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Modern ships are loud.

So loud in fact, that it is actively harming ocean life.

Now, this alone is a serious issue that needs to be addressed. However, in my world the issue is compounded due to the presence of Mer folk. They are your standard "anatomically and scientifically possible" mermaids that just so happen to have incredibly advanced bio-technology. Such as mollusk TV screens, neural internet connections, handheld dolphin sonar guns and enough biological WMDs to make any land power to take them seriously.

Thus, when they started complaining about the noises when faster ships that used propellers and not sails started to become more popular, the powers that be agreed to find another method of ship propulsion that didn't try to burst their ear drums 24/7. The issue of course was that sailing, while quiet, is both reliant on the wind and is not as fast or direct as propeller using ships.

Thus the question is: What propulsion method can match the performance of propeller ships while staying quiet?

Notes:

  • Lets assume the land powers want economic security in their shipping and trade routes from Mer folk control and would thus not ask for the Merfolk to ship goods in giant modified Humpback whales.
  • Some proposals that might in the near future or are theoretically possible are included. Thought the better understood it is, the better.
  • The land powers can't bully the Mer folk on the matter. They tried once with nuclear depth charges. It did not end well.
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  • $\begingroup$ The loud parts are typically propellers and engines. Try sailing, that eliminates both. $\endgroup$
    – Burki
    Commented Oct 7, 2022 at 10:24
  • $\begingroup$ you know you can just place the screw inside the ship, f.e. in a pipe that goes through its body. this will eliminate pretty much of the sound, plus you can build a box around it that devours further vibrations. But i guess what you really want to know is how the MOTORS of the ship can become more quiet - because i doubt the propeller makes that much noise on its own. and the answere to that would be nuclear-powered electronic motors instead of diesel motors $\endgroup$
    – user59660
    Commented Oct 7, 2022 at 15:19
  • $\begingroup$ if the ships go slower there is no cavitation thus negligible noise. if i had the use of boh hands i would make an answer of this $\endgroup$
    – John
    Commented Oct 7, 2022 at 20:17
  • $\begingroup$ Can it still use wind? They're developing container ships that use rotors. One example from Maersk. $\endgroup$
    – BruceWayne
    Commented Oct 7, 2022 at 21:29
  • $\begingroup$ Wait, what? "They tried once with nuclear depth charges" is highly incompatible with "when faster ships that used propellers and not sails started to become more popular". What technological period is this question about? Assuming a similar technological timeline as we have in the real world, is this question about what's possible when the Mer folk request this during the transition from sail to powered/propeller ships, or is it something which the Mer folk are requesting once the surface dwellers have nuclear weapons? Those are substantially different technology levels. $\endgroup$
    – Makyen
    Commented Oct 7, 2022 at 21:58

2 Answers 2

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Military submarines have means of being very quiet, sometimes even quieter than the background noise, thus becoming the equivalent of a black hole for sound.

With the needed pressure it's totally possible that military technology will be made available to the civil world, ship included. At the end, if those mer folks are so powerful that you don't want to bother them with ship noises, you don't want to bother them with sea battles either, so the need for having stealth submarines is strongly decreased.

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According to the webpage How does shipping affect ocean sound levels?, one major source of noise is cavitation due to the propeller. Cavitation is caused by changes in water pressure creating bubbles underwater, which then collapse, creating sound. However, propellers can be designed to rotate at certain speeds without causing cavitation.

Along with the propeller and other machinery, the very shape of the ship's hull changes how much sound it generates as it is underway. Again, the ship could be designed to minimize this sound.

Finally, as a rule of thumb, faster ships will generate more sound. Thus, slower cruising speeds will become the norm.

Putting all this together, mer-friendly shipping might involve ships with specially designed hulls and propellers traveling slower than the average ship does in our world. Imagine an angular hulled ship, which looks like the Zumwalt-class destroyer, crusing at 10 knots.

Sea Jet out of the water and showing the unique hull design

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