Do fish and jellyfish swim better than submarines? It depends a lot on what you mean by ‘better’. Some people have argued that fish are more efficient than subs, meaning that they waste a smaller fraction of the total propulsive energy. Others have argued that efficiency isn't all that important; instead, it's just the total energy that matters, regardless of what fraction of it is wasted.
One major difference is that jellyfish and fish produce thrust in pulses, whereas propellers on submarines produce thrust more or less continuously.
There's good reason to suppose that pulsed propulsion (jellyfish) might be more efficient than steady jets (propellers). For example, pulses produce vortex rings, which tend to pull in extra fluid because of their rotation, resulting in a jet that's effectively larger than an equivalent steady jet. But pulses may also take more energy to produce.
Unfortunately, it's rather hard to compare a jellyfish with a sub. One is fast; the other is slow. One uses muscle; the other uses motors. The body shapes tend to be different. There are so many differences that it's hard to know whether the mode of propulsion makes any difference. But Lydia Ruiz, Robert Whittlesey and John Dabiri at the California Institute of Technology have recently developed a way to make a direct comparison. They built an ingenious submarine that can propel itself with pulses, kind of like a jellyfish, or with a continuous jet, like a normal sub. They carefully designed the device so that it always uses the same motor, the same transmission and the same body, whether it's pulsing or producing a steady jet.
The sub looks like a torpedo, with a ring of holes in the hull. A normal propeller sits inside and pulls fluid in through the holes and pushes it out through the back end. That's basically like a torpedo. The ingenious part is that inside the hull is an extra spinning ring that also has holes. When the holes in the inner ring align with the holes in the hull, then fluid comes into the hull and passes through the propeller, which pushes it out the back, resulting in a pulse of thrust. If the inner ring has very large holes, then fluid can reach the propeller almost all the time, resulting in a steady jet. If the inner ring has smaller holes, then fluid only reaches the propeller when the holes align. By swapping just that one inner ring, the group was able to convert a sub with a steady jet to one with a pulsed jet – and keep everything else the same, so that they could compare the performance.
Indeed, the group found that pulsed propulsion was almost always 20–40% more efficient than the baseline steady jet. At low pulse rates, though, the energy saved through increased efficiency didn't make up for the extra energy required to spin the ring to make the pulsed jet. But at high pulse rates, the group saw an overall energy saving.
So jellyfish, and maybe also fish or biomimetic submarines with flapping fins, may in fact swim better than subs, but only if they're clever about how they produce their pulses.
