never heard of this- no consumer grade engines or boats that I know of have this.

A lot of the noise is from the combustion engines themselves- exhaust often releases underwater, and the engine vibration transmits through the hull.

Correct on both counts.

Nuclear submarines feature very large, many-bladed, specially-shaped propellers often turning very slowly to reduce both noise and wear (though more the former: stealth is key to submarine operations).

Cavitation on hydraulic and steam turbines, as well as other hydraulic mechanisms, is a major source of wear and failure. Keep in mind that cavitation is equivalent to boiling, and involves both high (if localised) temperatures and extreme pressure fluctuations as cavitation bubbles form and collapse.

A couple of discussions from both perspectives:

<https://archive.navalsubleague.org/2012/a-century-of-america...>

<https://www.sciencedirect.com/science/article/abs/pii/S00298...>

For nuclear submarines, overall efficiency isn't termendously important, as when the sub needs to move quickly the powerplant can turn out a heck of a lot of power.

Checking just now to confirm powerplant output, I'm learning that the Virginia class of hunter-killer submarines, for which both quiet operation and speed for pursuit are concerns) use pump-jet propulsors rather than traditional bladed propellors, specifically to reduce cavitation:

<https://en.wikipedia.org/wiki/Virginia-class_submarine#Propu...>

Top speed is in excess of 25 knots. (I suspect considerably in excess, though

Powerplant is a 210 MW nuclear reactor, 280,000 HP equivalent. By contrast the Maersk E-class containership (largest in the world until 2012) have an 81 MW (fossil-fuel based) powerplant, 109,000 HP equivalent. The still-larger Triple-Es have two 31 MW engines (for lower total output), and the current size giant CSCL Globe 69.7 MW.

<https://en.wikipedia.org/wiki/E-class_container_ship>

<https://en.wikipedia.org/wiki/Triple_E-class_container_ship>

<https://en.wikipedia.org/wiki/CSCL_Globe>

And the fastest known military submarine seems to have been the K-222 Soviet Project 661 "Anchar", which could reach 41-45 knots (about 50 mph) submirged. The K-222 is slightly shorter than the Virginia class, and as hull-length has a strong effect on top speed (see hull speed: <https://en.wikipedia.org/wiki/Hull_speed>, though how this relates to submarines I'm uncertain, see also Froude number: <https://en.wikipedia.org/wiki/Froude_number>), I suspect Virginia class boats could likely achieve similar speeds should the need exist.

Fascinating, thank you!

According to this (highly interesting) paper I just read, it seems that the operating sounds of wind turbines come from the gearbox, which makes more sense to me because the blades of offshore turbines are turning very slowly. Also according to the same paper, the operating noises of offshore wind farms are probably not significant.

https://www.int-res.com/articles/meps2006/309/m309p279.pdf

As an aside, I love discovering a researcher with such a delightful niche. How many people are out there researching "Acoustically mediated fin whale mating"? Amazing humans.

My experience disagrees with that claim. But I suggest an easier way to verify this, which is to just go to YouTube and look at videos of the sound produced by wind turbines on land. You will hear the distinct loud woosh sound of the actual blades and not the gearbox. Personally, I feel a lot of the academic denial around this noise issue is from people with an activist goal.

YouTube videos are not very difficult to edit. If you search YouTube for “noisy windmills,” I’m sure you’ll get… something.

Anyway windmills are all over the place, lots of people have had the experience of being near them. If they make too much noise, that’s a problem of course, but I think lots of people can verify the existence of non-noisy ones by virtue of having seen them in person. So, sure, loud designs should be avoided since there are perfectly good alternatives.

> YouTube videos are not very difficult to edit.

And academic papers are not difficult to fudge up with edited fraudulent data [1].

Even Ivy League colleges are committing rampant fraud sometimes for activism sometimes just to get attention and get more papers published.

It’s sad that we’re at a time where academia is as untrustworthy as random videos on YouTube.

