Nature does not use propellers. So why do people?

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.