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Old 05-08-2012, 05:25 AM
DSeid DSeid is online now
Join Date: Sep 2001
Location: Chicago, IL
Posts: 20,141
A relevant article:
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. ...

... 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.