"NO KNOWN SEA-CREATURE uses propellers. Perhaps that is because they are too difficult to evolve from existing animal body plans. Or perhaps it is because they are not particularly good at doing what they do. When pushing water around for propulsive purposes, bigger is not only more powerful but also more efficient. But the bigger a propeller is, the harder it is to accommodate to a hull and the more it risks adding to a ship’s draft and thus snagging the seabed. Even the biggest ships’ propellers are therefore only around ten metres in diameter.

Fins and flippers, by contrast, extend sideways, so do not suffer from such geometric restrictions. That means they can get big enough to push a lot more water around. Nor, unlike propellers, need they be rigid. In fact, being flexible is almost part of the definition (a rigid fin might better be described as an oar). They are therefore not easily damaged by contact with the seabed or other objects. Fins have thus become evolution’s go-to accoutrement for marine propulsion. From fish, via ichthyosaurs, to dolphins and whales, they turn up again and again. So, from plesiosaurs and turtles to seals and penguins, do their cousins, flippers.

In light of this evolutionary vote of confidence in fins, ships’ propellers look like a technology ripe for a bit of biomimetic disruption. And that may now have arrived in the shape of Benjamin Pietro Filardo, an ex-marine biologist and architect who was looking into ways of designing devices to extract power from water currents. His plan was to use flexible materials, so that they could easily shake off any debris which got entangled in them. He then realised that the undulations involved might also usefully be turned into thrust.

Mr Filardo has put his money where his mouth is. His firm, Pliant Energy Systems, based in New York, has developed Velox (pictured), a prototype propelled by flexible fins, port and starboard, that are reminiscent of yet another animal’s approach to swimming—the undulating mantle of a cuttlefish. Velox can travel on the surface, underwater, and also across mud or ice, with its fins then acting in the manner of a pair of robotic caterpillars."

https://www.economist.com/science-and-technology/nature-does-not-use-propellers-so-why-do-people/21806832