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?
- 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.