But i agree with noiseless windmills part, they should be promoted more instead of noisy ones that will discourage more clean energy initiatives.

[1](https://retractionwatch.com/)

I mean, nobody would say the academic publishing ecosystem is super healthy (every academic hates it, for one). But the existence of something like “retraction watch” points to the fact that these aren’t really comparable ecosystems. Nobody need bothers to catalogue the lies on YouTube because that’s an impossible and pointless task.

The academic process has always been messy and political, and yet universities do keep churning out research and students.

> The academic process has always been messy and political, and yet universities do keep churning out research and students.

The youtube process has always been messy and political, and yet new creators do keep churning out great content that people love watching and spreading awareness about various issues.

:)

The youtube show : “Undecided by Matt Ferrell” is my favourite

Lotta clickbait, too. They need it to keep earning, even if they don't like it.

I recall Tom Scott saying there was one video he basically couldn't make, because the climax was switching on a big light, which was actually silent, but it felt wrong unless he dubbed a sound over that and such dubbing was against his editorial standard.

That’s interesting. Without knowing what the switch was, I guess it is hard to say, but a switch that kicked off some big process silently seems like it could be really dramatic and cool.

You seem to have missed the point of the research, which is all about underwater sound propagation.

Wind turbines are moving a lot faster than they appear. E.g. Vestas v164 [1] rotates only at max 12 times per minute so looks slow. But because it has 80m blades, the tip speed is up to 100 meters per second (230mph). That's 1/3rd the speed of sound! That can generate a lot of noise.

[1] https://en.wind-turbine-models.com/turbines/1419-mhi-vestas-...

That explains why they would make broadband noises but not peaky high-q noises like the ones in the paper.

US Navy (and certainly others) kill many dolphins, whales and other sea life with their high powered sonar.

US also has high powered ultra low frequency radio (ELF) for communication to subs and that's got to be insane to wildlife.

https://en.wikipedia.org/wiki/Project_Sanguine

sound and radio are of course waves in completely different media. why would low frequency radio waves be insane to wildlife? it's not like it's microwave (which would cook them) or pressure (sound) which directly physically damages their organs. almost by definition low frequency radio waves are low energy, so I don't see how this would be more dangerous to Flipper than my Wi-Fi router is to me

Well, some people do think their Wi-Fi routers can hurt them ... maybe we should make tinfoil hats for the cephalopods.

Huh, sharks pick up EM signals, right? I wonder what they think of this stuff

But just to be clear, I don’t think we should suppose it is damaging and try to ask the military to stop using it (good luck, lol). It could be worth looking into though.

I mean, we all pick up EM signals. The frequency range is the distinction, I think.

Sure, but sharks use them as a sense. We all pick up photons in the sense that we can all feel the warmth of the sun on our skin, but also, it is sometimes interesting to think about how people who aren’t blind perceive light.

I’m talking about sound not electromagnetics. It’s like a torture for marine life to be surrounded by constant noise. Many also use sound to communicate or locate.

It's one of the things that I find fascinating about Naval tech. From the sonar from each boat to the network of devices like SOSUS net[0], you have to be very good and deliberate to sneak around in the oceans.

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

I've done some snorkeling on coral reefs in the Bahamas, and it's crazy how far you can hear boats. Once you learn what sound they make (it's completely different underwater), you can hear one very clearly but when you pop your head up it's way off in the distance. And this is just for smaller, outboard-powered boats.

I've never really had the chance to hear a big ship underwater, but my home is right by the St-Lawrence seaway (the one they talk about in the article) and when it's quiet at night I can feel the rumble they make through the ground when one passes in front of the house.

Even the above water noise is awful... I am sensitive to noise, and do engineless sailing partly because the sound of motors is awful, and ruins the fun of boating for me.

Try a modern electric motor. There are some really quiet ones now.

when i was a kid we often camped near a river. every morning i heard music but i couldn't identify the source. "it comes from the other side" my uncle said. that day i learned how far sound can travel in the water